Physical Properties of Materials

Physical Properties of Materials is part of Physics. Below are notes on 14 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Density and Specific Gravity

• Density (ρ) is mass per unit volume (ρ = m/ V). SI unit: kg/m³. CGS unit: g/cm³.
• Relative Density (Specific Gravity) is the ratio of a substance's density to water's density at 4°C. It is a dimensionless quantity.
• Water exhibits anomalous expansion: its density increases from 0°C to 4°C.
• Water has maximum density at 4°C (277 K).
• The maximum density of water at 4°C is 1000 kg/m³ or 1 g/cm³.
• Ice is less dense than liquid water, causing it to float.
• When ice floating in water melts, the water level remains unchanged.
• An object floats if its density is less than the fluid's density; it sinks if its density is greater.
• Example: Iron sinks in water (DensityIron > DensityWater) but floats in mercury (DensityIron < DensityMercury).
• Common substances in decreasing order of density: Iron > Honey > Water > Air.
• Common substances in increasing order of density (examples): Acetone (approx. 0.79 g/ cm³) < Water (1 g/cm³) < Milk (approx. 1.03 g/cm³) < Copper (approx. 8.96 g/cm³) < Lead (approx. 11.34 g/cm³) < Iron (approx.

< Gold (approx. 19.3 g/cm³) < Platinum (approx. 21.45 g/cm³).

• For two miscible liquids of densities ρ₁ and ρ₂ mixed in equal volumes, the density of the mixture is (ρ₁ + ρ₂) / 2.
• For two miscible liquids of densities ρ₁ and ρ₂ mixed in equal masses, the density of the mixture is 2ρ₁ρ₂ / (ρ₁ + ρ₂).
• The average density of a composite object is its Total Mass / Total Volume.
• States of Matter and their properties:
• Solid: Fixed shape and fixed volume; least compressible.
• Liquid: Fixed volume but no fixed shape; more compressible than solids but less than gases. (Example: Kerosene)
• Gas: No fixed shape and no fixed volume; most compressible.
• Bose-Einstein Condensate (BEC): The fifth state of matter, formed by cooling a gas of extremely low density to very low temperatures.
• Compressibility order: Solid < Liquid < Gas.
• Air's low density and compressibility make it suitable for vehicle tyres.
• Helium (low density, non-flammable) is used in meteorological balloons and in hospital oxygen mixtures (often with oxygen) to aid respiration.
• Atmospheric pressure decreases with increasing altitude; low atmospheric pressure can cause nosebleeds.
• When a solute (e.g., sugar) dissolves in a solvent (e.g., water), the total mass, volume, density, and concentration of the solution generally change.
• Pure water is a non-conductor of electricity. Its latent heat of fusion is high. Its specific heat is abnormally high.
• Institute of Nano Science and Technology is located in Mohali.
• When a neutral atom forms a cation (loses electrons), its size decreases.
• A Face-Centered Cubic (FCC) unit cell contains 4 atoms per unit cell.

Diamagnetism,

Paramagnetism,

and

Ferromagnetism

• Diamagnetism:
• Exhibited by all materials, but often masked.
• Caused by induced magnetic dipoles opposing external field (orbital electron motion).
• Weakly repelled by external magnetic field.
• No permanent magnetic dipoles; all electrons are paired.
• Magnetic susceptibility (χ) is small, negative, and independent of temperature.
• Examples: Water (H₂O), Gold, Silver, Copper, Mercury, Bismuth, inert gases, organic compounds.
• Paramagnetism:
• Exhibited by materials with unpaired electrons (possess permanent magnetic dipoles).
• Weakly attracted by external magnetic field.
• Magnetic dipoles align with external field.
• Magnetic susceptibility (χ) is small, positive, and inversely proportional to absolute temperature (Curie's Law: χ ∝ 1/T).
• Examples: Platinum (Pt), Aluminium (Al), Oxygen (O₂), Sodium (Na), Chromium(III) (Cr³⁺), Copper(II) (Cu²⁺), Manganese (Mn), Gadolinium (Gd), Ferric Chloride (FeCl₃).
• Liquids and gases can exhibit diamagnetic or paramagnetic properties.
• Ferromagnetism:
• Exhibited by materials with strong, permanent magnetic dipoles that spontaneously align in magnetic domains.
• Strongly attracted by external magnetic field.
• Large, positive magnetic susceptibility (χ).
• Displays magnetic hysteresis (magnetization lags behind applied field).
• Retentivity (Remanence): Ability to retain magnetization after the external field is removed, even when the applied magnetic field is reduced to zero.
• Coercivity: Reverse magnetic field required to reduce residual magnetization to zero.
• Can form permanent magnets due to high retentivity and coercivity.
• Becomes paramagnetic above its Curie Temperature (Tc), leading to loss of magnetism upon heating.
• Examples: Iron (Fe), Nickel (Ni), Cobalt (Co), Gadolinium (Gd), alloys like Alnico, some Stainless Steel grades.
• Natural magnet: Lodestone (Fe₃O₄).
• Permanent Magnets: Made from materials with high retentivity and coercivity (e.g., Steel, Alnico).
• Electromagnets / Induction Cores: Made from materials with low retentivity and coercivity (e.g., Soft Iron, Soft Ferrites like Ni-Zn ferrite).
• Tape recorder tapes are coated with ferromagnetic powder (e.g., iron oxides).
• Liquids and gases generally do not exhibit ferromagnetic properties (requires ordered domain structure, typically solid-state).
• Magnetic Properties and Nanomaterials: Nanostructured materials can have different magnetic properties compared to bulk forms due to size and surface effects.
• Materials generally NOT attracted by common magnets (diamagnetic/paramagnetic/ non-magnetic): Plastic, Carbon, Wood, Gold, Copper, Silver, Aluminium (very weak attraction, often considered non-magnetic in practical contexts).

Dielectric Properties

• Dielectrics are electrical insulators that can be polarized by an applied electric field.
• Dielectric Constant (κ or εr): Ratio of permittivity of material (ε) to permittivity of free space (ε₀). κ = ε/ε₀.
• The dielectric constant (κ) is a dimensionless quantity (M⁰L⁰T⁰).
• When a dielectric of constant κ fills the space between capacitor plates, the new capacitance (C') becomes C' = κC, where C is the capacitance with air/vacuum.
• At the boundary between two dielectric media:
• The tangential component of the electric field (Et) is continuous (E₁t = E₂t).
• The normal component of the electric displacement (Dn) is continuous (D₁n = D₂n), which implies κ₁E₁n = κ₂E₂n.
• Insulators (dielectrics) can be charged by static electricity (e.g., friction).
• Bundled conductors in high voltage transmission lines reduce corona discharge (ionization of surrounding air/dielectric) by lowering the electric field gradient at the conductor surface.

Elasticity and Plasticity

Elasticity is the property of a material to 1. regain its original shape and size after the removal of deforming forces. Plasticity is the property of a material to 2. undergo permanent deformation without fracture after removal of deforming forces. Hooke's Law states that within the elastic 3. limit, stress is directly proportional to strain. It is valid only for the proportional region of the stress-strain curve. Stress ($\sigma$) = Force/Area; Strain ($ 4. \varepsilon$) = Change in dimension/ Original dimension. Young's Modulus (E) = Longitudinal Stress/ 5. Longitudinal Strain. Bulk Modulus (K) = Volumetric Stress/Volu6. metric Strain = -ΔP / (ΔV/V). Rigidity Modulus (N or G) = Shearing Stress/ 7. Shearing Strain. The relation connecting Young's modulus (E), 8. Bulk modulus (K) and Rigidity modulus (N) is: $\frac{9}{E} = \frac{3}{N} + \frac{1}{K} $. A perfectly rigid body experiences no de9. formation, hence its Young's Modulus is infinite. Perfectly plastic bodies undergo permanent 10. deformation without further increase in stress beyond the yield point; examples include paraffin wax and putty. Steel is considered more elastic than rubber, 11. copper, or sponge because it has a higher Young's Modulus and a greater elastic limit, allowing it to store more elastic energy. This makes steel suitable for springs. Elastic potential energy per unit volume (W) 12. = $\frac{1}{2} \times \text{Stress} \times \text{Strain}$ (W = $\frac{1} {2}\sigma\varepsilon$). The spring constant of a spring depends on 13. its material, length, and diameter/thickness. For a rod of length L and cross-sectional 14. area A under force F, elongation ($\Delta L$) = (FL)/(YA), where Y is Young's Modulus. $ \Delta L$ is inversely proportional to the cross-sectional area (A = $\pi r^2$). Thus, doubling the radius quadruples the area and reduces elongation by a factor of four. When materials are melted and reshaped into 15. different forms (e.g., spheres into a wire), their total volume remains conserved. Anisotropic materials exhibit properties that 16. vary with direction (e.g., Quartz). Isotropic materials have uniform properties in all directions (e.g., Glass, Rubber, Plastic are generally considered isotropic). (Civil Engineering context) IS : 456 : 2000 17. specifies the modulus of elasticity of concrete (Ec) as $5000\sqrt{f{ck}}$, where $f{ck}$ is the characteristic compressive strength in N/ mm². (Civil Engineering context) In limit state 18. design of concrete structures, the strain distribution is assumed to be parabolic up to a certain strain and then rectangular. (Civil Engineering context) Nominal concrete 19. cover for durability (M45 grade): Mild exposure is 20 mm, Severe exposure is 45 mm, Very Severe exposure is 50 mm. (Civil Engineering context) For a 6 m overall 20. height cantilever retaining wall, the base slab thickness may be around 450 mm as per design practices.

