A relay is used to switch the electric starter motor in a car on and off. It has a plastic case and two flexible springy strips, X and Y, made of soft iron. When a circuit is connected between terminals W and Z, these iron strips act as the switch.
Fig. 7.1 shows X, Y and the plastic case.
Fig. 7.2 shows the apparatus from Fig. 7.1 placed inside a magnetising coil. The magnetising coil is in series with the 12 V car battery and switch S, which is open.
(a)[3]
Explain the changes that occur to the springy iron strips X and Y.
(b(i))[2]
Calculate the current in the starter motor when it is operating.
(b(ii))[2]
Explain why copper wires with a large cross-sectional area are chosen for this circuit.
(c)[2]
Complete the circuit diagram of the motor circuit.
Worked solution & mark scheme
This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: “X and Y are magnetised / strips develop poles” …