Magnetic fields from currents and solenoids

Pass a long, straight insulated wire vertically through a hole in a horizontal sheet of card. Connect the wire to a low-voltage direct-current supply, with a switch and a variable resistor in series to control the current. Keep the other connecting leads away from the card so that their magnetic fields do not confuse the pattern.

Place plotting compasses on the card around the wire. With the switch open, note their directions: they respond mainly to the Earth's field. Close the switch briefly. The needles change direction, showing that the current has produced a magnetic effect. When the wire's field dominates, the needles lie approximately along tangents to circles centred on the wire.

Mark the direction of the north-seeking end at several positions around the wire. These directions reveal which way the field circles. Open the switch, reverse the supply connections and switch on again: the field direction reverses.

To reveal the shape more clearly, remove the compasses, sprinkle iron filings thinly onto the card and tap it gently while the current flows. The filings form a circular pattern. They do not show which way round the field points; compasses are needed for that.

Use a controlled, low-voltage supply and switch off between observations because the wire can become hot. Keep filings away from eyes.

Finding the direction around the wire

The magnetic field around a long straight conductor forms concentric circles: circles sharing a centre, with the wire at that centre. The circles lie in planes perpendicular to the wire. The current flows along the wire, but the magnetic field goes around it.

Use the right-hand grip rule to find the direction:

• Point your right thumb along the conventional current.
• Curl your fingers around the wire.
• Your fingers show the direction of the magnetic field.

Conventional current travels from the positive terminal towards the negative terminal through the external circuit. Electrons in a metal move in the opposite direction.

For example, viewed from above a vertical wire, an upward current comes towards you and produces an anticlockwise field. A downward current goes away from you and produces a clockwise field. Reversing the current reverses the field direction without changing its circular shape.

What changes the field strength?

At the same distance from a straight wire, a larger current produces a stronger magnetic field. With the current kept constant, the field becomes weaker as the distance from the wire increases.

Field-line drawings represent strength by the concentration of lines. Around a straight wire, lines are drawn closer together near the wire and further apart farther away. These are qualitative relationships; no equation is needed here.

Combining fields in a solenoid

A solenoid is a long coil of insulated wire. Each loop is called a turn. Insulation prevents neighbouring turns from making electrical contact, so the current follows the wire around every turn.

Every turn produces a magnetic field. The field of the whole solenoid is the combined effect of these individual fields, making the solenoid an example of an electromagnet.

Along the centre of the solenoid, the fields from the turns point in the same direction, so they add together. This produces a very strong, almost uniform field along the centre. ‘Almost uniform’ means that its strength and direction are nearly constant in this central region. On a diagram, it is represented by straight, parallel, closely and evenly spaced field lines. Near the ends, the lines spread out, so the field is less uniform there.

Outside the solenoid, contributions from different turns oppose one another and partly cancel. The resulting field is much weaker, not absent. Its lines spread out and curve around the coil.

A switchable, bar-magnet-like field

The overall field resembles that of a bar magnet: one end of the solenoid acts as a north pole and the other as a south pole. Outside, field lines run from north to south; inside, they return from south to north, forming continuous loops.

Reversing the current reverses the magnetic field and swaps the poles. Switching off the current removes the field produced by the coil. Unlike a permanent magnet, the solenoid's magnetism can therefore be controlled by its electrical circuit.

• A current produces a magnetic field around the conductor.
• Shape: concentric circles centred on the wire, in planes perpendicular to it.
• Direction: right-hand grip rule — thumb along conventional current, fingers around the field.
• Current towards you → anticlockwise field; current away from you → clockwise field.
• Reverse the current → reverse the field direction.

Demonstrating the field

• Compare plotting-compass directions with the current off and on.
• Reverse the current and observe the reversed field direction.
• Iron filings reveal the circular shape; compasses reveal direction.

Field strength around a straight wire

• Larger current → stronger field at the same distance.
• Greater distance → weaker field at the same current.
• More concentrated field lines represent a stronger field.

Solenoid

• Inside: individual turns' fields add → very strong, almost uniform field along the centre.
• Outside: fields partly cancel → weaker field.
• Central field lines are straight, parallel and evenly spaced; outside lines spread out.
• Overall pattern resembles a bar magnet, with north and south poles.
• Reversing current swaps the poles; switching off removes the coil's field.