Electrical Conductivity and Resistivity

• Tungsten (W) is commonly used for electric bulb filaments due to its high melting point and high resistivity, enabling incandescent light emission.
• Gradual decrease in bulb brightness over time is caused by the evaporation of tungsten metal from the filament.
• Nichrome (an alloy of Nickel and Chromium) is typically used for heating elements in electric irons due to its high resistivity, high melting point, and resistance to oxidation at high temperatures.
• Fuse wire is an alloy of lead and tin, chosen for its low melting point and relatively high resistivity, designed to melt and break a circuit under excessive current.
• For semiconductors, resistivity (ρ) decreases as temperature (T) increases because higher thermal energy generates more free charge carriers.
• Pure Silicon (Si) at 0 K acts as an insulator; it functions as a semiconductor at higher temperatures or when doped with impurities.
• Silicon (Si) is the most commonly used semiconductor material for solar cells, transistors, and integrated circuits.
• Germanium (Ge) was prominently used in early transistors and is also a semiconductor.
• Light Emitting Diodes (LEDs) are made from semiconductor compounds such as Gallium Nitride (GaN), Gallium Arsenide (GaAs), or Indium Gallium Nitride (InGaN).
• LASER diodes are often made of Gallium Arsenide (GaAs).
• Material classification by bandgap (Eg): Conductors (Eg ≈ 0), Semiconductors (0.2 eV < Eg < 3 eV), Insulators (Eg > 3 eV, typically > 5 eV).
• A material with a bandgap of 5.0 eV is classified as an insulator.
• Electron-hole pairs in semiconductors can be generated by thermal excitation, impact ionization, and photo excitation. Impurity injection (doping) alters carrier concentration but does not directly generate intrinsic electronhole pairs from the host material.
• Rubidium is used in photoelectric cells (to convert light energy into electrical energy) due to its low work function and high photosensitivity.
• Graphene is a crystalline allotrope of carbon with two-dimensional properties. It exhibits the highest electrical conductivity at room temperature among known materials and is an excellent conductor of heat.
• Lead-acid storage cells utilize Sulfuric acid (H₂SO₄) as their electrolyte.

Hardness, Toughness, and Brittleness

• Hardness: Resistance of a material to scratching, indentation, or abrasion.
• Mohs' Scale of Hardness: Used to determine the relative hardness of minerals. Diamond is the hardest substance known (Mohs 10).
• Diamond's Hardness: Attributed to its crystal lattice where each carbon atom is bonded to four others by strong covalent bonds, forming a giant covalent structure.
• Tungsten: A metal known for being both very hard and very ductile.
• Toughness: The ability of a material to absorb energy and deform plastically up to the point of fracture. It measures the energy absorbed before failure, especially for ductile materials.
• Brittleness: The tendency of a material to fracture with little or no prior plastic deformation, absorbing minimal energy before failure. Brittle materials break suddenly.
• Ductility: The property of a material (primarily metals) to be drawn into thin wires without fracturing. Examples include Gold (Au) and Copper.
• Malleability: The property of a material (primarily metals) to be hammered or rolled into thin sheets without cracking. Examples include Gold (Au) and Aluminium.
• Nanoparticle: A particle with at least one dimension in the nanoscale range, typically defined as 1 to 100 nanometres (nm). (1 nm = 10⁻⁹ m).
• Quantum Dot: A semiconductor nanostructure that confines charge carriers (electrons, holes) in all three spatial dimensions, exhibiting quantum mechanical properties.
• Quantum Well: A semiconductor structure that confines charge carriers in one dimension, allowing free movement in the other two dimensions.
• Graphene: A 2D nanomaterial composed of a single layer of carbon atoms arranged in a hexagonal lattice. Its applications include bendable electronics, super lenses, quantum computing, and advanced water purification.

Magnetic Susceptibility and Permeability

• Bar Magnets: Fundamental properties essential for understanding magnetic materials.
• Magnetic poles always exist in pairs (North & South); magnetic monopoles do not exist.
• Pole strength of North pole is equal in magnitude to that of South pole, but opposite in nature.
• If a bar magnet is broken or bisected (either parallel or perpendicular to its axis), each fragment acts as a new complete magnet with its own North and South poles.
• Magnetic Permeability (μ):
• Measures a material's ability to support the formation of a magnetic field within itself.
• Defined as the ratio of magnetic flux density (B) to magnetic field intensity (H): μ = B / H.
• Unit: Henry per meter (H/m).
• Permeability of free space (μ₀): A constant value, 4π × 10⁻⁷ H/m.
• Relative Permeability (μr): Dimensionless ratio of a material's permeability to the permeability of free space: μr = μ / μ₀.
• Magnetic Susceptibility (χm):
• Measures how easily a material can be magnetized in an external magnetic field.
• Defined as the ratio of magnetization (M) to magnetic field intensity (H): M = χm H.
• It is a dimensionless quantity.
• Relationship with relative permeability: μr = 1 + χm.
• Classification of Materials Based on Magnetic Properties (μr, χm):
• Diamagnetic Materials:
• Weakly repelled by magnetic fields.
• χm is small and negative (e.g., -10⁻⁵ to -10⁻⁶).
• μr is slightly less than 1 (e.g., 0.9999).
• Examples: Copper, Bismuth, Water, Gold, Mercury.
• Paramagnetic Materials:
• Weakly attracted by magnetic fields.
• χm is small and positive (e.g., +10⁻³ to +10⁻⁵).
• μr is slightly greater than 1 (e.g., 1.0001).
• Examples: Aluminum, Platinum, Oxygen, Sodium.
• Ferromagnetic Materials:
• Strongly attracted by magnetic fields; can be permanently magnetized.
• χm is large and positive (e.g., +10² to +10⁵).
• μr is very large, much greater than 1 (e.g., 100 to 100,000).
• Exhibit magnetic hysteresis and domain structure.
• Become paramagnetic above their specific Curie temperature.
• Examples: Iron, Nickel, Cobalt, Gadolinium.
• Antiferromagnetic Materials:
• Adjacent magnetic moments align antiparallel and are of equal magnitude, resulting in zero net magnetization.
• χm is small and positive, temperature-dependent.
• Examples: MnO, Cr₂O₃.
• Ferrimagnetic Materials:
• Adjacent magnetic moments align antiparallel but are of unequal magnitude, resulting in a net magnetization.
• Properties are intermediate between ferromagnetic and antiferromagnetic materials.
• Examples: Ferrites.

Melting Point and Boiling Point

• Melting Point (MP): Temperature at which a solid converts into a liquid at a given atmospheric pressure.
• Boiling Point (BP): Temperature at which a liquid converts into a gas, when its vapor pressure equals the surrounding atmospheric pressure.
• States of Matter: Primarily Solid, Liquid, Gas. The Fourth state is Plasma, and the Fifth is Bose-Einstein Condensate (BEC).
• Change of State: A physical change, not a chemical change (e.g., evaporation of water). Occurs by changing temperature and/or pressure.
• Evaporation: A surface phenomenon where particles from the surface of a liquid change into the vapor state below its boiling point.
• Boiling: A bulk phenomenon where the entire liquid changes into the vapor state at its boiling point.
• Sublimation: The direct conversion of a solid into a gas without passing through the liquid state (e.g., camphor, naphthalene, dry ice).
• Deposition: The direct conversion of a gas into a solid without passing through the liquid state.
• Plasma State: The fourth fundamental state of matter, consisting of highly energetic and super-excited particles (ionized gas, free electrons). Examples include stars (Sun) and fluorescent light bulbs.
• Tungsten: Has a very high melting point, making it suitable for electric bulb filaments.
• Electric Bulbs: Tungsten filaments are usually filled with inert gases like Nitrogen and Argon to prevent the filament's evaporation/ sublimation and extend its lifespan.
• Liquid Nitrogen: Possesses an extremely low boiling point and is used for storing biological tissues (cryopreservation).
• Bromine: Is a red liquid at room temperature.
• Galinstan: An alloy of gallium, indium, and tin, which remains liquid at room temperature due to its low melting point, used as a non-toxic replacement for mercury.
• Iron-Carbon Phase Diagram: The eutectoid reaction, where austenite transforms into ferrite and cementite, occurs at approximately 727 °C.
• Thermit Welding: Uses a mixture of aluminium and iron oxides in a 3:1 ratio.

Refractive Index and Optical Transparency

• Refractive Index (n) is the ratio of the speed of light in vacuum (c) to the speed of light in a specific medium (v), i.e., n = c/v.
• It quantifies how much light slows down and bends (refracts) when entering a material.
• Optical Transparency is the ability of a material to transmit light without significant absorption or scattering.
• Materials are classified as transparent, translucent, or opaque based on their light transmission properties.
• 'Litracón' (Light Transmitting Concrete) is a building material that achieves optical transparency by embedding optical fibers within concrete.
• Selenium is a photoconductor, meaning its electrical resistance decreases significantly upon exposure to light.
• Selenium's photoconductive property is essential for the drum of a Photostat machine (xerography) and in certain types of solar cells.
• The working principle of a solar cell is based on the Photovoltaic Effect, where semiconductor materials convert light energy directly into electrical energy.
• Droplets form on the outside of a glass of cold water in a warm room because water vapor in the warmer air condenses into liquid water when it comes into contact with the colder glass surface, which cools the air below its dew point.

Specific Heat Capacity

Specific Heat Capacity (c): Amount of heat 1. energy required to raise the temperature of a unit mass (e.g., 1 kg) of a substance by one degree Celsius (or Kelvin). Formula: Q = mcΔT, where Q is heat energy, 2. m is mass, c is specific heat capacity, and ΔT is the change in temperature. SI Unit: Joules per kilogram per Kelvin (J/ 3. kg·K) or Joules per kilogram per degree Celsius (J/kg·°C). Nature: An intensive property, specific to the 4. material and its physical state (phase). Variation with State: The specific heat 5. capacity of a substance varies significantly with its state of matter (e.g., different for solid, liquid, and gas phases). Common States of Matter: Solid, Liquid, 6. Gas. Fourth State of Matter: Plasma (ionized gas, 7. found in stars, lightning). Fifth State of Matter: Bose-Einstein Con8. densate (BEC) (a state of matter formed by cooling a gas of extremely low density bosons to temperatures very close to absolute zero). Water's High Specific Heat Capacity: Ap9. proximately 4186 J/kg·°C, making it an excellent coolant and a moderator of global temperatures. Thermal Inertia: Substances with high specif10. ic heat capacity require more energy to change their temperature, thus exhibiting higher thermal inertia.

