(a) State the two types of electric charge. (b) State the rule that describes which charges attract and which repel. [2 marks]
Positive charge and negative charge.
Like charges repel (push apart) and unlike (opposite) charges attract (pull together).
For each pair of charges, state whether they attract or repel. [3 marks]
Repel (like charges).
Repel (like charges).
Attract (unlike charges).
When a solid is charged by friction, what is transferred from one object to the other, and is its charge positive or negative? [2 marks]
Electrons are transferred from one object to the other. (1 mark)
Electrons carry a negative charge. (1 mark)
Define (a) an electrical conductor and (b) an electrical insulator. [2 marks]
A material that allows electric charge to pass through it easily (for example, a metal such as copper).
A material that does not allow electric charge to pass through it (for example, plastic or rubber).
State the unit used to measure electric charge, and give its symbol. Supplement [1 mark]
Electric charge is measured in coulombs, symbol C.
Describe a simple experiment to produce an electrostatic charge by friction, and a simple way to detect that the object is charged. [4 marks]
Producing the charge:
- Take a plastic rod (for example a polythene rod) and a dry cloth.
- Rub the rod with the cloth several times. The rubbing gives the rod an electric charge.
Detecting the charge:
- Hold the charged rod near tiny pieces of paper — the paper jumps up and sticks to the rod.
- (Or) hold the rod near a thin stream of water — the water bends towards the rod.
Because the rod attracts these light objects, it must be charged.
A polythene rod is rubbed with a wool cloth. The polythene becomes negative and the wool becomes positive. Explain, in terms of electrons, how this happens. [3 marks]
- During rubbing, electrons (negative charge) are transferred from the wool to the polythene. (1 mark)
- The polythene gains electrons, so it has extra negative charge → it becomes negatively charged. (1 mark)
- The wool loses electrons, so it has fewer electrons than normal → it becomes positively charged. (1 mark)
Describe an experiment to find out whether a material is a conductor or an insulator. State what you would observe in each case. [4 marks]
- Make a circuit with a cell (battery), a lamp, and connecting wires, leaving a gap in the circuit.
- Place the material to be tested across the gap.
- If the lamp lights up, charge can pass through → the material is a conductor.
- If the lamp stays off, charge cannot pass through → the material is an insulator.
Repeat with different materials to compare them.
Using a simple electron model, explain why copper is a good conductor while plastic is an insulator. Give one further example of each. [4 marks]
- A conductor (such as copper) has some electrons that are free to move through the material. These free electrons carry the charge through, so charge can flow easily. (1 mark)
- An insulator (such as plastic) has electrons that are not free to move — they are held in place. There are no free electrons to carry charge, so charge cannot flow. (1 mark)
Further examples:
- Conductor: iron (or aluminium / any other metal). (1 mark)
- Insulator: rubber (or glass / wood). (1 mark)
(a) Describe what is meant by an electric field. (b) State what is meant by the direction of an electric field at a point. Supplement [3 marks]
An electric field is a region in which an electric charge experiences a force.
The direction of an electric field at a point is the direction of the force on a positive charge placed at that point.
This question is about charging solids by friction.
The charge that moves is carried by electrons, which are negative and are able to move from one object to the other. (1 mark)
The positive charges cannot move — they stay where they are. So charging by friction is only a transfer of negative charge (electrons). (1 mark)
Electrons are transferred from the Perspex to the wool. (1 mark)
The Perspex loses electrons, so it has fewer electrons than normal → it becomes positively charged. (1 mark)
The wool gains these electrons, so it becomes negatively charged. (1 mark)
No. No charge is created or destroyed — the electrons are simply moved from one object to the other.
This question is about the forces between charges.
They repel (push apart). (1 mark)
This is because both rods carry the same (negative) charge, and like charges repel. (1 mark)
It shows that the rod is charged (it can attract a light object without touching it).
They attract (unlike charges attract).
A student tests four materials in a circuit with a cell and a lamp (Copper wire — Yes; Plastic ruler — No; Iron nail — Yes; Rubber band — No).
| Material | Lamp lights? | Conductor or insulator? |
|---|---|---|
| Copper wire | Yes | Conductor |
| Plastic ruler | No | Insulator |
| Iron nail | Yes | Conductor |
| Rubber band | No | Insulator |
(1 mark for the two conductors, 1 mark for the two insulators.)
The materials that let the lamp light (copper, iron) have free electrons that can move, so charge can flow through them — they are conductors. (1 mark)
The materials that did not light the lamp (plastic, rubber) have electrons that are held in place (not free to move), so charge cannot flow — they are insulators. (1 mark)
Conductor: aluminium (or any other metal). Insulator: glass (or wood).
This question is about electric field patterns. Supplement
The field lines are straight and spread out evenly (like the spokes of a wheel), pointing away from the positive charge.
Award 1 mark for straight, evenly spaced radial lines; 1 mark for arrows pointing away from the positive charge.
The lines are straight and spread out evenly, starting at the surface of the sphere (the same shape as for a point charge). (1 mark)
Because the sphere is negative, the field lines point inward — towards the sphere. (1 mark)
This question is about the field between two oppositely charged parallel plates. Supplement
Two features of the field lines (any two):
- The lines are straight and parallel to each other.
- The lines are evenly spaced (the field is uniform).
- They run from the positive plate to the negative plate.
Award 1 mark for the sketch, 1 mark each for two correct features.
From the positive plate to the negative plate.
Place a small positive charge at the point. (1 mark)
The direction of the force on that positive charge is the direction of the electric field at that point. (1 mark)
