Final answer:
The sponge pad becomes charged when rubbing plastic due to triboelectric charging, where electrons are transferred to the plastic, making it negatively charged. Metal parts do not become charged because they are earthed, allowing any developed charge to dissipate. A polished car attracts dust due to the insulating properties of wax and tires that hold static charges.
Step-by-step explanation:
The phenomenon where the sponge pad becomes charged when rubbing the plastic parts of a car but not when rubbing the metal parts can be explained by the concept of triboelectric charging and earthing or grounding.
When a material like plastic is rubbed with another material, such as a sponge pad, electrons can be transferred from one material to the other. This happens because different materials have different affinities for electrons; for instance, plastic tends to gain electrons and become negatively charged.
This process is a form of triboelectric charging. However, when a sponge pad is rubbed against metal parts of the car, the metal does not become charged. This is because metals are conductors and the metal parts of the car are earthed, meaning they are connected to the ground which can accept or supply excess charge easily. Therefore, any charge developed is immediately dissipated through the grounding, preventing the accumulation of charge on the surface of the metal.
Furthermore, a car often attracts dust after being polished because the insulating properties of the wax and the tires can hold static charges, which in turn attract dust particles. Dust particles can carry either positive or negative charges and are attracted to surfaces that have an opposite charge.