Tensile Strength and Yield Strength

• Tensile Strength (Ultimate Tensile Strength UTS): Maximum stress a material can endure under tension before breaking/fracture. It is the peak stress on the engineering stressstrain curve.
• Yield Strength: Stress at which a material begins to deform plastically (permanently). Below this stress, deformation is elastic.
• Stress: Internal force per unit area, typically measured in Pascals (Pa) or N/m².
• Graphene: Possesses the highest known tensile strength among all materials.
• Carbon Fibres: Exhibit high tensile strengthto-weight ratio, making them ideal for lightweight, high-strength applications like aircraft and automobile components.
• Aircraft Materials: Aluminium and Titanium alloys are commonly used due to their superior strength-to-weight ratios.
• Stress Concentration: The localized increase in stress that occurs around geometric discontinuities (e.g., holes, notches) in a loaded material. For a circular hole in a plate under tension, the maximum stress can be approximately 3 times the nominal stress.
• Hoop Stress (Thin Cylinders): The circumferential stress in a thin-walled cylinder subjected to internal pressure, calculated as `σh = (P r) / t` (where P = internal pressure, r = inner radius, t = wall thickness).
• Net Sectional Area: For a tensile member with holes, the net area is the minimum crosssectional area available to resist the tensile load, calculated as `(Gross Width - Sum of Hole Diameters) Thickness`.
• Wahl's Correction Factor: Used in the design of helical springs to account for both direct shear stress (transverse) and stress concentration due to the curvature of the spring wire.
• Average Bond Stress (τbd) in RCC: Its value depends on the concrete strength and the type of reinforcing bar used.
• IS 456-2000 (Plain and Reinforced Concrete): Stipulates that lap splices for reinforcing bars should not be used for bars larger than 32 mm diameter.
• IS 800 (General Construction in Steel): Specifies design criteria for steel structures, including minimum fillet weld sizes (e.g., 5 mm for connecting two 10 mm thick plates).
• IS 13920 (Ductile Detailing of RC Structures for Seismic Forces): Provides guidelines for earthquake-resistant design, including minimum column dimensions (e.g., 200 mm for seismic zones).

Thermal Conductivity

• Thermal conductivity (k) quantifies a material's ability to transfer heat. Higher k indicates faster heat transfer.
• Metals are generally good thermal conductors due to the presence of free electrons. Non-metals typically act as poor conductors or insulators.
• Order of thermal conductivity (decreasing, general): Silver > Copper > Gold > Aluminium > Iron > Stainless Steel.
• Among carbon nanoclusters, Graphene exhibits the highest known thermal conductivity at room temperature, surpassing Diamond.
• Mica is a good thermal conductor but an excellent electrical insulator, making it suitable for applications like electric irons to transfer heat while preventing electric shock.
• Pots made of materials with higher thermal conductivity (e.g., Silver, Copper) facilitate faster cooking by efficiently transferring heat to food.
• When objects are at the same temperature, materials with higher thermal conductivity (e.g., copper vs. glass) transfer heat to or from the hand more rapidly, resulting in a more intense sensation of hotness (if warmer than hand) or coldness (if cooler than hand).
• Inert gases, such as Argon, are used in incandescent light bulbs to reduce the rate of evaporation (sublimation) of the tungsten filament at high operating temperatures, thereby significantly increasing the bulb's lifespan.
• Examples of good heat conductors include Iron, Copper, and Aluminium. Examples of poor heat conductors (insulators) include Rubber, Glass, and Wood.

Thermal Expansion Coefficients

• Thermal Expansion: Tendency of matter to change in volume, area, or length in response to a change in temperature.
• Coefficient of Linear Expansion (α): Measures change in length per unit original length per unit temperature change (ΔL = L₀αΔT). Unit: /°C or /K.
• Coefficient of Area/Superficial Expansion (β): Measures change in area per unit original area per unit temperature change (ΔA = A₀βΔT). For isotropic materials, β ≈ 2α.
• Coefficient of Volume/Cubical Expansion (γ): Measures change in volume per unit original volume per unit temperature change (ΔV = V₀γΔT). For isotropic materials, γ ≈ 3α.
• Expansion of Holes: When a solid with a hole is heated, the hole expands as if it were made of the same material.
• Anomalous Expansion of Water: Water contracts when heated from 0°C to 4°C (density maximum at 4°C) and expands when cooled from 4°C to 0°C. It expands further upon freezing into ice, which can cause containers to break.
• Gas Expansion (Pressure Effect): Gases expand significantly when external pressure decreases (e.g., a balloon rising in the atmosphere due to reduced atmospheric pressure).
• Imbibition: A process where certain solid materials (e.g., wood, seeds) adsorb water, leading to a significant increase in their volume (swelling). This is due to water absorption, not thermal expansion.

Viscosity and Surface Tension

• Viscosity: Internal friction within a fluid, measuring its resistance to flow.
• Liquid Viscosity & Temperature: Decreases as temperature increases (e.g., glycerine's viscosity decreases with rising temperature).
• Gas Viscosity & Temperature: Increases as temperature increases.
• Honey has a relatively high viscosity compared to water, air, or blood.
• Ball pens operate on the principle of ink viscosity.
• Reynolds Number (Re): The transition from laminar to turbulent flow typically occurs in the range 2000 < Re < 4000.
• Viscosity (and surface tension) are properties inherent to fluids and would still exist in the absence of gravity.
• Surface Tension: The property of a liquid surface at rest that makes it behave like a stretched elastic membrane, tending to minimize its surface area. It arises from cohesive forces between liquid molecules.
• Molecules on the surface of a liquid have higher potential energy than those in the bulk (inside the liquid).
• Cohesive Forces: Attractive forces between molecules of the same substance (e.g., waterwater).
• Adhesive Forces: Attractive forces between molecules of different substances (e.g., waterglass).
• Wetting: A liquid wets a surface if adhesive forces are stronger than cohesive forces (e.g., water on clean glass plate, it spreads).
• Non-Wetting: A liquid does not wet a surface if cohesive forces are stronger than adhesive forces (e.g., mercury on glass).
• Mercury remains almost spherical on a clean glass plate because its cohesive force is greater than its adhesive force with glass; it does not wet glass.
• The contact angle of mercury with glass is greater than 90°.
• Spherical Shape: Raindrops and liquid drops (especially in space/reduced gravity) are spherical due to surface tension, which minimizes their surface area.
• Capillary Action (Capillarity): The rise or fall of a liquid in a narrow tube (capillary) due to the combined effects of surface tension, cohesion, and adhesion.
• Examples of Capillary Action:
• Blotting paper absorbing ink.
• Kerosene rising in a lamp wick.
• Water rising from plant roots to foliage (though transpiration pull is the primary driver, capillary action contributes significantly due to cohesive forces in water).
• Water rising in capillary tubes.
• Without capillarity, blotting paper and kerosene lamps would fail to function.
• Soaps & Detergents: Reduce the surface tension of water, allowing it to spread more easily, penetrate fabrics, and remove dirt.
• Floating Objects: Small, dense objects like an iron needle can float on the surface of water due to surface tension supporting their weight.
• Oil and Water: Do not mix due to differences in surface tension. Oil spreads on water because it generally has lower surface tension than water.
• Shaving Brush: Hairs of a shaving brush draw together when taken out of water due to the surface tension of water acting on the wet hairs.
• Soap Bubbles: The pressure inside a soap bubble is greater than the atmospheric pressure due to surface tension.
• Energy Changes with Drops:
• When two liquid drops merge to form a single larger drop, the total surface area decreases, and energy is released.
• When a single drop breaks into two, the total surface area increases, and energy is absorbed.
• When a drop breaks into smaller drops, the total volume remains constant.
• Particle Interactions: Particles of matter intermix on their own (diffusion) and have forces acting between them (cohesive and adhesive forces).
• Liquid water is denser than ice due to hydrogen bonding.

6. Optics

Diffraction:

Fraunhofer

and

Fresnel

diffraction, diffraction grating

• Diffraction: Phenomenon of bending of light waves around obstacles or spreading out of light after passing through an aperture. Occurs when obstacle/aperture size is comparable to or smaller than the wavelength of light.
• Fraunhofer Diffraction: Occurs when both the light source and the observation screen are effectively at infinite distances from the diffracting object. Characterized by plane wavefronts and typically observed using lenses.
• Fresnel Diffraction: Occurs when either the light source or the observation screen (or both) are at a finite distance from the diffracting object. Characterized by spherical or cylindrical wavefronts.
• Diffraction Grating: An optical component with a periodic structure (multiple parallel slits or grooves) that diffuses light, separating it into different wavelengths (spectra).
• Compact Discs (CDs) and DVDs: Display rainbow-like colors in sunlight due to the combined phenomena of reflection and diffraction from their microscopic grooves, which act as a reflection diffraction grating.
• Scattering: Redirection of light by particles in its path.
• Tyndall Effect: Scattering of light by colloidal particles or very fine suspensions, making the path of light visible. Responsible for the cloudy appearance of soap micelles.
• Rayleigh Scattering: Occurs when scattering particles are much smaller than the wavelength of light (particle size << λ). The intensity of scattered light is inversely proportional to the fourth power of the wavelength (I ∝ 1/λ^4).
• Rayleigh Scattering explanations:
• Blue color of the sky: Shorter blue wavelengths are scattered more effectively than longer red wavelengths.
• Reddening of the Sun at sunrise and sunset: When the sun is low, light travels through more atmosphere, and blue light is scattered away, leaving more red light to reach the observer.
• Diffusion of light in the atmosphere: Primarily caused by scattering from atmospheric dust particles and water vapours.
• Sharpest Shadow: Produced by a point source of light and an opaque screen, as it minimizes the penumbra (partial shadow) region.
• X-rays: Electromagnetic radiation with wavelengths typically in the range of 0.01 nm to 10 nm (e.g., 1 nm).
• X-ray Diffraction (XRD): A technique that utilizes the diffraction pattern of X-rays passing through a material to determine the atomic and molecular structure of crystals.

Practice MCQs: Physical Properties of Materials

Density and Specific Gravity

1. A form of matter has no fixed shape but it has a fixed volume. An example of this form of matter is

   Asked in: BPSC Prelims Exam 68th

   1. krypton
   2. kerosene
   3. carbon steel
   4. More than one of the above
   Show answer

   Answer: B. kerosene

   The description "no fixed shape but it has a fixed volume" perfectly matches the properties of the liquid state of matter. - Solid (e.g., carbon steel): Has a fixed shape and a fixed volume. - Liquid (e.g., kerosene): Takes the shape of the container but has a fixed volume. - Gas (e.g., krypton): Has neither a fixed shape nor a fixed volume; it expands to fill the entire container. Krypton is a gas, kerosene is a liquid, and carbon steel is a solid. Therefore, kerosene is the example that fits the description.

