When two different materials are rubbed together, electrons can be transferred from one material to the other due to differences in their electron affinities. This process is called charging by friction or triboelectric effect. If the rod gains a negative charge, it means it has received negative charge carriers, which are electrons. Since positive charges (protons) are fixed within the nucleus of atoms in the solid material, they are generally not transferred during rubbing. Therefore, the negative charge acquired by the rod must come from the wool. - If the rod becomes negatively charged, it has gained electrons. - These electrons must have come from the wool. So, negative charges (electrons) are transferred from the wool to the rod. The wool loses electrons and becomes positively charged.