Magnetic Fields from Currents and Solenoids

Showing that a current produces a magnetic field

A magnetic field is a region in which a magnet experiences a force. Permanent magnets are not the only things that produce magnetic fields: an electric current in a wire produces a field around the wire too.

A plotting compass provides a way to detect this effect. Its needle is a small magnet, so it turns to align with the magnetic field at its position.

Place a straight section of wire close above a plotting compass and connect the wire to a low-voltage power supply through a switch. With the switch open, note the needle's direction. Close the switch briefly: when current flows, the compass needle deflects. Open the switch again and it returns towards its original direction. This change shows that the current has created a magnetic effect.

Reverse the connections to the supply and repeat. The needle deflects in the opposite direction because reversing the current reverses the magnetic field. Keep nearby magnets away so that their fields do not confuse the observations. Use a current-limited, low-voltage supply and switch off between observations because the wire can become hot; never connect a wire directly across an unrestricted supply.

The field circles around a straight wire

To investigate the pattern, pass a straight wire vertically through a horizontal card. With current flowing, place a plotting compass at several positions around the wire. Mark the direction of the north-seeking end of its needle at each position. Together, these directions trace circles centred on the wire.

The field lines are concentric circles, lying in planes perpendicular to the wire. At each position, the compass points along the circle, not towards or away from the wire.

Use the right-hand grip rule to find the direction: point your right thumb in the direction of conventional current, and curl your fingers around the wire. Your fingers show the direction of the magnetic field. For example, viewed from above, a current coming upwards towards you produces an anticlockwise field. A current going downwards away from you produces a clockwise field.

The strength of this field depends on two things:

• Current: a larger current produces a stronger magnetic field.
• Distance: the field becomes weaker farther from the wire.

Field-line drawings represent stronger fields with more closely spaced lines. Around a straight wire, the circles are drawn farther apart as distance increases. These lines are a model of the field, not physical rings around the wire.

Turning a wire into a solenoid

A solenoid is a wire wound into a long coil. When current flows, each turn produces a magnetic field. Inside the coil, the effects of the turns reinforce one another, producing a strong field that is approximately uniform away from the ends. A uniform field has approximately the same strength and direction throughout that region.

The overall field resembles that of a bar magnet: the solenoid has a north pole at one end and a south pole at the other. Inside it, the field lines are approximately straight, parallel and evenly spaced. Outside it, they curve from north to south.

Reversing the current swaps the poles. Viewed directly from one end, anticlockwise current makes that end a north pole; clockwise current makes it a south pole. Switching off removes the magnetic field produced by the current.

Making the magnetic effect stronger

There are three main ways to strengthen a solenoid's magnetic effect:

• Increase the current. Each turn then produces a stronger field.
• Increase the number of turns in the same length of coil. More turns contribute to the field in that region.
• Insert an iron core through the coil. The solenoid's field magnetises the iron, making it an induced magnet. Its field reinforces the coil's field, producing a much stronger electromagnet.

The iron core strengthens the magnetic effect; it does not supply the current. The electrical supply provides the current that creates the coil's field.

Straight current-carrying wire

• Current produces a magnetic field: a nearby compass deflects when the current is switched on.
• Field pattern: concentric circles centred on the wire.
• Right-hand grip rule: thumb = conventional current; curled fingers = field direction.
• Reverse current → reverse field direction.
• Larger current → stronger field; greater distance → weaker field.

Solenoid

• A current-carrying coil has a bar-magnet-like field, with north and south poles.
• Inside: strong, approximately uniform field.
• Reverse current → swap poles.
• Strengthen it by increasing current, adding turns in the same length, or inserting an iron core.