2. If two miscible liquids of same volume but different densities P1 and P2 are mixed, then the density of the mixture is given by

   Asked in: UPSC CAPF 2024 · UPSC CDS 2 2018

   1. (P1 + P2) / 2
   2. 2P1P2 / (P1 + P2)
   3. 2P1P2 / (P1 - P2)
   4. P1P2 / (P1 + P2)
   Show answer

   Answer: D. P1P2 / (P1 + P2)

   Understanding projectile motion under gravity, calculating time of flight, speed before impact, calculating speed after an inelastic collision (loss of speed), and calculating the maximum height reached with a new initial velocity. Ans: A - Density is mass per unit volume. - When mixing, total mass is the sum of individual masses, and total volume is the sum of individual volumes (assuming no volume change upon mixing, which is typical for miscible liquids unless otherwise specified). - The calculation is based on the given condition that the liquids have the 'same volume'.

3. When water is heated from 0°C to 4°C, its density

   Asked in: UPSC CDS 2 2022

   1. remains constant.
   2. increases.
   3. decreases.
   4. first increases then decreases to its original value.
   Show answer

   Answer: B. increases.

   Water exhibits anomalous expansion between 0°C and 4°C. Unlike most substances whose density decreases upon heating (as volume increases), water's volume decreases when heated from 0°C to 4°C. Density is mass per unit volume (ρ = m/V). Since the mass remains constant and the volume decreases in this temperature range, the density of water increases.

4. Which one of the following is the correct sequence of substances having densities in decreasing order?

   Asked in: UPSC Geoscientist 2020

   1. Iron, Water, Honey, Air
   2. Iron, Honey, Water, Air
   3. Iron, Honey, Air, Water
   4. Water, Iron, Air, Honey
   Show answer

   Answer: B. Iron, Honey, Water, Air

   Density is defined as mass per unit volume. Generally, solids are denser than liquids, and liquids are denser than gases. Among liquids, denser substances have more mass packed into the same volume.

5. Which one of the following has the least density at room temperature?

   Asked in: UPSC Geoscientist 2022

   1. Milk
   2. Acetone
   3. Red soil
   4. Iron
   Show answer

   Answer: B. Acetone

   - Approximate densities at room temperature: - Milk: Approx. 1.03 g/cm³ (slightly denser than water). - Acetone: Approx. 0.79 g/cm³ (a lightweight organic solvent). - Red soil: Bulk density varies greatly but is typically in the range of 1.2 to 1.6 g/cm³ or higher, depending on composition and compaction. Even the solid mineral components 7.87 g/cm³ (a dense metal). - Comparing these values, Acetone (0.79 g/cm³) has the lowest density.

6. Institute of Nano Science and Technology is located in

   Asked in: GPSC Prelims CSP 2 2017

   1. New Delhi
   2. Mohali
   3. Cochin
   4. Mumbai
   Show answer

   Answer: B. Mohali

   The Institute of Nano Science and Technology (INST) is an autonomous institute under the Department of Science and Technology (DST), Government of India. It is located in Mohali, Punjab.

7. A hollow iron pipe is 21 cm long with external diameter 8 cm. If iron weighs 8 gm/cm³ and weight of the pipe is 3.696 kg, then what is its internal radius?

   Asked in: GPSC Prelims CSP 1 2023

   1. 2.75 cm
   2. 3 cm
   3. 3.25 cm
   4. None of the above
   Show answer

   Answer: B. 3 cm

   The volume of a hollow cylinder is the difference between the volumes of the outer and inner cylinders with the same height. Weight is calculated by multiplying volume by density.

Diamagnetism, Paramagnetism, and Ferromagnetism

8. The paramagnetic theory of magnetism applies to

   Asked in: BPSC Prelims Exam 67th Re Exam

   1. mercury
   2. iron
   3. platinum
   4. nickel
   Show answer

   Answer: C. platinum

   Paramagnetism is a form of magnetism where materials are weakly attracted to a magnetic field and lose their magnetism when the field is removed. This property arises from the presence of unpaired electrons in the material.

9. Which one of the following is not a ferromagnetic material?

   Asked in: UPSC CDS 1 2024

   1. Cobalt
   2. Iron
   3. Silver
   4. Ferric chloride
   Show answer

   Answer: C. Silver

   Ferromagnetic materials are strongly attracted to magnetic fields and can be permanently magnetized. Common examples include iron, cobalt, nickel, and their alloys. Silver is a diamagnetic material, which means it is weakly repelled by a magnetic field. Ferric chloride is a paramagnetic material, which is weakly attracted by a magnetic field. Neither diamagnetic nor paramagnetic materials are classified as ferromagnetic.

10. Liquids and gases never show

    Asked in: UPSC CDS 2 2016

    1. diamagnetic property
    2. paramagnetic property
    3. ferromagnetic property
    4. electromagnetic property
    Show answer

    Answer: C. ferromagnetic property

    Ferromagnetism requires a specific long-range ordering of magnetic moments and formation of magnetic domains, which is typically a property of crystalline solids. Liquids and gases lack the necessary atomic structure for this phenomenon.

11. Which among the following pair is an example of paramagnetic and ferromagnetic substances ?

    Asked in: CGPSC Prelims Exam 2024

    1. H₂O and Fe
    2. Cu²⁺ and Gd
    3. Cr³⁺ and MnO
    4. Fe and Cu²⁺
    Show answer

    Answer: B. Cu²⁺ and Gd

    Paramagnetic substances are weakly attracted by an external magnetic field due to the presence of unpaired electrons. Ferromagnetic substances are strongly attracted and can become permanently magnetized due to ordered magnetic domains.

12. The magnetization in ferromagnetic materials

    Asked in: UPSC Geoscientist 2022

    1. can never vanish
    2. is always opposite to the direction of applied magnetic field
    3. is always in the direction perpendicular to the applied magnetic field
    4. may not vanish even when the applied mag‐ netic field is reduced to zero
    Show answer

    Answer: D. may not vanish even when the applied mag‐ netic field is reduced to zero

    - Ferromagnetic materials show hysteresis in their magnetization curve. - Remanence is the residual magnetization left after the applied field is removed. - This property is essential for creating permanent magnets.

13. What happens to a bar magnet when it is heated?

    Asked in: UPSC Geoscientist 2022

    1. Its polarity reverses
    2. Its magnetism is increased
    3. Its magnetism remains unchanged
    4. Its magnetism is either reduced or lost
    Show answer

    Answer: D. Its magnetism is either reduced or lost

    - There is a specific temperature, called the Curie temperature, above which a ferromagnetic material loses its ferromagnetism and becomes paramagnetic. - Heating below the Curie temperature can still weaken the magnet's field, especially if the magnet is not uniformly heated or has structural imperfections.

14. Consider the following statements regarding Nanotechnology. 1. Nanotechnology is the understanding and control of matter at the Nano scale, at dimensions between approximately 1 and 1000 nanometres. 2. Stained-glass windows are an example of how nanotechnology was used in the pre-modern era. 3. Nanostructured materials can have different magnetic properties compared to other forms or sizes of the same material. Which of the above statement/s is/are correct?

    Asked in: GPSC Prelims CSP 2 2024

    1. 1, 2
    2. 2 only
    3. 2, 3
    4. 1, 2, 3
    Show answer

    Answer: D. 1, 2, 3

    - Statement 1 is incorrect. Nanotechnology is defined as the understanding and control of matter at the nanoscale, typically at dimensions between approximately 1 and 100 nanometres (nm). The range up to 1000 nm (or 1 micrometer) is generally outside the core definition of nanotechnology, although nanoscale effects can be relevant up to that size in some contexts. - Statement 2 is correct. Ancient stained-glass windows used gold and silver nanoparticles to create vibrant colours, demonstrating an early, albeit unintentional, use of nanomaterials. - Statement 3 is correct. Nanostructured materials often exhibit properties (like magnetic, optical, electrical, or mechanical) that are significantly different from their bulk counterparts due to quantum effects and increased surface area-to- volume ratio at the nanoscale.

Dielectric Properties

15. A parallel-plate capacitor, with air in between the plates, has capacitance C. Now the space between the two plates of the capacitor is filled with a dielectric of dielectric constant 7. Then the value of the capacitance will become

    Asked in: UPSC CDS 1 2017

    1. C
    2. C/7
    3. 7C
    4. 14C
    Show answer

    Answer: C. 7C

    The capacitance of a parallel-plate capacitor is given by C = ε A / d, where ε is the permittivity of the material between the plates, A is the area of the plates, and d is the distance between them. When there is air (or vacuum) between the plates, the permittivity is approximately ε₀, and the capacitance is C = ε₀ A / d. When the space is filled with a dielectric material of dielectric constant κ, the permittivity changes to ε = κ ε₀. Therefore, the new capacitance C' becomes C' = (κ ε₀) A / d = κ (ε₀ A / d). Given that the dielectric constant κ = 7, the new capacitance is C' = 7 C.

16. In a bipolar junction transistor

    Asked in: UPSC CDS 2 2016

    1. all the three regions (the emitter, the base and the collector) have equal concentrations of impur‐ ity
    2. the emitter has the least concentration of im‐ purity
    3. the collector has the least concentration of im‐ purity
    4. the base has the least concentration of impur‐ ity
    Show answer

    Answer: D. the base has the least concentration of impur‐ ity

    - A BJT consists of three semiconductor regions: the emitter, the base, and the collector, separated by two p-n junctions. - These regions are doped with impurities (donors or acceptors) to create n-type and p-type semiconductors. - The doping levels are designed specifically for the transistor's operation: - Emitter: Heavily doped (high concentration) to efficiently inject charge carriers (electrons or holes) into the base. - Base: Lightly doped (low concentration) and made very thin to allow most injected carriers from the emitter to reach the collector and to minimize recombination within the base. - Collector: Moderately doped (intermediate concentration, less than emitter but more than base) and typically larger in size to collect the carriers from the base and dissipate heat.

17. Two dielectric media D1 and D2 have dielectric constants 3 and 2, respectively. They are separated by x - z plane. A uniform electric field $\vec{E} = 3\hat{i} + 2\hat{j}$ exists in D1. Which one among the following is the correct electric field in D2 at the xz plane ?

