# 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

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 should be coiled to form a solenoid
• The magnetic field around the solenoid is similar to that of a bar magnet

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

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:

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.
Close