What shape is the field around a long straight wire?

Concentric circles around the wire.

What does the field of a solenoid look like?

Like the field of a bar magnet, strong and uniform inside the coil.

How do you find the north pole of a solenoid?

Curl the fingers of the right hand in the direction of the current, and the thumb points to the north pole.

Name three ways to make an electromagnet stronger.

Increase the current, add more turns, add a soft iron core.

Why is soft iron used for the core?

It is easily magnetised and loses its magnetism when the current is switched off.

A switch operated by an electromagnet so a small current can control a large current.

How does an electric bell work?

An electromagnet pulls a hammer to strike the gong and breaks the circuit, so the hammer springs back and it repeats.

How does a circuit breaker use an electromagnet?

A large current makes the electromagnet pull open a switch and break the circuit.

Give one advantage of an electromagnet over a permanent magnet.

It can be switched on and off, or its strength changed.

ElectromagnetsIB MYP Physics: Flashcards

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What produces a magnetic field around a wire?

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What produces a magnetic field around a wire?: An electric current.
What shape is the field around a long straight wire?: Concentric circles around the wire.
What happens to the field if the current is reversed?: The field direction reverses.
What is a solenoid?: A long coil of wire.
What does the field of a solenoid look like?: Like the field of a bar magnet, strong and uniform inside the coil.
How do you find the north pole of a solenoid?: Curl the fingers of the right hand in the direction of the current, and the thumb points to the north pole.
Name three ways to make an electromagnet stronger.: Increase the current, add more turns, add a soft iron core.
Why is soft iron used for the core?: It is easily magnetised and loses its magnetism when the current is switched off.
What is a relay?: A switch operated by an electromagnet so a small current can control a large current.
How does an electric bell work?: An electromagnet pulls a hammer to strike the gong and breaks the circuit, so the hammer springs back and it repeats.
How does a circuit breaker use an electromagnet?: A large current makes the electromagnet pull open a switch and break the circuit.
Give one advantage of an electromagnet over a permanent magnet.: It can be switched on and off, or its strength changed.
Give one advantage of a permanent magnet over an electromagnet.: It needs no power supply.

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

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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).