    Asked in: UPSC Geoscientist 2024

    1. $\vec{E} = 2\hat{j}$
    2. $\vec{E} = 3\hat{i} + 3\hat{j}$
    3. $\vec{E} = 3\hat{i} - 2\hat{j}$
    4. $\vec{E} = 2\hat{i} + 3\hat{j}$
    Show answer

    Answer: B. $\vec{E} = 3\hat{i} + 3\hat{j}$

    - The interface between the two dielectric media D1 and D2 is the x-z plane, which is defined by $y=0$. The normal vector to this interface is along the y- axis, i.e., $\hat{j}$. - Medium D1 has dielectric constant $\epsilon_{r1} = 3$, and medium D2 has dielectric constant $\epsilon_{r2} = 2$. Let's assume D1 is in the region $y0$. - The electric field in D1 is given as $\vec{E}_1 = 3\hat{i} + 2\hat{j} $. - At the boundary between two dielectric media, two boundary conditions for the electric field and displacement field must be satisfied: 1. The tangential component of the electric field ($\vec{E}_{tan} $) is continuous across the boundary: $\vec{E} _{1,tan} = \vec{E}_{2,tan}$. 2. The normal component of the electric displacement field ($\vec{D} _{norm}$) is continuous across the boundary (assuming no free charges on the surface): $\vec{D} _{1,norm} = \vec{D}_{2,norm}$. - The tangential components of the electric field are those parallel to the x-z plane (along $\hat{i}$ and $\hat{k}$). From $\vec{E}_1 = 3\hat{i} + 2\hat{j}$, the tangential component is $\vec{E}_{1,tan} = 3\hat{i}$. - Therefore, the tangential component of the electric field in D2 is $\vec{E}_{2,tan} = 3\hat{i}$. This means the x-component of $\vec{E}_2$ is 3 ($E_{2x} = 3$) and the z-component is 0 ($E_{2z} = 0$). - The normal component of the electric field is along the y-axis ($\hat{j}$). From $\vec{E}_1$, the normal component is $E_{1y} = 2$. - The electric displacement field is given by $\vec{D} = \epsilon_0 \epsilon_r \vec{E}$. - The normal component of the displacement field in D1 is $D_{1y} = \epsilon_0 \epsilon_{r1} E_{1y} = \epsilon_0 \times 3 \times 2 = 6\epsilon_0$. - The normal component of the displacement field in D2 is $D_{2y} = \epsilon_0 \epsilon_{r2} E_{2y} = \epsilon_0 \times 2 \times E_{2y}$. - Applying the boundary condition $\vec{D}_{1,norm} = \vec{D} _{2,norm}$, we have $D_{1y} = D_{2y}$. - $6\epsilon_0 = 2\epsilon_0 E_{2y}$. - $6 = 2 E_{2y} \implies E_{2y} = 3$. - The electric field in D2 is $ \vec{E}_2 = E_{2x} \hat{i} + E_{2y} \hat{j} + E_{2z} \hat{k}$. - Substituting the values we found: $\vec{E}_2 = 3\hat{i} + 3\hat{j} + 0\hat{k} = 3\hat{i} + 3\hat{j}$.

18. Light Emitting Diode (LED) work on principle of

    Asked in: JPSC CS Prelims 2021

    1. Electroluminescence
    2. Laser
    3. Thermionic emission
    4. Photoelectric code
    Show answer

    Answer: A. Electroluminescence

    LEDs are made of semiconductor materials where electrons and holes recombine at a p-n junction, releasing energy in the form of photons. The color of the emitted light depends on the band gap energy of the semiconductor material used.

19. Which one of the following can charge an insulator ?

    Asked in: UPSC NDA 1 2019

    1. Current electricity
    2. Static electricity
    3. Magnetic field
    4. Gravitational field
    Show answer

    Answer: B. Static electricity

    Insulators are materials that resist the flow of electric current. However, they can be charged through the accumulation of static electric charge on their surface or within their volume. This typically happens through processes like friction (triboelectric effect), contact with a charged object, or induction, which involve the transfer or redistribution of electrons, creating an imbalance of charge.

20. The dimension of dielectric constant is A) M⁰L⁰T⁰ B) ML⁰T⁰ C) MLT⁰ D) M⁰L⁰T

    Asked in: cgpsc 2025 2025

    1. M⁰L⁰T⁰
    2. ML⁰T⁰
    3. MLT⁰
    4. M⁰L⁰T
    Show answer

    Answer: A. M⁰L⁰T⁰

    Physical quantities can be expressed in terms of fundamental dimensions: Mass (M), Length (L), Time (T), Electric Current (A), Temperature (K), Amount of Substance (mol), and Luminous Intensity (cd). Dimensionless quantities have zero powers for all fundamental dimensions.

21. A common fluorescent tube contains

    Asked in: FINANCE AND ACCOUNTS OFFICER(LDCE) 2017

    1. Sodium vapours
    2. Argon at low pressure
    3. Mercury vepour low low pressure
    4. Mercury vapours at high pressure
    Show answer

    Answer: C. Mercury vepour low low pressure

    Fluorescent tubes rely on the excitation of mercury vapour by an electric current to produce UV light, which is then converted to visible light by a phosphor coating.

Elasticity and Plasticity

22. The Hooke's law is valid for

    Asked in: UPSC CDS 1 2019

    1. only proportional region of the stress-strain curve
    2. entire stress-strain curve
    3. entire elastic region of the stress-strain curve
    4. elastic as well as plastic region of the stress- strain curve
    Show answer

    Answer: A. only proportional region of the stress-strain curve

    Hooke's law (Stress ∝ Strain) is strictly valid only in the proportional region of the stress-strain curve.

23. The spring constant of a spring depends on its

    Asked in: UPSC CDS 2 2016

    1. length only
    2. material only
    3. length and its diameter
    4. thickness, its diameter and its material
    Show answer

    Answer: D. thickness, its diameter and its material

    - The spring constant (k) quantifies the stiffness of a spring. - For a helical spring, the spring constant is given by the formula k = (G d⁴) / (8 D³ n), where G is the shear modulus of the material, d is the wire diameter (thickness), D is the coil diameter, and n is the number of active turns (related to length). - Therefore, k depends on the material (shear modulus G), the wire thickness (d), the coil diameter (D), and the number of turns (n, which determines the overall length for a given coil diameter and pitch). - Option D includes thickness (wire diameter), diameter (coil diameter), and material, which are the key determinants of the spring constant.

24. A steel rod having radius r and length L gets stretched along its length by ΔL, when a force F is applied to it. If another rod made of the same material having radius 2r and length L is subjected to the same force F, then the elongation observed for the second rod is

    Asked in: UPSC Geoscientist 2023

    1. 4ΔL
    2. 2ΔL
    3. ΔL /4
    4. ΔL /2
    Show answer

    Answer: C. ΔL /4

    Elongation under a given force is inversely proportional to the cross-sectional area of the rod, assuming the material, length, and force are constant. Area is proportional to the square of the radius.

25. After melting four metallic spheres of diameter 15 cm in a factory, wire of diameter of the cross-section of 0.2 cm was drawn. What would be the length of wire so obtained?

    Asked in: HAS Prelims 2023

    1. 1,950 m
    2. 2,040 m
    3. 2,170 m
    4. 2,250 m
    Show answer

    Answer: D. 2,250 m

    The principle is that the total volume of the metal remains constant when it is melted and reshaped. The volume of the four spheres is equated to the volume of the wire (cylinder).

26. The relation connecting Young's modulus (E), Bulk modulus (K) and Rigidity modulus (N) is

    Asked in: TNPSC CS Prelims 2015

    1. $\frac{9}{E} = \frac{3}{N} + \frac{1}{K}$
    2. $\frac{9}{N} = \frac{3}{E} + \frac{1}{K}$
    3. $\frac{9}{N} = \frac{1}{E} + \frac{3}{K}$
    4. $\frac{9}{E} = \frac{1}{N} + \frac{3}{K}$
    Show answer

    Answer: A. $\frac{9}{E} = \frac{3}{N} + \frac{1}{K}$

    The elastic moduli are related to each other and Poisson's ratio ($\nu$) by several equations. The standard relations are $E = 3K(1 - 2\nu)$ and $E = 2N(1 + \nu)$. Eliminating Poisson's ratio from these two equations leads to the combined relation: $E = \frac{9KN}{3K+N}$, which can be rearranged to $\frac{1}{E} = \frac{3K+N}{9KN} = \frac{3K}{9KN} + \frac{N}{9KN} = \frac{1}{3N} + \frac{1}{9K}$. Multiplying by 9 gives $9/E = 3/ N + 1/K$.

27. Which one of the following is anisotropic in nature?

    Asked in: UPSC CAPF 2018

    1. Glass
    2. Rubber
    3. Plastic
    4. Quartz
    Show answer

    Answer: D. Quartz

    Anisotropic materials have properties that vary with direction, typically due to their ordered internal structure (like crystals). Isotropic materials have properties that are the same in all directions, typically due to their disordered or randomly oriented structure (like amorphous solids or polycrystalline aggregates with randomly oriented grains).

28. Which one of the following is more elastic

    Asked in: WBCS Main Paper 4 2023

    1. Copper
    2. Rubber
    3. Sponge
    4. Steel
    Show answer

    Answer: D. Steel

    In physics, elasticity refers to a material's ability to deform under stress and return to its original shape when the stress is removed. A material is considered more elastic if it requires a greater amount of stress to produce a given amount of strain (deformation). This property is quantified by the elastic modulus (e.g., Young's Modulus). Steel has a significantly higher Young's modulus compared to copper, rubber, or sponge, meaning it is much stiffer and resists deformation more effectively within its elastic limit. While rubber can undergo large deformations and return to its shape, steel requires much greater force to deform even slightly within its elastic range.

Electrical Conductivity and Resistivity

29. The filament string of an electric bulb is made of

    Asked in: APSC CCE Prelims Exam 2020

    1. copper
    2. silver
    3. platinum
    4. tungsten
    Show answer

    Answer: D. tungsten

    Tungsten is used due to its extremely high melting point and strength at high temperatures, allowing it to withstand the heat generated when electricity passes through it, causing it to glow (incandesce).

30. Which one of the following schematic graphs correctly represents qualitatively the variation of resistivity ρ with respect to temperature T for a semiconductor?

    Asked in: UPSC Geoscientist 2023

    1. Graph (a)
    2. Graph (b)
    3. Graph (c)
    4. Graph (d)
    Show answer

    Answer: D. Graph (d)

    For a semiconductor, resistivity generally decreases as temperature increases. This is because increasing thermal energy excites more electrons into the conduction band and creates more holes in the valence band, increasing the number of free charge carriers available for conduction. More charge carriers lead to higher conductivity and thus lower resistivity. Graph (b) qualitatively shows this inverse relationship where resistivity decreases with increasing temperature.

