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15.3 Electromagnets in Devices

GCSE Physics · AQA · Chapter 15: Electromagnetism

A free GCSE Physics lesson for AQA (8463) from OBA School: physics only. Lifting magnets, circuit breakers, electric bells and relays, and testing the strength of an electromagnet. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain why electromagnets are useful: they can be switched on and off and their strength can be changed
  • Describe how to investigate the factors that affect the strength of an electromagnet and interpret the results
  • Interpret diagrams of a lifting magnet, a circuit breaker, an electric bell and a relay to explain how each one works

Key words

  • Armature: A piece of iron that is pulled towards an electromagnet when the current is switched on.
  • Circuit breaker: An automatic switch that opens a circuit when the current is too large. It can be reset.
  • Relay: A switch worked by an electromagnet. A small current in one circuit switches on a large current in another.
  • Make-and-break switch: A contact that is opened by the moving armature, cutting off the current, then closes again when the armature springs back.
  • Pivot: The point a lever or armature turns about.

Why use an electromagnet?

A permanent magnet is always on. An electromagnet only works while there is a current, so it can be switched on and off, and its strength can be controlled by the current. Every device below uses the same idea: current on → iron core magnetised → an iron armature or load is pulled towards it.

  • Lifting magnet (scrapyard crane); What the electromagnet attracts steel car bodies and scrap; Result lifts the load; switching off drops it where it is wanted
  • Circuit breaker; What the electromagnet attracts an iron catch; Result releases a spring-loaded switch, cutting off a dangerous current
  • Electric bell / buzzer; What the electromagnet attracts an iron armature; Result the armature moves to and fro, so the hammer strikes the gong repeatedly
  • Relay; What the electromagnet attracts an iron armature; Result closes the contacts of a second circuit

Key fact: The core is always iron (magnetically soft) so that it loses its magnetism as soon as the current stops. With a steel core the scrap would stay stuck and the bell would not reset.

Investigating the strength of an electromagnet

Hang an iron plate from the bottom of an electromagnet with a known weight on it. Start with a large current, then slowly reduce it with a variable resistor until the plate just falls. Record the smallest current that holds the load. Repeat for different weights, then repeat everything with more turns on the coil.

  • Independent variable: the weight of the load (and, in a second run, the number of turns).
  • Dependent variable: the smallest current needed to hold the load (read on an ammeter).
  • Control variables: the same core, the same iron plate, the same coil length.
  • Safety: use a low-voltage supply and switch off between readings because the coil gets hot.

Worked example: reading the graph

What current does the 50-turn coil need to hold 5.0 N?
Read up from 5.0 N to the 50-turn line, then across: 0.80 A.
For the same weight, the 100-turn coil needs 0.40 A, half as much.

Answer: more turns or more current make the electromagnet stronger

The circuit breaker

A circuit breaker protects a circuit, like a fuse, but it does not melt and it can be reset. The electromagnet is in series with the switch contacts, so all the current passes through its coil.

1. Too much current passes through the coil.

2. The electromagnet becomes strong enough to pull the iron catch towards it.

3. The catch releases the switch arm and the spring pulls the contacts apart.

4. The circuit is broken and the current stops. The switch stays open until someone resets it by hand.

The electric bell

1. Switch on: current flows through the contact screw, the armature and the coil.

2. The electromagnet attracts the iron armature, so the hammer hits the gong.

3. The armature has moved away from the contact screw, so the circuit is broken and the electromagnet switches off.

4. The springy strip pulls the armature back, the contact closes again and the cycle repeats many times a second.

Exam tip: A buzzer works the same way but has no hammer or gong. Its armature is lighter and moves a shorter distance, so it vibrates at a higher frequency and you hear a buzz.

The relay

A relay lets a small current switch on a much larger current in a separate circuit. For example, a small current from a car’s ignition switch can turn on the starter motor, which needs a current of over 100 A. It also keeps the person using the switch safely away from high-voltage or high-current circuits.

1. A small current flows in the input circuit and through the coil.

2. The iron core is magnetised and attracts the iron armature, which turns about its pivot.

3. The armature pushes the springy contact across the gap and closes the output circuit, so the motor runs.

4. When the input current stops, the core loses its magnetism and the springy contact moves back, opening the gap.

Worked example: current in a relay coil

A relay coil has a resistance of 60 Ω and is connected to a 12 V battery. Calculate the coil current.
I = V ÷ R = 12 ÷ 60

Answer: 0.20 A (much smaller than the 10 A or more it can switch)

Common mistakes

  • Saying the electromagnet attracts copper contacts. It attracts the iron armature or catch, which then moves the contacts.
  • Forgetting the step where the circuit is broken in a bell. The armature leaving the contact screw is what switches the electromagnet off.
  • Saying a relay “makes the current bigger”. It uses a small current to switch on a separate circuit that already has a large supply.
  • Saying a circuit breaker melts like a fuse. It opens a switch and can be reset.

Revision questions

Try these, then check your answers below each one.

Question 1. Why does a scrapyard crane use an electromagnet instead of a permanent magnet? (Choose from: It can be switched off to drop the load, It works without electricity, It attracts copper and aluminium, It is never hot.)

Answer: It can be switched off to drop the load

Working: An electromagnet can be switched on to pick up steel and switched off to release it where it is needed.

Question 2. In an electric bell, what breaks the circuit each time the hammer hits the gong? (Choose from: The armature moves away from the contact screw, The gong melts a fuse, The battery switches itself off, The electromagnet reverses its poles.)

Answer: The armature moves away from the contact screw

Working: When the armature is pulled towards the electromagnet, it leaves the contact screw. The circuit is broken, the electromagnet switches off and the springy strip pulls the armature back.

Question 3. A relay coil has a resistance of 40 Ω and is connected to a 6.0 V battery. Calculate the current in the coil.

Answer: 0.15 A

Working: I = V ÷ R = 6.0 ÷ 40 = 0.15 A. This small current can switch on a much larger current in the relay’s output circuit.

Related lessons

Continue in the full course

This lesson is section 15.3 (AGCPH1503) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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