Magnetic field of a straight current-carrying wire

A current in a straight wire produces a magnetic field around the wire. The field lines form concentric circles centred on the wire. Each circle lies in a plane perpendicular to the wire, so the lines appear circular when viewed along the wire.

The field is strongest near the wire and becomes weaker farther away. Field-line diagrams show this by drawing the lines farther apart as the distance from the wire increases.

Direction of the magnetic field

The field circles the wire in a direction determined by the direction of conventional current. To find it, use the right-hand grip rule: point your right thumb along the current and curl your fingers around the wire. Your fingers point in the direction of the magnetic field.

If the current is reversed, the magnetic field circles the wire in the opposite direction.

Magnetic field of a solenoid

A solenoid is a coil of wire. When current flows through it, each turn produces a magnetic field. The fields from the turns combine to produce the field of the solenoid.

Field pattern of a solenoid

The field pattern resembles that of a bar magnet: the solenoid has a north pole at one end and a south pole at the other. Outside the solenoid, magnetic field lines curve from north to south. Inside, they run from south to north.

Inside a long solenoid, the field lines are nearly parallel and evenly spaced. This represents an approximately uniform field: the field has nearly the same direction and strength throughout that region.

Direction of the solenoid’s field

The field direction depends on the direction of conventional current around the turns. To find the north pole, curl the fingers of your right hand in the direction of the current around the solenoid. Your thumb points towards its north pole.

If the current is reversed, the magnetic field also reverses. The end that was the north pole becomes the south pole, and the end that was the south pole becomes the north pole.

Mapping current-produced magnetic fields

A current in a conductor produces a magnetic field around it. A plotting compass can be used to find the field’s direction at different positions. The north-seeking end of the compass needle points in the direction of the magnetic field at its position.

Mapping the field with a compass

For a straight wire, pass the wire vertically through a horizontal card and connect it to a power supply in a circuit. The card provides a surface on which to place the compass and draw the field pattern.

For a solenoid, connect the coil to a power supply and arrange it so that the compass can be placed at positions around the coil.

1. Close the circuit so that current flows through the conductor.
2. Place the compass at a position near the conductor. Mark the direction in which the north-seeking end points.
3. Move the compass to successive positions and mark the direction at each one. Join the marks to draw field lines, adding arrows in the directions indicated by the compass.
4. Repeat at other positions to build up the pattern. Switch off the current when measurements are not being taken to limit heating.

Around a straight wire, the field lines form circles centred on the wire. Around a solenoid, field lines pass through the coil and curve around outside it.

Iron filings and field direction

Iron filings sprinkled on a suitable surface around the conductor can show the shape of the field pattern. They do not show which way the field points, so a compass is needed to establish its direction.

If the current is reversed and the compass observations are repeated, the compass points in the opposite direction at the same positions. This shows that reversing the current reverses the magnetic field direction.

Electromagnetic relay

An electromagnetic relay uses a current in one circuit to operate a switch in a separate circuit. This allows a small control current to switch a circuit carrying a larger current.

How an electromagnetic relay operates

When current flows through the relay’s coil, the coil produces a magnetic field that magnetises an iron core. The magnetised core attracts an iron armature: a movable piece of iron connected to the switch contacts.

As the armature moves, it moves the contacts. Depending on the arrangement, the contacts either close or open the controlled circuit. When the coil is switched off, the magnetic attraction is removed and a spring returns the armature to its original position, returning the contacts with it.

For example, a vehicle starter relay uses a control circuit to switch the separate circuit that supplies the starter motor.

Moving-coil loudspeaker

A moving-coil loudspeaker converts an electrical signal into sound. It contains a coil of wire attached to a lightweight cone. The coil sits in the magnetic field of a permanent magnet.

When current flows through the coil, the current produces a magnetic field. This field interacts with the permanent magnet’s field, producing a force on the coil. Because the cone is attached to the coil, the force moves both of them.

How the loudspeaker produces sound

In an audio signal, the current changes in size and direction. The force on the coil changes in size and direction too, so the coil and cone move back and forth: they vibrate. The vibrating cone makes the surrounding air vibrate, producing sound waves.

Moving-coil loudspeakers reproduce sound in devices such as radios and public-address systems.

Strength of current-produced magnetic fields

A current in a wire produces a magnetic field. For both a straight wire and a solenoid, a larger current produces a stronger magnetic field; a smaller current produces a weaker field. A solenoid is a coil of wire carrying a current.

Field strength around a straight wire

The magnetic field around a straight wire is strongest close to the wire. As the distance from the wire increases, the field becomes weaker. If the current is increased, the field becomes stronger at a given distance from the wire.

Field strength inside and outside a solenoid

Inside a long solenoid, the magnetic field is approximately uniform: its strength and direction are nearly the same at different points inside. The field is stronger inside than outside the solenoid. Outside, it spreads out and becomes weaker.