Magnetic Fields in Wires & Solenoids (AQA GCSE Physics): Revision Note

Magnetic Field Around a Wire

• When a current flows through a conducting wire a magnetic field is produced around the wire

  • A conducting wire is any wire that has current flowing through it

• The shape and direction of the magnetic field can be investigated using plotting compasses

  • The compasses would produce a magnetic field lines pattern that would like look the following

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

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 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

Diagram showing the magnetic field around a flat circular coil

• To increase the strength of the magnetic field around the wire, it can be coiled to form a solenoid

• The magnetic field around the solenoid is similar to that of a bar magnet

Magnetic field 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, and the other end 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

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 turns in the coil in a given length

  • reducing the length of the wire and maintaining the number of turns

  • 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 by 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:

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