Electric Charge
6 marks · NormalA student investigates materials for electrostatic paint spraying. She rubs a plastic rod with a woollen cloth, then uses the rod to try to move small pieces of aluminium foil. Next, she describes how electrostatic spraying works to paint car parts efficiently.
Explain, using ideas about electrons, how rubbing the rod with the cloth causes them to become charged.
The student finds the aluminium foil is attracted to the charged rod. State whether the foil must be charged or uncharged, and explain your answer.
Describe how electrostatic paint spraying makes painting more efficient than using an ordinary spray.
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) Electrons are transferred from one material to the other (e.g., from the rod to the cloth), leaving one positively charged (loss of electrons) and the other negatively charged (gain of electrons). · must show reference to electrons moving, making one object positive by electron loss, making other object negative by electron gain
(b) The foil must be uncharged. The charged rod induces an opposite charge in the nearest side of the foil, leading to attraction. · must show foil is initially uncharged, induction of opposite charge, attraction due to induced charges
(c) In electrostatic spraying, paint droplets are given a charge so they repel each other and spread out. The object to be painted has the opposite charge, so paint is attracted evenly, reducing waste and ensuring thorough coverage. · must show paint droplets charged, object opposite charge, repulsion spreads paint, attraction ensures coverage
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly describes electron transfer between materials
A1 (a) · 1 mark · Identifies which object is positive/negative after transfer
B1 (b) · 1 mark · States foil is uncharged
B2 (b) · 1 mark · Explains attraction by induced charge/charge redistribution
M2 (c) · 1 mark · Describes paint droplets made charged, attracted to oppositely charged object
A2 (c) · 1 mark · States spray is more efficient because paint spreads and is attracted to hard-to-reach areas/object
Worked solution
a) Rubbing the rod with the cloth causes electrons to move from the rod to the cloth (or vice versa, depending on materials). The rod loses electrons and becomes positively charged. The cloth gains electrons and becomes negatively charged.
b) The foil is initially uncharged. When the charged rod is brought near, it induces a charge on the foil: electrons in the foil move so the side nearest the rod has an opposite charge. This causes the foil to be attracted to the rod.
c) In electrostatic paint spraying, the paint droplets are given one type of charge and the object to be painted is given the opposite charge. Charged paint droplets repel each other and spread out, and are attracted to the object. This attracts paint even into corners and reduces waste.