31. With reference to 'graphene', which following statements is/are correct ?

    Asked in: KPSC KAS Prelims 2015

    1. Graphene conducts electricity faster at room temperature than any other known material.
    2. Graphene is a crystalline allotrope of carbon with two-dimensional properties.
    3. It is a bad conductor of heat. Select the code for the correct answer from the options given below : A. A and B only B. A only C. B only
    4. A, B and C
    Show answer

    Answer: A. Graphene conducts electricity faster at room temperature than any other known material.

    Graphene is a 2D allotrope of carbon known for its exceptional electrical and thermal conductivity and high electron mobility.

32. Which one of the following metals is used in the filaments of photo-electric cells that convert light energy into electric energy ?

    Asked in: UPSC NDA 1 2018

    1. Tungsten
    2. Copper
    3. Rubidium
    4. Aluminium
    Show answer

    Answer: C. Rubidium

    Alkali metals have low work functions, making them suitable for use as photoemissive materials in devices that rely on the photoelectric effect.

33. In an incandescent electric bulb, the filament of the bulb is made up of which metal ?

    Asked in: UPSC NDA 2 2020

    1. Aluminium
    2. Copper
    3. Tungsten
    4. Silver
    Show answer

    Answer: C. Tungsten

    The principle behind an incandescent bulb is resistive heating: an electric current passes through the filament, heating it to incandescence (glowing white hot) and emitting light. The bulb is usually filled with an inert gas (like argon or nitrogen) to reduce the evaporation of the tungsten filament, prolonging its life.

34. Which one of the following metals is most commonly used for making filament of incandescent electric bulbs?

    Asked in: UPSC NDA 2 2022

    1. Aluminium
    2. Silver
    3. Copper
    4. Tungsten
    Show answer

    Answer: D. Tungsten

    - The filament in an incandescent bulb needs to glow white-hot (reach a very high temperature) to produce light. - This requires a material with a very high melting point that can withstand such extreme temperatures without melting or vaporizing quickly. - Tungsten (chemical symbol W) has the highest melting point of all known metals (3422°C or 6192°F). - It also has high tensile strength, even at high temperatures, and relatively low vapor pressure, reducing evaporation of the filament.

35. The heating element in an electric iron is usually made of

    Asked in: UPSC NDA 2 2023

    1. Constantan
    2. Tungsten
    3. Nichrome
    4. Copper
    Show answer

    Answer: C. Nichrome

    Nichrome is an alloy primarily composed of nickel and chromium. It is chosen for heating elements because it has a high electrical resistance, allowing it to efficiently convert electrical energy into heat. It also has a high melting point and resistance to oxidation at high temperatures, ensuring durability and safety during operation.

Hardness, Toughness, and Brittleness

36. Which one of the following metals has both malleability and ductility properties ?

    Asked in: UPSC CDS 1 2021

    1. Na
    2. Au
    3. Ce
    4. Hg
    Show answer

    Answer: B. Au

    - Malleability is the ability of a material to be deformed under compression without fracturing (hammered into sheets). - Ductility is the ability of a material to be deformed under tensile stress without fracturing (drawn into wires). - These properties are characteristic of metals due to the nature of metallic bonding.

37. Mohs' Scale is used to determine

    Asked in: GPSC Prelims CSP 2 2017

    1. Brightness of substance
    2. Viscosity of a fluid
    3. Hardness of minerals
    4. Elasticity of minerals
    Show answer

    Answer: C. Hardness of minerals

    - Mohs' scale of mineral hardness is a relative scale that measures the scratch resistance of minerals. - Developed by German mineralogist Friedrich Mohs in 1812, the scale ranks minerals from 1 (softest, like Talc) to 10 (hardest, like Diamond). - The test involves observing whether a mineral is scratched by another substance of known hardness or whether it can scratch that substance. - This scale is widely used by geologists and mineralogists for the identification and classification of minerals.

38. A particle having at least one dimension less than 10⁻⁷ metre, is known as

    Asked in: RPSC RAS Prelims 2018

    1. Micro particle
    2. Milli particle
    3. Nano particle
    4. Macro particle
    Show answer

    Answer: C. Nano particle

    Nanoparticles are typically defined as particles with at least one dimension in the size range of 1 to 100 nanometres (nm). One nanometre is equal to 10⁻⁹ metres. Therefore, 100 nm is equal to 100 x 10⁻⁹ m = 10⁻⁷ m. A particle having at least one dimension less than 10⁻⁷ metre falls within this nanoscale range.

39. A quantum dot is -

    Asked in: RPSC RAS Prelims 2021

    1. Electron microscopy image of nanostructures smaller than 1 nanometre
    2. Nanoscale analog of radio antennas
    3. A fictional nanorobot
    4. A semiconductor nanostructure
    Show answer

    Answer: D. A semiconductor nanostructure

    A quantum dot is a tiny semiconductor crystal that is so small (typically a few nanometers in diameter) that its electronic properties are significantly altered by quantum mechanics.

40. If in a material one dimension is reduced to the Nano range while the other two dimensions remain large, the structure so obtained is known as

    Asked in: RPSC RAS Prelims 2023

    1. Quantum step
    2. Quantum well
    3. Quantum dot
    4. Quantum wire
    Show answer

    Answer: B. Quantum well

    Nanostructures are classified based on the number of dimensions confined to the nanorange (typically 1-100 nm). A Quantum well is a structure where carriers (like electrons or holes) are confined in one dimension (e.g., a very thin layer or film), while being free to move in the other two dimensions.

41. Graphene is used for manufacturing bendable smart phones. What is the other field for which it can be used?

    Asked in: TNPSC CS Prelims 2014

    1. Medical Radiation therapy
    2. Super lenses
    3. Quantum computers
    4. Ultimate water purifier
    Show answer

    Answer: D. Ultimate water purifier

    Graphene oxide membranes can be engineered with tiny pores that allow water molecules to pass through but block larger salt ions and other impurities, making them highly effective filters.

42. Which one of the following materials is very hard and very ductile?

    Asked in: UPPCS Prelims 2000

    1. Carborundum
    2. Tungsten
    3. Cast iron
    4. Nichrome
    Show answer

    Answer: D. Nichrome

    - Carborundum (Silicon carbide) is extremely hard but very brittle. - Tungsten is very hard and has a high melting point, but it is also brittle at room temperature, although it can be made ductile at high temperatures. - Cast iron is relatively hard but is known for being brittle due to its high carbon content. - Nichrome is an alloy of nickel and chromium, often with iron. It is known for its high electrical resistance, resistance to oxidation at high temperatures, high melting point, and good mechanical properties. It is both hard and ductile, particularly useful for drawing into thin wires (ductility) that can withstand high temperatures (hardness/ strength).

Magnetic Susceptibility and Permeability

43. Which one of the following statements about bar magnet is correct ?

    Asked in: UPSC CDS 1 2016

    1. The pole strength of the north-pole of a bar magnet is larger than that of the south-pole
    2. When a piece of bar magnet is bisected per‐ pendicular to its axis, the north and south poles get separated
    3. When a piece of bar magnet is bisected paral‐ lel to its axis, two new bar magnets are formed
    4. The poles of a bar magnet are unequal in magnitude and opposite in nature
    Show answer

    Answer: C. When a piece of bar magnet is bisected paral‐ lel to its axis, two new bar magnets are formed

    Magnetic poles always exist in pairs. If you cut a bar magnet, you don't separate the north and south poles; instead, each resulting piece becomes a new magnet with its own north and south pole. Cutting parallel to the axis divides the magnet into two thinner magnets, each retaining its original length and thus forming new poles on the cut surface.

Melting Point and Boiling Point

44. Bromine is a

    Asked in: BPSC Prelims Exam 63rd

    1. colourless gas
    2. brown solid
    3. highly inflammable gas
    4. red liquid
    Show answer

    Answer: D. red liquid

    - Bromine (Br) is a chemical element from the halogen group (Group 17). - At standard room temperature and pressure, bromine is a volatile, corrosive, reddish-brown liquid. - It readily evaporates to form a reddish-brown gas with a strong, irritating odor. - It is one of only two elements that are liquid at room temperature (the other being mercury).

45. What is the maximum number of states of matter?

    Asked in: UPSC CDS 1 2017

    1. Three
    2. Four
    3. Five
    4. Variable
    Show answer

    Answer: C. Five

    Beyond the classical solid, liquid, gas, and plasma states, physics recognizes many other distinct states (phases) of matter. The number of these states is not fixed and can vary depending on the substance and the environmental conditions, as well as how a "state" is defined.

46. Which one of the following is used for storing biological tissues?

    Asked in: UPSC CDS 2 2020

    1. Liquid nitrogen
    2. Liquid helium
    3. Liquid argon
    4. Liquid bromine
    Show answer

    Answer: A. Liquid nitrogen

    Liquid nitrogen boils at -196°C (-320°F). This extremely low temperature effectively halts biological activity, including enzymatic reactions and cellular degradation, allowing long-term storage of samples such as sperm, eggs, embryos, blood cells, tissue biopsies, and even whole organs for transplantation in some cases.

47. What are true about plasma state? (i) Particles in this state are in ionic state. (ii) Light in the LED bulb is related to plasma state. (iii) Sunlight is also an example of plasma state.

    Asked in: CGPSC Prelims Exam 2022

    1. (i), (ii) and (iii)
    2. (i) and (ii)
    3. (i) and (iii)
    4. None of the above
    Show answer

    Answer: C. (i) and (iii)

    - Statement (i) is true. Plasma is often called the fourth state of matter and consists of ionized gas containing a mixture of ions and free electrons, which are in an ionic state. - Statement (ii) is false. Light in LED (Light Emitting Diode) bulbs is produced through electroluminescence in a semiconductor material, which is a solid-state phenomenon, not related to the plasma state. Light from fluorescent lamps or neon signs is produced by plasma. - Statement (iii) is true. The Sun and other stars are composed of plasma. While sunlight itself is electromagnetic radiation (light), the source of sunlight (the Sun) is in the plasma state. Therefore, sunlight originates from plasma.

48. Which of the following is correct? 1. Evaporation is a surface phenomenon, which means particles from the surface change into the vapour state. 2. Boiling is a bulk (whole) of the liquid change into vapour state. 3. Deposition is the change of gaseous state directly to solid without converting into the liquid state. 4. The state of matter can be changed by changing its temperature only.

