- The brightness of a bulb (assumed to be resistive) is proportional to the power dissipated, P. Power can be calculated as P = V²/R or P = I²R, where V is the voltage across the bulb, I is the current through the bulb, and R is its resistance. For a given bulb (fixed R), maximum brightness corresponds to maximum voltage or maximum current. - Assuming each battery provides a voltage V: - Diagram (a) (one battery, one bulb): Voltage across bulb = V. - Diagram (b) (two batteries in series, one bulb): The voltages of batteries in series add up. Total voltage = 2V. This voltage is across the bulb. - Diagram (c) (two batteries in parallel, one bulb): Batteries in parallel maintain the same voltage as a single battery (assuming identical ideal batteries). Total voltage = V. This voltage is across the bulb. (Parallel connection increases total current capacity, but not voltage). - Diagram (d) (one battery, two bulbs in series): The voltage V from the battery is divided between the two bulbs. Assuming identical bulbs, voltage across each bulb = V/