ElectromagnetsIB MYP Physics: Revision notes
Section 1
The magnetic field around a current
An electric current produces a magnetic field. Around a long straight wire the field lines are concentric circles centred on the wire. The field is strongest close to the wire and gets weaker further away.
Use the right-hand grip rule to find the direction: point the right thumb along the current direction and the fingers curl in the direction of the field lines.
If the current is reversed, the field direction reverses. A larger current gives a stronger field.
Section 2
The solenoid
A solenoid is a long coil of wire. When a current flows, each turn adds to the field. The result is a field like that of a bar magnet: field lines loop from one end of the coil to the other, and inside the solenoid the field is strong and uniform.
To find the north pole of a solenoid use the grip rule again: curl the fingers of the right hand in the direction of the current round the coil, and the thumb points to the north end.
Reversing the current swaps the north and south poles.
For a solenoid the fingers follow the current and the thumb gives the north pole. For a straight wire the thumb follows the current and the fingers give the field.
Section 3
Making an electromagnet stronger
The strength of an electromagnet can be increased by:
- increasing the current (e.g. a higher voltage)
- adding more turns of wire
- putting a soft iron core inside the coil, because iron is easily magnetised and strengthens the field
A soft iron core is used because it loses its magnetism when the current is switched off. Steel would stay magnetised.
An iron core does not make the magnet permanent. Soft iron loses its magnetism as soon as the current is switched off.
Section 4
Uses of electromagnets
- Cranes: lift scrap steel and release it by switching off the current.
- Relays: a small current in a coil makes an electromagnet pull an iron armature, which closes a switch in a second circuit with a large current.
- Electric bells: the current magnetises an electromagnet that pulls a hammer to strike the gong. This also breaks the circuit, so the hammer springs back and the process repeats.
- Circuit breakers: if the current is too large, the electromagnet pulls open a switch and breaks the circuit.
Section 5
Electromagnets and permanent magnets
An electromagnet can be switched on and off, its strength can be changed by altering the current, and its poles can be reversed. A permanent magnet is always magnetic, needs no power supply, and its strength cannot be changed.
The cost of an electromagnet includes the electricity it uses, and a power cut makes it lose its field.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Electromagnets
- A scrapyard in Johannesburg uses a crane with an electromagnet to lift old steel car bodies. The electromagnet is a coil of insulated copper wire wound round a soft iron core and connected to a d.c. supply.Explain why a soft iron core is used in the crane's electromagnet.2 marks
- In a car, the driver's ignition switch controls the starter motor through a relay. The starter motor needs a very large current, while the circuit with the ignition switch carries only a small current.Explain why a relay is used to switch the starter motor instead of the driver's ignition switch.2 marks
- A technician in Cairo makes a solenoid, which is a long coil of insulated copper wire, and connects it to a d.c. power supply through a switch. She uses a small plotting compass to investigate the magnetic field.Describe the magnetic field around the solenoid when a current flows, and state how the right-hand grip rule shows which end is the north pole.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).