Electric Charge
4 marks · EasyA plastic ruler is rubbed with a woollen cloth. It is then held near small pieces of paper, which jump towards the ruler. Later, this ruler is used to paint-spray a metal lamp using electrostatic paint spraying.
Explain why the plastic ruler becomes charged when rubbed with the woollen cloth.
State what happens to the electrons during this process.
Explain why the electrostatic paint is attracted to the metal lamp after being sprayed.
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.
Show answer
(a) Friction causes electrons to be transferred between the two materials, making one positively and the other negatively charged.
(b) Electrons move from the ruler to the cloth (or vice versa), depending on the materials used.
(c) The paint droplets and metal lamp have opposite charges, so the paint is attracted to the lamp due to electrostatic forces.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that friction transfers (or separates) charge by moving electrons.
A1 (a) · 1 mark · Correctly describes that objects end up with opposite charges.
B1 (b) · 1 mark · States which way electrons move or simply that electrons are transferred between objects.
B2 (c) · 1 mark · Explains opposite charges create attraction between paint and object.
Worked solution
a) When the ruler and cloth are rubbed together, friction causes electrons to transfer from one material to the other.
This leaves one object positively charged (losing electrons) and the other negatively charged (gaining electrons).
b) Electrons are transferred from the plastic ruler to the cloth, or vice versa, depending on their electron affinities.
c) In electrostatic paint spraying, the paint droplets are given a charge opposite to that of the metal lamp. Like charges repel, so the droplets spread out, and unlike charges attract, so the paint is drawn to all parts of the oppositely charged lamp surface.