AQA GCSE Physics

Revision Notes

7.2.1 Magnetic Fields in Wires & Solenoids

Magnetic Field Around a Wire

  • When a current flows through a conducting wire a magnetic field is produced around the wire
  • The shape and direction of the magnetic field can be investigated using plotting compasses

straight-wire-magnetised, IGCSE & GCSE Physics revision notes

Diagram showing the magnetic field around a current-carrying wire

  • The magnetic field is made up of concentric circles
    • A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
  • As the distance from the wire increases the circles get further apart
    • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases
  • The right-hand thumb rule can be used to work out the direction of the magnetic field

Right hand thumb rule, downloadable IGCSE & GCSE Physics revision notes

The right-hand thumb rule shows the direction of current flow through a wire and the direction of the magnetic field around the wire

  • Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field

Side and Top View of Current and the Magnetic Field Produced, downloadable IGCSE & GCSE Physics revision notes

Side and top view of the current flowing through a wire and the magnetic field produced

  • If there is no current flowing through the conductor there will be no magnetic field
  • Increasing the amount of current flowing through the wire will increase the strength of the magnetic field
    • This means the field lines will become closer together

Magnetic Field Around a Solenoid

  • When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it

flat-celular-coil, IGCSE & GCSE Physics revision notes

Diagram showing the magnetic field around a flat circular coil

  • To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid
  • The magnetic field around the solenoid is similar to that of a bar magnet

Magnetic field around a solenoid, downloadable IGCSE & GCSE Physics revision notes

Magnetic field around and through a solenoid

  • The magnetic field inside the solenoid is strong and uniform
  • One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole
    • To work out the polarity of each end of the solenoid it needs to be viewed from the end
    • If the current is travelling around in a clockwise direction then it is the south pole
    • If the current is travelling around in an anticlockwise direction then it is the north pole
  • If the current changes direction then the north and south poles will be reversed
  • If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid

Direction of field solenoid, downloadable AS & A Level Physics revision notes

Poles of a Solenoid

Magnetic Field Strength Around a Solenoid

  • The strength of the magnetic field produced around a solenoid can be increased by:
    • Increasing the size of the current which is flowing through the wire
    • Increasing the number of coils
    • Adding an iron core through the centre of the coils
  • The iron core will become an induced magnet when current is flowing through the coils
  • The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall

Electromagnets

  • An electromagnet is a solenoid with an iron core
  • The magnetic field produced by the electromagnet can be switched on and off
    • When the current is flowing there will be a magnetic field produced around the electromagnet
    • When the current is switched off there will be no magnetic field produced around the electromagnet
  • The strength of the electromagnet can be changed by:
    • Increasing the current will increase the magnetic field produced around the electromagnet
    • Decreasing the current will decrease the magnetic field produced around the electromagnet

Examples of Electromagnetic Devices

  • Electromagnets are used in several devices, for example, a scrapyard crane or an electric bell
  • Scrapyard cranes:
    • When the electromagnet is switched on it will attract magnetic materials
    • When the electromagnet is switched off it will drop the magnetic materials
  • Electric bell:

SME, IGCSE & GCSE Physics revision notes

Animation showing an electric bell in operation

  • When the button K is pressed:
    • A current passes through the electromagnet E creating a magnetic field
    • This attracted the iron armature A, causing the hammer to strike the bell B
    • The movement of the armature breaks the circuit at T
    • This stops the current, destroying the magnetic field and so the armature returns to its previous position
    • This re-establishes the circuit, and the whole process starts again

Exam Tip

When trying to figure out how an electromagnetic device works:

  • Look for a coil / solenoid – this is going to act as an electromagnet
  • Look for a piece of iron – this will be attracted to the solenoid

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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