Describe the magnetic field of a current in a wire and a solenoid, and how an electromagnet works

What this dot point is asking

SEAB wants you to describe the magnetic field produced by a current in a straight wire and in a solenoid (coil), to use the right-hand grip rule for direction, and to explain how an electromagnet works and how to make it stronger. The big idea is that an electric current creates a magnetic field, which is the link between electricity and magnetism.

The answer

The field around a straight wire

When a current flows in a straight wire, it produces a magnetic field made of concentric circles around the wire, lying in planes perpendicular to it. The field is stronger (circles closer together) near the wire and weaker further away. Reversing the current reverses the direction of the field.

The right-hand grip rule gives the direction: point the right thumb along the conventional current, and the curled fingers show the way the circular field lines point.

The field of a solenoid

A solenoid is a long coil of wire. When a current flows, the field inside is strong and uniform, running along the axis, and the field outside looks just like that of a bar magnet, with a north pole at one end and a south pole at the other.

The right-hand grip rule for a solenoid: curl the right fingers the way the current flows around the coil, and the thumb points to the north pole.

The electromagnet

An electromagnet is a solenoid wound around a soft iron core. The current produces a magnetic field, and the soft iron core greatly increases the strength because it becomes strongly magnetised. The key advantage is that the magnetism can be switched on and off with the current, and reversed by reversing the current.

Making an electromagnet stronger

The magnetic field of an electromagnet can be increased by:

• increasing the current in the coil,
• increasing the number of turns on the coil,
• using a soft iron core (or winding the turns more closely).

Examples in context

Example 1. The electric bell. An electric bell uses an electromagnet to attract an iron armature, which strikes the bell and breaks the circuit; the electromagnet then switches off, the armature springs back, the circuit remakes, and the cycle repeats. The switchable nature of the electromagnet makes the rapid ringing possible.

Example 2. A relay. A relay uses a small current in an electromagnet's coil to switch on a much larger current in a separate circuit. This lets a low-power switch (or sensor) safely control a high-power device, such as a car's starter motor, by using the electromagnet to close the heavy-current contacts.

Try this

Q1. Describe the magnetic field around a straight wire carrying a current. [2 marks]

• Cue. Concentric circles around the wire, perpendicular to it, closer together (stronger) near the wire.

Q2. State three ways to increase the strength of an electromagnet. [3 marks]

• Cue. Increase the current, increase the number of turns, and use a soft iron core.

Q3. Explain why a scrapyard crane uses an electromagnet rather than a permanent magnet. [2 marks]

• Cue. An electromagnet can be switched on to lift and off to release the load, which a permanent magnet cannot do.