# AQA A2 Physics A Unit 4 Magnetic Fields

• Created by: lauren
• Created on: 22-04-14 17:07

Magnetic Fields

A magnetic field is a force field surrounding a magnet or current-carrying wire which acts on magnetic materials or magnetically susceptible materials.

Magnetic field lines go from NORTH to SOUTH (a line in the direction a 'free' north pole would move in the field). The closer together the field lines, the stronger the field.

Magnetic Fields Around a Wire

A current flowing in a wire induces a magnetic field around the wire, the field lines are concentric circles around the wire. The direction of the magnetic field is worked out using the right-hand rule.

Force on a Current-Carrying Wire

A current-carrying wire will experience a force if it is placed at a non-zero angle to the field lines of another magnetic field. The force is perpendicular to the wire and to the field lines, it is known as the MOTOR EFFECT. The field lines from the wire and the external magnetic field interact, this causes the field lines from the external magnetic field to contract so the field lines are closer together, causing a force on the wire.

The magnitude of the force experienced by the wire depends on the:

• Current

• Strength of the magnetic field

• Length of the wire

• Angle between the lines of force of the field and the direction of the current.

The force is largest when the wire is perpendicular to the magnetic field. The force is zero when the wire is parallel to the magnetic field.

Fleming's left-hand rule can be used to relate the directions of the force, current and field. If the current or magnetic field direction is reversed, the direction of the force will also reverse. Passing an alternating current through a wire in a magnetic field causes the wire to vibrate as the direction of the current is