Practise comparing metallic wires by length and cross-sectional area, using diagrams, tables and qualitative resistance changes in exam questions.
The electric starter motor in a car is switched on and off using a relay.
The relay consists of a plastic case and two flexible springy strips, X and Y, which are made of soft iron. These iron strips act as the switch when a circuit is connected between the terminals W and Z.
Fig. 7.1 shows \(\mathrm{X
Fig. 7.2 shows the equipment from Fig. 7.1 inside a magnetising coil. The magnetising coil is in series with the 12 V car battery and switch S , which is open.
The power of the starter motor is 1.8 kW and it is also operated by the car battery.
The starter motor circuit is connected between terminals W and Z .
Explain why copper wires with a large cross-sectional area are used for this circuit.