A student tries to demagnetise a permanent magnet using a solenoid. She places the magnet inside the solenoid, connects the solenoid to an alternating current (a.c.) supply, switches it on, and then simply switches off the current while the magnet is still inside.
State and explain whether this method will reliably demagnetise the magnet, and describe the correct procedure.
An electric motor has a flat coil of wire placed between the poles of a permanent magnet. The coil is connected to a d.c. voltage source through a split-ring commutator.
State the function of the split-ring commutator in this motor and explain why it is necessary to maintain continuous rotation.
The initial setup of the motor has a coil with 4 turns of wire. The magnetic field strength of the magnet is BBB. With a current of 1.5 A flowing through the coil, the coil experiences a maximum turning effect τ\tauτ.
Complete the table below to show the turning effect for each of the changes made to the setup. Select your answers from the box:
{τ4,τ2,τ,2τ,3τ,4τ,8τ} \left\{ \frac{\tau}{4}, \frac{\tau}{2}, \tau, 2\tau, 3\tau, 4\tau, 8\tau \right\} {4τ,2τ,τ,2τ,3τ,4τ,8τ}| Number of turns | Current in the coil / A | Magnetic field strength | Turning effect |
|---|---|---|---|
| 4 | 1.5 | BBB | τ\tauτ |
| 4 | 4.5 | BBB | |
| 8 | 1.5 | BBB | |
| 4 | 1.5 | B4\displaystyle \frac{B}{4}4B |
275 exam-style questions on CIE IGCSE Physics 4.5 Electromagnetic effects, covering 4.5.1a Electromagnetic induction, 4.5.1b Direction of an induced e.m.f., 4.5.2 The a.c. generator, 4.5.3a Magnetic effect of a current and its applications, 4.5.3b Strength and direction of the field around wires and solenoids, 4.5.4a Force on a current-carrying conductor, 4.5.4b Force on beams of charged particles, 4.5.5 The d.c. motor, 4.5.6a Construction and operation of a transformer, 4.5.6b Turns ratio and current in a transformer, and 4.5.6c High-voltage transmission of electricity. Each one has a worked solution and a mark scheme showing where the marks go.