Electromagnetism
5 marks · NormalA solenoid is used as an electromagnet and is part of a moving-coil loudspeaker. The speaker is powered by an alternating current produced by a generator.
Describe the magnetic field pattern produced around a solenoid when current flows through it.
Explain why the core of the solenoid in an electromagnet should be made from iron and not steel.
Explain briefly how the movement of a generator's coil produces an alternating current to power the speaker.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show field lines inside are parallel and close together, field pattern like a bar magnet (north/south poles, field lines from north to south outside)
(b) — · must show iron is easily magnetised and demagnetised, steel retains magnetism (not suitable for switching on/off)
(c) The coil rotates in a magnetic field, cutting field lines and inducing an alternating voltage (and so current).
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Field lines are parallel and close together inside the solenoid.
A2 (a) · 1 mark · Pattern is similar to a bar magnet (named poles, field lines emerge from one end and curve around to the other).
A3 (b) · 1 mark · Iron core is easily magnetised and demagnetised; responds rapidly to current changes (for electromagnetic effect).
A4 (b) · 1 mark · Steel retains magnetism and is not suitable for switches or variable magnets.
A5 (c) · 1 mark · States that coil cuts field lines as it rotates in a magnetic field, inducing ac.
Worked solution
a) The field inside a solenoid with current is strong, almost uniform (lines are parallel and close together). The field outside is like a bar magnet; field lines leave from one end (north pole), curve round and enter the other end (south pole).
b) Iron becomes magnetised and demagnetised quickly which is needed for electromagnets that must be switched on/off. Steel stays magnetised, so is unsuitable for temporary magnets.
c) When a generator's coil rotates in a magnetic field, it cuts field lines and induces an alternating voltage, producing an alternating current.