Electric charge questions

Electric charge syllabus

Questions on electric charge can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 2.22P identify common materials that are electrical conductors or insulators, including metals and plastics
• 2.23P practical: investigate how insulating materials can be charged by friction
• 2.24P explain how positive and negative electrostatic charges are produced on materials by the loss and gain of electrons
• 2.25P know that there are forces of attraction between unlike charges and forces of repulsion between like charges
• 2.26P explain electrostatic phenomena in terms of the movement of electrons
• 2.27P explain the potential dangers of electrostatic charges, e.g. when fuelling aircraft and tankers
• 2.28P explain some uses of electrostatic charges, e.g. in photocopiers and inkjet printers

How to answer electric charge questions

1. Identify whether the circuit is series or parallel before combining currents, voltages or resistances.
2. Draw standard circuit symbols cleanly and place ammeters in series and voltmeters in parallel.
3. Show unit conversions, especially milliamps, kilohms, kilowatt-hours and seconds.
4. In explanations, follow charge or energy through the circuit rather than only naming a rule.

Electric charge example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Charge transfer and aircraft refuelling

January 2020 · Paper 2P · Question 4(c)

Question 1

A student uses this apparatus to demonstrate the effect of electric charge.

He pours some fine powder into a funnel.

The fine powder moves through a length of plastic tubing and falls into a metal can.

The metal can rests on a metal cap.

The metal cap is connected to a thin piece of metal via a metal rod.

When the powder lands in the can, the thin piece of metal moves away from the metal rod.

The student suggests that this demonstration is similar to refuelling an aircraft.

The powder represents the fuel and the metal can represents the fuel tank in the aircraft.

Explain how the student should modify this apparatus to demonstrate how to minimise the dangers when refuelling an aircraft.

You may add to the diagram to help your answer.

Final answer

Connect the metal can, representing the fuel tank, to the ground with a wire. Charge can then flow to or from the ground, so it does not build up on the tank. This reduces the risk of a spark causing a fire or explosion.

Mark scheme points

Any three of these four points are needed for full marks:

1. M1 Identify a relevant danger, such as a spark, fire or explosion.
2. M2 Earth the metal can or aircraft; for example, connect it to the ground with a wire.
3. M3 State that charge flows to or from the ground.
4. M4 State that charge will not build up on the metal can or aircraft.

Explanation

Charge building up on the fuel tank could produce a spark. If the spark ignites fuel vapour, it could cause a fire or explosion. Connecting the metal can to the ground with a wire provides a path for charge to flow. Charge may flow either way, and this prevents it from building up on the can.

Common mistakes

• Candidates generally recalled the need for earthing and the hazards well. Remember that charge can flow either towards or away from the ground through the wire.
• Naming a spark or fire alone does not explain how the danger is reduced; include the earth connection and what it does to the charge.

Example 2: Charging on a plastic slide

June 2017 (R) · Paper 2P · Question 2(b)(i)

Question 2

A girl slides from point A to point B on a plastic slide.

As the girl slides down, she becomes charged and her hair stands on end.

© Chris Darling (Wikipedia)

The passage explains how the girl becomes charged.

Use words from the box to complete the passage.

Each word may be used once, more than once, or not at all.

conduction        electrons        friction        negative        positive        protons

The girl becomes charged because of ________ between the slide and her clothes.

As the girl travels down the slide, the slide loses ________.

When the girl reaches point B, the slide has a ________ charge.

Final answer

The girl becomes charged because of friction between the slide and her clothes. As the girl travels down the slide, the slide loses electrons. When the girl reaches point B, the slide has a positive charge.

Mark scheme points

1. M1 friction
2. M2 electrons
3. M3 positive. The answers must be in this order.

Explanation

Friction between the clothes and the slide allows electrons to transfer. The slide loses electrons, leaving it with more positive charge than negative charge, so the slide becomes positively charged.

Common mistakes

• Do not write “negative” for the slide’s charge: losing electrons makes the slide positively charged.
• Keep the answers in the required order: friction, electrons, positive.

Practise Electric charge questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.