Charging materials by friction
3 marks · NormalA metal spoon and a plastic comb are rubbed with a woolen jumper. Only the comb becomes charged.
Explain why the plastic comb becomes charged but the metal spoon does not.
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The plastic comb is an insulator so the charge remains where it is rubbed. The metal spoon is a conductor and the charge moves away and disperses, so it does not become charged.
(a) Plastic is an insulator, so the electrons that move stay in place, making the comb charged. Metal is a conductor, so electrons that move quickly spread through the spoon or move away to the environment.
Also accept: Insulators retain charge; conductors lose charge.
Common mistakes: Suggesting both become charged
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Identifies plastic as an insulator, metal as conductor.
M1 (a) · 1 mark · States charge remains on insulators but not on conductors.
M1 (a) · 1 mark · Explains that, in conductors, charge flows away, so charging by friction is ineffective.
Worked solution
The plastic comb is an insulator, so when it is rubbed, electrons that are moved by friction cannot leave and the comb keeps its charge. Metal, being a conductor, allows any moved electrons to quickly spread throughout the spoon and away, so the spoon does not get charged.