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AQA · GCSE · Combined Science · Higher

Balance reading and force on a wire

Q07.1 2 marks

A student clamped a wire between the poles of a permanent magnet. The student investigated how the force on the wire varied with the current in the wire. Figure 13 shows the equipment used. The top pan balance was used to determine the force on the wire.

Diagram of a wire clamped between supports and placed between the poles of a permanent magnet on a top pan balance. The circuit includes a switch and cell. Labels identify wire, supports, permanent magnet and top pan balance.

Figure 13

Enlarged question image

Diagram of a wire clamped between supports and placed between the poles of a permanent magnet on a top pan balance. The circuit includes a switch and cell. Labels identify wire, supports, permanent magnet and top pan balance.
Figure 13

When the switch was closed the reading on the top pan balance increased. Explain why the increased reading showed that there was an upward force on the wire.

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Marking points

  1. 1 State that the increased balance reading means the downward force on the balance increased.
  2. 2 Use the equal and opposite force idea to explain that the wire must have an upward force on it.

Full-mark answer

The increased balance reading shows that the downward force on the balance increased. The force on the wire is equal and opposite to this force, so the force on the wire must be upwards.

Why this answer loses marks

The balance reading increased, so the force was upward.

This gives the conclusion but misses the necessary reasoning: the balance experienced a greater downward force and the wire experienced the equal and opposite upward force.

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