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Physics · Magnetism and electromagnetism

Electromagnetism

Describe the magnetic field around a wire carrying a current and in a solenoid, and explain how the strength of the field can be changed.

  • 6 key terms
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Warm-up

Answer each one, then check.

  1. 1

    What is an electric current?

    Show answerHide answer

    A flow of charge

  2. 2

    What is a circuit?

    Show answerHide answer

    A complete loop for current to flow round

  3. 3

    What is a coil of wire called?

    Show answerHide answer

    A solenoid

  4. 4

    Name a magnetic material.

    Show answerHide answer

    Iron

  5. 5

    What does a battery do?

    Show answerHide answer

    Provides a potential difference

Learning Objectives

  1. 1Describe the magnetic field around a straight wire carrying a current.
  2. 2Explain how the field changes with current and distance.
  3. 3Describe the field of a solenoid and compare it with a bar magnet.
  4. 4Explain how to increase the strength of a solenoid's field and how an iron core affects it.
  5. 5Describe an electromagnet.

CURRENT AND MAGNETISM

When a current flows in a wire it produces a magnetic field around the wire. The field is stronger with a bigger current and closer to the wire.

The field lines around a straight wire are concentric circles. Reversing the current reverses the direction of the field.

Changing the Strength

Learn what makes a stronger field.

  • Bigger current

    Effect: Stronger field

  • More turns of wire in the coil

    Effect: Stronger field

  • Iron core inside the solenoid

    Effect: Much stronger field

  • Reverse the current

    Effect: Field direction reverses

  • Move further from the wire

    Effect: Weaker field

Making an Electromagnet

Describe how to make a strong electromagnet.

Show the solutionHide the solution
  1. 1 Wind a coil Many turns of insulated wire round an iron core (nail)
  2. 2 Connect to a cell Current in the coil magnetises the core
  3. 3 Increase the strength More turns, larger current, or an iron core

AnswerCoil with an iron core and a large current.

Why Use an Electromagnet?

Give two advantages of an electromagnet over a permanent magnet.

Show the solutionHide the solution
  1. 1 Switch on and off The field disappears when the current is turned off
  2. 2 Change the strength Change the current to change the field

AnswerIt can be switched off and its strength controlled.

Uses

Where are electromagnets used?

  • Scrapyards

    Lift and drop scrap iron.

  • Electric bells

    Current magnetises an iron core that attracts a hammer.

  • Relays

    A small current switches a large current.

  • Maglev trains

    Strong electromagnets lift and move the train.

Build One

Wrap insulated wire round a nail and connect it to a cell. Count how many paper clips it picks up with 10, 20 and 30 turns.

1. Keep the current the same.

2. Do not leave it connected: the wire gets hot.

A good answer shows: More turns pick up more paper clips. Stop current flowing and the clips drop.

Can I...?

  1. 1Describe the field round a wire.
  2. 2State that the field circles the wire.
  3. 3Say what a solenoid is.
  4. 4Compare with a bar magnet.
  5. 5List ways to strengthen the field.
  6. 6Say what an iron core does.
  7. 7Describe an electromagnet.
  8. 8Give uses.

Summary & Exam Focus

  • Current produces a magnetic field.
  • More turns, more current or an iron core gives a stronger field.
  • A solenoid field is like a bar magnet's field.
  • Reversing the current reverses the field.

Exam focus

State two ways to increase the strength of an electromagnet. (2 marks) (2 marks)

Number of turns, current, iron core.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Electromagnet
A solenoid with an iron core that is magnetic only when a current flows.
Solenoid
A coil of wire.
Magnetic field
The region around a magnet or current where a force acts on magnetic materials.
Current
The rate of flow of charge.
Core
A piece of iron inside a solenoid.
Turns
Loops of wire in a coil.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 State 2 marks

    State two ways of increasing the strength of the magnetic field of a solenoid.

    Show answerHide answer

    Model answer

    Increase the current. Increase the number of turns. Put an iron core inside the coil. (any two)

    Mark scheme

    • One way — 1 mark
    • A second way — 1 mark
  2. Question 2 Describe 2 marks

    Describe the magnetic field around a straight wire carrying a current.

    Show answerHide answer

    Model answer

    The field lines are concentric circles around the wire, and the field gets weaker further from the wire.

    Mark scheme

    • Circles around the wire — 1 mark
    • Weaker with distance — 1 mark
  3. Question 3 Explain 3 marks

    Explain why a solenoid with an iron core is used in a scrapyard to lift cars and then drop them.

    Show answerHide answer

    Model answer

    The iron core makes the magnetic field much stronger so it can lift heavy iron. The magnet can be switched off by turning off the current, so the car is released.

    Mark scheme

    • Iron core strengthens the field — 1 mark
    • Electromagnet only works when a current flows — 1 mark
    • Switching off releases the car — 1 mark
  4. Question 4 Describe 3 marks

    Describe the magnetic field of a solenoid and explain how it is similar to the field of a bar magnet.

    Show answerHide answer

    Model answer

    Field lines pass through the coil and loop round outside from one end to the other. The shape is the same as a bar magnet, with a north and a south end. Inside the coil the field is strong and uniform.

    Mark scheme

    • Lines pass through and loop round outside — 1 mark
    • Same shape as a bar magnet with two poles — 1 mark
    • Strong and uniform inside the coil — 1 mark
  5. Question 5 Explain 3 marks

    A student increases the current in a solenoid. Explain what happens to the field and to the number of paper clips the solenoid can lift.

    Show answerHide answer

    Model answer

    The field gets stronger. The solenoid exerts a greater force on the paper clips so it can lift more of them.

    Mark scheme

    • Field is stronger — 1 mark
    • Greater force on the paper clips — 1 mark
    • More clips lifted — 1 mark

Quick check

  1. A current in a wire produces a...

    1. Agravitational field
    2. Bsound wave
    3. Cmagnetic field
    4. Dstatic charge
    Show answerHide answer

    C: magnetic field

    Electromagnetism.

  2. An iron core in a solenoid makes the field...

    1. Aweaker
    2. Bzero
    3. Cuniform but weaker
    4. Dstronger
    Show answerHide answer

    D: stronger

    Iron is magnetic.

  3. Reversing the current reverses the field...

    1. Adirection
    2. Bstrength only
    3. Cnothing
    4. Dcolour
    Show answerHide answer

    A: direction

    Direction changes.

  4. A coil of wire is called a...

    1. Aturbine
    2. Bsolenoid
    3. Ctransformer
    4. Dresistor
    Show answerHide answer

    B: solenoid

    Solenoid.

  5. An electromagnet can be switched off by...

    1. Aremoving the iron
    2. Bheating it
    3. Cstopping the current
    4. Dreversing it
    Show answerHide answer

    C: stopping the current

    No current, no field.

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