Physics

8.6 - Electromagnetism

8 min

Field around a wire

Coils and electromagnets

Electromagnet strength

note 1 of 2

The magnetic field around a current-carrying wire

Whenever an electric current flows through a wire, a magnetic field is created around it.
Around a long, straight current-carrying wire the field is made up of concentric circles, lying in planes perpendicular to the wire, with the wire at their centre.
This pattern can be shown with small plotting compasses placed around the wire: as current flows, each compass lines up along a circle, tracing the field direction.

The right-hand thumb rule

Point the thumb of the right hand in the direction of the current and curl the fingers around the wire.
The fingers then point in the direction of the magnetic field.
Reversing the current reverses the field.

A right hand gripping a straight wire, thumb pointing left along it, with circular field lines round the wire and labels for the direction of current and the fingers curling with the field lines.

Field strength

The field strength depends on two things:

Size of the current - A larger current produces a stronger field.
Distance from the wire - The field is stronger closer to the wire and weaker further away.

note 2 of 2

Solenoids and electromagnets

Coiling the wire strengthens the field

The magnetic field of a single current-carrying wire is fairly weak. It can be made much stronger by wrapping the wire into a coil.
A coil of wire is called a solenoid.
The field lines around each turn line up and add together, giving:
- A strong, uniform field inside the solenoid, where the field lines are parallel and evenly spaced.
- A field outside the coil that has the same shape as a bar magnet's.
One end of the solenoid behaves as a north pole and the other as a south pole. Reversing the current reverses the poles.
To find the north pole, curl the fingers of the right hand around the coil in the direction of the current. The thumb then points to the north pole (the right-hand grip rule).

A wire wound into a coil, field lines running straight through the middle and looping round outside, S at the left end and N at the right, labelled coil, current and magnetic field.

Increasing the field strength

Increasing the current through the coil.
Increasing the number of turns of wire.
Adding an iron core inside the coil.

The electromagnet

Placing a soft iron core inside the solenoid greatly increases the magnetic field.
The soft iron is easily magnetised to become an induced magnet while the current flows, and quickly loses its magnetism when the current is switched off.
An electromagnet is a solenoid with an iron core: a magnet whose field can be switched on and off, and varied in strength by changing the current.

A grey cylinder with an orange wire wound round it as a coil, current arrows running up one end and down the other, titled "electromagnet" and labelled "current" and "soft iron core".

Plotting the field of a solenoid

Place a piece of card across the middle of the solenoid.
With a current flowing, sprinkle iron filings or use a plotting compass on the card to trace the magnetic field lines.

key terms

TermDefinition

SolenoidA coil of wire that produces a magnetic field when an electric current flows through it.

ElectromagnetA coil of wire (a solenoid), usually with an iron core, whose magnetic field can be switched on and off by changing the current.

end of notes

Key points to remember

The magnetic field around a current-carrying wire

Whenever an electric current flows through a wire, a magnetic field is created around it.

Around a long, straight current-carrying wire the field is made up of concentric circles, lying in planes perpendicular to the wire, with the wire at their centre.

This pattern can be shown with small plotting compasses placed around the wire: as current flows, each compass lines up along a circle, tracing the field direction.

The right-hand thumb rule

Point the thumb of the right hand in the direction of the current and curl the fingers around the wire.

The fingers then point in the direction of the magnetic field.

Reversing the current reverses the field.

Solenoids and electromagnets

Coiling the wire strengthens the field

The magnetic field of a single current-carrying wire is fairly weak. It can be made much stronger by wrapping the wire into a coil.

A coil of wire is called a solenoid.

The field lines around each turn line up and add together, giving:

A strong, uniform field inside the solenoid, where the field lines are parallel and evenly spaced.
A field outside the coil that has the same shape as a bar magnet's.

One end of the solenoid behaves as a north pole and the other as a south pole. Reversing the current reverses the poles.

To find the north pole, curl the fingers of the right hand around the coil in the direction of the current. The thumb then points to the north pole (the right-hand grip rule).