Charging materials by friction
4 marks · NormalTwo students rub a plastic rod with a cloth, then observe that small pieces of paper are attracted to the rod.
State the type of material (conductor or insulator) the plastic rod must be to become charged by friction.
Explain why only certain materials can become charged by friction.
Describe how the rod attracts the small pieces of paper.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Insulator
(b) Only insulators become charged by friction because their electrons cannot easily move away. In conductors, electrons can flow away so they do not get charged locally by friction.
(c) The charged rod induces a charge on the paper, attracting the opposite charge and causing the paper to move towards the rod.
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Marking points
B1 (a) · 1 mark · Insulator correctly identified.
M1 (b) · 1 mark · States that in conductors, electrons are free to move; in insulators, electrons are not.
M1 (b) · 1 mark · Explains that only insulators can hold charge where they are rubbed.
M1 (c) · 1 mark · Correctly describes charge induction/attraction process.
Worked solution
a) The rod must be an insulator.
b) Only insulators can hold charge where they are rubbed because in conductors, the electrons move freely and any gained or lost charge escapes quickly. In insulators, the electrons can't move far, so charge stays on the surface.
c) The charged rod causes the electrons in the paper to move slightly, inducing an opposite charge on the side closest to the rod. This causes an attractive force between the rod and the paper.