    Asked in: HAS Prelims 2021

    1. 1, 3 and 4
    2. 1, 2 and 3
    3. 1, 2 and 4
    4. 2, 3 and 4
    Show answer

    Answer: B. 1, 2 and 3

    Evaporation is a surface process, boiling is a bulk process. Deposition is the direct transition from gas to solid. Phase changes depend on both temperature and pressure.

49. A sample of any of the following substances | disappears after sometime when exposed to air like camphor, naphthalene or dry ice. This phenomenon is called

    Asked in: UPPCS Prelims 2019

    1. Sublimation
    2. Evaporation
    3. Diffusion
    4. Radiation
    Show answer

    Answer: A. Sublimation

    Sublimation is the phase transition of a substance directly from the solid to the gas state, without passing through the liquid state. Camphor, naphthalene (mothballs), and dry ice (solid carbon dioxide) are classic examples of substances that readily undergo sublimation at room temperature and atmospheric pressure, causing them to "disappear" or turn directly into vapour.

50. The filament of electric bulb is generally made of tungsten because

    Asked in: UPSC CAPF 2013

    1. tungsten is cheap
    2. durability of tungsten is high
    3. light-emitting power of tungsten is high
    4. melting point of tungsten is high
    Show answer

    Answer: D. melting point of tungsten is high

    Incandescent light bulbs produce light by heating a wire filament to a very high temperature (around 2500-3000 °C) until it glows (incandescence). Tungsten has the highest melting point of all metals (3422 °C), allowing it to withstand these extreme temperatures without melting. This property is crucial for the filament's operation.

Refractive Index and Optical Transparency

51. Which one of the following elements the drum of a Photostat machine is made up of?

    Asked in: GPSC Prelims CSP 2 2019

    1. Aluminium
    2. Selenium
    3. Barium
    4. Caesium
    Show answer

    Answer: B. Selenium

    The core component of xerography (the process used in photocopiers) is the photoreceptor drum or belt. This drum is coated with a photoconductive material, which is electrically insulating in the dark but becomes conductive when exposed to light. Amorphous selenium was historically a common material for this purpose due to its excellent photoconductivity. Modern copiers often use organic photoconductors (OPC).

52. Working of a solar cell is based on which one of the following ?

    Asked in: Assistant Conservator of Forest Exam 2019

    1. Laser Technology
    2. Tyndall effect
    3. Thermal Emission
    4. None of these
    Show answer

    Answer: D. None of these

    Solar cells operate on the photovoltaic effect, not laser technology, Tyndall effect, or thermal emission.

53. 'Litracón' is applied in which of the following aspects?

    Asked in: Assistant Auditor (Direct) 2021

    1. Building material
    2. Classroom communication
    3. Deep sea navigation
    4. Advanced Flu research
    Show answer

    Answer: A. Building material

    Litracón is a trademarked material, often described as a type of lightweight concrete or engineered cementitious composite (ECC). Its key feature is its ability to withstand significant tensile strain, making it more durable and less prone to cracking than traditional concrete.

54. Why does a glass of cold water develop droplets on the outside in a warm room?

    Asked in: Paper I 5

    1. The glass is releasing moisture
    2. Water vapour in the air condenses on the cold surface
    3. The glass absorbs water from the surroundings
    4. Water is leaking through the glass
    Show answer

    Answer: B. Water vapour in the air condenses on the cold surface

    The phenomenon is due to the condensation of water vapor from the atmosphere onto a cold surface.

Specific Heat Capacity

55. Bose-Einstein Condensate is

    Asked in: UPSC CDS 2 2019

    1. solid state of matter
    2. fifth state of matter
    3. plasma
    4. state of condensed matter
    Show answer

    Answer: B. fifth state of matter

    Bose-Einstein Condensate represents a state of matter exhibiting unique quantum properties at extremely low temperatures.

56. In case of a 0.8% carbon steel, during eutectoid reaction at 727 °C with lever arm extending upto ferrite (0.0% Carbon) at one end and up to cementite (6.67 % C) at other end, the fraction of ferrite is given by - (5) Question not attempted

    Asked in: Assistant Eng. (Mechanical) Competitive Examination, 2023

    1. 0.88
    2. 0.12
    3. 0.75
    4. 0.25
    Show answer

    Answer: B. 0.12

    The lever rule is used to determine the relative amounts of two phases in a two-phase region of a phase diagram. For the eutectoid reaction in the Fe- C system, at the eutectoid temperature (727 °C), a steel with 0.8% carbon will transform into ferrite and cementite.

57. The natural source from which energy can be harnessed, which is essentially made up mainly of one element is

    Asked in: ASSISTANT AUDIT OFFICER 2017 18 2017

    1. Geothermal energy
    2. Air
    3. Water
    4. Coal
    Show answer

    Answer: D. Coal

    Energy sources can be categorized based on their origin and composition. The question asks for a natural source that is predominantly made up of a single element.

58. What was the fourth state of matter called?

    Asked in: Inspector(Legal Metrology & Consumer Affairs) 2020

    1. Solid
    2. Liquid
    3. Gas
    4. Plasma
    Show answer

    Answer: D. Plasma

    The commonly known states of matter are solid, liquid, and gas. Plasma is a high-energy state of matter where atoms are ionized.

Tensile Strength and Yield Strength

59. Consider the following statements : 1. Carbon fibres are used in the manufacture of components used in automobiles and aircraftes. 2. Carbon fibres once used cannot be recycled. Which of the statements give above is/are correct?

    Asked in: UPSC IAS 2023

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Show answer

    Answer: A. 1 only

    Statement 1 is correct. Carbon fibres and carbon fibre composites (CFRPs) are widely used in high- performance applications due to their high strength- to-weight ratio, stiffness, and corrosion resistance. They are integral in the manufacture of structural components for aircraft (e.g., wings, fuselage parts) and high-end automobiles (chassis, body panels), as well as sports equipment and wind turbine blades. Statement 2 is incorrect. While challenging and less developed than traditional material recycling, carbon fibre composites can be recycled using various methods, including mechanical recycling (shredding/milling), pyrolysis (thermal decomposition), and solvolysis (chemical decomposition). Recycled carbon fibres can then be used in new composite materials. The infrastructure and economic viability are still developing, but it is not true that they cannot be recycled.

60. Which one of the following materials is strongest?

    Asked in: UPPSC Upper Mains 2015

    1. German silver
    2. Brass
    3. Steel
    4. Graphene
    Show answer

    Answer: D. Graphene

    Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, is known for its exceptional tensile strength. Pound for pound, it is significantly stronger than steel and other conventional materials. While steel is a strong alloy widely used in construction and engineering, graphene's intrinsic strength at the atomic level surpasses it.

61. A thin cylinder of inner radius 500 mm and thickness 10 mm is subjected to an internal pressure of 5 MPa. The average circumferential (hoop) stress in MPa is 0 - Question not attempted

    Asked in: Assistant Eng. (Mechanical) Competitive Examination, 2023

    1. 100
    2. 250
    3. 500
    4. 1000
    Show answer

    Answer: A. 100

    The hoop stress in a thin cylinder is calculated using the formula $\sigma_h = \frac{Pr}{t}$, where P is the internal pressure, r is the inner radius, and t is the thickness.

62. A plate with a circular hole in the centre is subjected to a tensile load. The maximum stress induced at the edge of hole will be ______ of the normal stress. - (5) Question not attempted

    Asked in: Assistant Eng. (Mechanical) Competitive Examination, 2023

    1. double
    2. 2.5 times
    3. four times
    4. 3 times
    Show answer

    Answer: C. four times

    The presence of a hole or notch in a component under load causes a localized increase in stress, known as stress concentration. The ratio of the maximum stress at the discontinuity to the nominal stress applied is called the stress concentration factor (Kt).

63. In the calculation of induced shear stress in helical spring, the Wahl's correction factor is used to take care of combined effect of - (5) Question not attempted

    Asked in: Assistant Eng. (Mechanical) Competitive Examination, 2023

    1. Transverse shear stress and bending stress in wire.
    2. Transverse shear stress and curvature of wire.
    3. Torsional shear stress and transverse shear stress of wire.
    4. Bending stress and curvature of wire.
    Show answer

    Answer: B. Transverse shear stress and curvature of wire.

    Wahl's factor corrects the basic torsional shear stress calculation for helical springs to include the effects of stress caused by the transverse shear and the curvature of the spring wire.

64. Ultimate tensile strength is the

    Asked in: Civil Engg GA 24 2023 CO 1505 2024

    1. highest stress at which tensile specimen fails
    2. lowest stress at which tensile specimen fails
    3. normal stress at which tensile specimen fails
    4. highest stress at which compressive specimen fails
    Show answer

    Answer: A. highest stress at which tensile specimen fails

    Ultimate Tensile Strength (UTS) is the maximum stress a material can endure in tension before it starts to significantly deform and eventually fracture.

65. The value of average bond stress $T_{bd}$ depends on

    Asked in: Civil Engg GA 24 2023 CO 1505 2024

    1. steel strength and cross-sectional area of bar
    2. concrete strength and the type of bar
    3. concrete strength and reinforcement strength
    4. concrete strength and cross-sectional area of bar
    Show answer

    Answer: B. concrete strength and the type of bar

    Bond stress is the stress developed at the interface between concrete and reinforcement steel to ensure composite action. Higher concrete strength generally leads to higher bond strength. Deformed bars provide better bond resistance than plain bars due to their mechanical interlock with the concrete.

Thermal Conductivity

66. Why is argon gas used along with tungsten wire in an electric bulb?

    Asked in: UPSC CDS 1 2018

    1. To increase the life of the bulb
    2. To reduce the consumption of electricity
    3. To make the emitted light colored
    4. To reduce the cost of the bulb
    Show answer

    Answer: A. To increase the life of the bulb

    - Tungsten filaments operate at very high temperatures (around 2500-3000°C) to produce light by incandescence. - High temperatures cause the tungsten metal to sublime (evaporate) from the filament. - Evaporated tungsten atoms deposit on the cooler glass bulb wall, causing blackening. - Sublimation thins the filament over time, eventually causing it to break. - Inert gases like argon, krypton, or xenon, when present at pressure, impede the movement of tungsten atoms away from the filament, reducing the evaporation rate and thus prolonging the filament's life and preventing rapid blackening of the bulb.

