A hard-rubber or plastic comb rubbed against wool can often be shown to have a static charge. Why does a metal comb not readily show a static charge when rubbed against wool?
plastics are insulators
When a hard-rubber or plastic comb is rubbed against wool, it gains electrons from the wool through a process called triboelectric charging. This results in the comb becoming negatively charged, and it can exhibit static electricity.
In contrast, a metal comb does not easily show a static charge when rubbed against wool because metals are conductors. Conductors, like metal, have free electrons that can move easily within the material. When the metal comb is rubbed against the wool, the excess electrons are quickly dispersed throughout the metal, neutralizing any charge imbalance and preventing the buildup of static charge.
To put it simply, the metal comb allows the excess electrons to flow away easily due to its conductive nature, preventing the accumulation of static charge. Whereas, hard-rubber or plastic combs, being insulators, trap the excess electrons, resulting in a static charge buildup.
So, the difference in the behavior of hard-rubber or plastic combs and metal combs when rubbed against wool is due to the difference in conductivity between the materials.