1.1. Higher Tier Only

The apparatus shown in Figure 3 below can be used to measure the force acting on a current-carrying wire placed in a magnetic field.

Figure 3

The balance is initially zeroed so that when the power supply is off, it shows no reading.

When the power supply is turned on, the balance shows a reading.

As the current is increased, the reading increases.

Explain in terms of forces why the reading increases.

1.2. Higher Tier Only

For the above apparatus, when the current in the wire was 0.25 A, a force of 1.96 mN was exerted on the wire

The length of wire between the poles of the magnet was 0.042 m

Calculate the magnetic flux density between the poles of the magnet.

Give your answer to 2 significant figures.

(You may need to look up the appropriate formula on the Physics Equations Sheet).