67. Mica is used in the iron for pressing clothes. In this connection, which of the following two statements is/are correct? a. Mica is a good conductor of electricity. b. Mica is a good conductor of heat.

    Asked in: MPSC CS Prelims 2016

    1. Only a
    2. Only b
    3. Both a and b
    4. Neither a nor b
    Show answer

    Answer: D. Neither a nor b

    - a. Mica is a good conductor of electricity. This statement is incorrect. Mica is widely used as an excellent electrical insulator due to its high dielectric strength. - b. Mica is a good conductor of heat. This statement is also incorrect, or at best misleading. While mica can withstand high temperatures and allows heat transfer from the heating element to the soleplate, it is a relatively poor thermal conductor compared to metals like iron or aluminium. It facilitates the transfer of heat while providing electrical insulation, but it is not classified as a "good" conductor of heat in the way metals are.

68. Thermal conductivity of aluminium, copper and stainless steel increases in the order

    Asked in: UPSC NDA 1 2015

    1. Copper < Aluminium < Stainless Steel
    2. Stainless Steel < Aluminium < Copper
    3. Aluminium < Copper < Stainless Steel
    4. Copper < Stainless Steel < Aluminium
    Show answer

    Answer: B. Stainless Steel < Aluminium < Copper

    Different materials have different thermal conductivity values; pure metals like copper and aluminium are typically better conductors than alloys like stainless steel.

69. Assertion (A): A piece of copper and a piece of glass are heated to the same temperature. When touched, thereafter, the copper piece appears hotter than the glass piece. Reason (R): The density of copper is more than that of glass.

    Asked in: UPPCS Prelims 2001

    1. Both A and R are individually true and R is the correct explanation of A
    2. Both A and R are individually true but R is NOT a correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Show answer

    Answer: B. Both A and R are individually true but R is NOT a correct explanation of A

    Assertion (A) is true. When objects at the same temperature are touched, how hot or cold they feel depends on their thermal conductivity. Copper is a good thermal conductor, meaning it transfers heat efficiently. Glass is a poor thermal conductor (an insulator). When touched, copper transfers heat from your hand (or to your hand if colder) much faster than glass, making it feel hotter (or colder) even if both are at the same temperature. Reason (R) is true. The density of copper (approx. 8.96 g/ cm³) is indeed significantly greater than the density the reason why the copper piece feels hotter than the glass piece when both are at the same elevated temperature is due to their difference in thermal conductivity, not their density. A material's density affects its heat capacity per unit volume, but the rate of heat transfer to your hand is primarily governed by thermal conductivity.

70. Which Carbon nano cluster among the following has the highest thermal conductivity near room temperature ? - Question not attempted

    Asked in: Asst. Engineer Comb. Comp. Exam (Pre) 2024

    1. Graphene
    2. Graphite
    3. Carbon nano tubes
    4. Diamond
    Show answer

    Answer: D. Diamond

    Diamond is known for its exceptionally high thermal conductivity, significantly surpassing other carbon allotropes and materials. This property arises from its strong covalent bonds and rigid crystal structure, which allow for efficient vibration (phonon) transfer.

71. In which kind of pot food will be cooked faster?

    Asked in: Civil Judge 2018

    1. Steel
    2. Copper
    3. Silver
    4. Bronze
    Show answer

    Answer: B. Copper

    This question assesses knowledge of material properties, specifically thermal conductivity, and its application in cooking.

72. Which of the following is a good conductor of heat?

    Asked in: STAFF NURSE UNANI 2023

    1. Rubber
    2. Glass
    3. Iron
    4. Wood
    Show answer

    Answer: C. Iron

    Iron is a metal and metals are generally good conductors of heat and electricity. Rubber, glass, and wood are insulators, meaning they do not conduct heat or electricity well.

Thermal Expansion Coefficients

73. Wooden doors often swell up and get stuck during rainy season. This is due to the phenomenon of

    Asked in: APSC CCE Prelims Exam 2020

    1. endosmosis
    2. imbibition
    3. endocytosis
    4. capillarity
    Show answer

    Answer: B. imbibition

    Imbibition is a special type of diffusion where water is absorbed by solids - colloids - causing them to increase in volume.

74. When water is filled in a bottle and allowed to freeze, the bottle breaks because :

    Asked in: UPPCS Prelims 1995

    1. Water expands on freezing -
    2. Bottle contracts at freezing point
    3. Temperature outside the bottle is more than in‐ side
    4. Water expands on heating
    Show answer

    Answer: A. Water expands on freezing -

    Water exhibits anomalous expansion upon freezing; it expands rather than contracts.

75. A hydrogen inflated polythene balloon is released from the surface of the earth. As the balloon rises to an altitude up in the atmosphere, it will

    Asked in: UPPCS Prelims 2003

    1. Decrease in size
    2. Flatten into a disc-like shape
    3. Increase in size
    4. Maintain the same size and shape
    Show answer

    Answer: C. Increase in size

    - Atmospheric pressure decreases with altitude. - The pressure inside the balloon is influenced by external pressure. - Lower external pressure allows the gas inside the balloon to expand. - Expansion of the gas increases the size of the balloon.

76. A metal sheet with a circular hole is heated. The hole

    Asked in: WBCS Main Paper 4 2021

    1. gets larger
    2. gets smaller
    3. remains of the same size
    4. gets deformed
    Show answer

    Answer: A. gets larger

    - Metals expand when heated (thermal expansion). - The expansion is uniform throughout the material. - A hole in a heated object expands as if it were filled with the same material that is expanding.

Viscosity and Surface Tension

77. Raindrops are spherical in shape because of the phenomenon called

    Asked in: APSC CCE Prelims Exam 2020

    1. viscosity
    2. velocity
    3. residual pressure
    4. surface tension
    Show answer

    Answer: D. surface tension

    Surface tension is the property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules. This force causes the liquid surface to contract to the minimum possible surface area for a given volume, which is a sphere.

78. The highest viscosity among the following is of

    Asked in: BPSC Prelims Exam 66th

    1. water
    2. air
    3. blood
    4. honey
    Show answer

    Answer: D. honey

    Higher viscosity means the fluid offers greater resistance to shear stress or deformation.

79. Consider the following statements: Particles of matter intermix on their own. Particles of matter have force acting between them. Which of the statements given above is/are correct ?

    Asked in: UPSC CDS 2 2019

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Show answer

    Answer: C. Both 1 and 2

    Matter is composed of particles that are in constant motion (at temperatures above absolute zero) and interact with each other through forces.

80. On a clean glass plate a drop of water spreads to form a thin layer whereas a drop of mercury remains almost spherical because

    Asked in: KPSC KAS Prelims 2014

    1. Cohesive force for water is greater than adhesive force with glass.
    2. Cohesive force for mercury is greater than adhesive force with glass.
    3. Density of mercury is greater than water.
    4. Adhesive force for mercury is greater than cohesive force with glass.
    Show answer

    Answer: B. Cohesive force for mercury is greater than adhesive force with glass.

    The behavior of a liquid drop on a surface is determined by the balance between the liquid's cohesive forces (attraction between its own molecules) and the adhesive forces (attraction between the liquid molecules and the surface molecules).

81. Consider the following statements : If capillarity phenomenon did not exist : The blotting paper would fail to function It would be difficult to use kerosene lamp The trees that we see around would not have grown on earth The water cannot flow through pipes in cities Which of the given statements are correct ?

    Asked in: MPSC CS Prelims 2013

    1. (a), (b) and (c) only
    2. (b), (c) and (d) only
    3. (a), (c) and (d) only
    4. (a), (b) and (d) only
    Show answer

    Answer: A. (a), (b) and (c) only

    Capillarity is the phenomenon of a liquid rising or falling in a narrow tube or porous material due to the interplay of adhesive and cohesive forces. (a) Blotting paper absorbs ink or liquid through capillary action within its porous structure. Without capillarity, it would not function effectively. This statement is correct. (b) Kerosene lamps rely on a wick that draws fuel up to the flame by capillary action. Without capillarity, the fuel would not reach the burning part. This statement is correct. (c) Trees transport water from their roots to their leaves through xylem vessels. While transpiration pull is the main force over long distances, capillary action plays a significant role in the upward movement of water, especially within the narrow xylem tubes and against gravity. Without this, the vast majority of plants, particularly large trees, would not be able to survive. This statement is correct. (d) Water flow through pipes in cities is primarily driven by pressure differences created by pumps or gravity from reservoirs. Capillary action is negligible in large pipes and not the mechanism for municipal water distribution. This statement is incorrect.

82. Which one of the following is not a result of surface tension ?

    Asked in: UPSC NDA 1 2015

    1. Nearly spherical drop of rain
    2. Capillary rise
    3. Removal of dirt by soap or detergent
    4. Flow of a liquid
    Show answer

    Answer: D. Flow of a liquid

    Surface tension is a property of the surface of a liquid that allows it to resist an external force due to the cohesive nature of water molecules. It causes liquid surfaces to behave like stretched elastic membranes. A) Nearly spherical drop of rain: Surface tension minimizes the surface area of the liquid, and a sphere has the minimum surface area for a given volume. This is a direct result of surface tension. B) Capillary rise: This phenomenon, where a liquid rises in a narrow tube, is caused by the combined effects of surface tension and adhesion between the liquid and the tube walls. Surface tension contributes to the upward force. C) Removal of dirt by soap or detergent: Soaps reduce the surface tension of water. This reduced surface tension allows the water to wet fabrics and dirt particles more effectively, helping in their removal. While it involves the change in surface tension, the ability to remove dirt in this way is linked to the surface properties facilitated by the detergent's effect on surface tension. D) Flow of a liquid: The bulk flow of a liquid (like water flowing in a pipe or a river) is driven by pressure differences or gravity, and it is resisted by viscosity. Surface tension primarily affects the liquid-air interface and phenomena occurring there (like drop formation, ripples), but it is not the fundamental cause or main characteristic of the bulk flow itself.

83. Which one among the following does not wet the walls of the glass vessel in which it is kept ?

    Asked in: UPSC NDA 1 2015

    1. Water
    2. Alcohol
    3. Mercury
    4. Phenol
    Show answer

    Answer: C. Mercury

    Wetting is determined by the relative strengths of adhesive forces between the liquid and the solid and cohesive forces within the liquid.