How does an electric current produce magnetism, and how is this used in motors?
Describe the magnetic field of a current, the electromagnet, and the force on a current in a field
Describe the magnetic field around a current-carrying wire and coil, how an electromagnet works, and the turning force on a current in a magnetic field that drives a motor at N(A)-Level.
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What this dot point is asking
SEAB wants you to describe the magnetic field produced by an electric current, to explain how an electromagnet works and how to make it stronger, and to describe the force on a current-carrying wire in a magnetic field (the motor effect). The big idea is that electricity and magnetism are linked: a current makes a magnetic field, and a current in a field feels a force.
The answer
A current makes a magnetic field
Whenever an electric current flows, it produces a magnetic field around it. A straight wire carrying a current has circular field lines around it. The larger the current, the stronger the field. This was the first clue that electricity and magnetism are connected.
The field of a coil (solenoid)
If the wire is wound into a coil (a solenoid), the fields from each turn add together. The result is a magnetic field very like that of a bar magnet, with a north pole at one end and a south pole at the other. Inside the coil the field is strong and fairly uniform.
Electromagnets
An electromagnet is a coil wound around a core of soft iron. When current flows, the coil's field magnetises the iron core, which greatly strengthens the overall field. The key feature is that the magnetism can be switched on and off with the current.
You can make an electromagnet stronger by:
- increasing the current;
- adding more turns to the coil;
- using a soft iron core (or a larger one).
The big advantage over a permanent magnet is control: it can be turned on, turned off, or have its strength changed.
The motor effect
When a wire carrying a current is placed in a magnetic field, the wire feels a force. This is called the motor effect. The force is greatest when the wire is at right angles to the field.
You can increase the force by:
- increasing the current;
- using a stronger magnet (a stronger field);
- using a longer length of wire in the field.
If you reverse the current, or reverse the magnetic field, the force acts in the opposite direction.
How a simple motor works
In an electric motor, a coil carrying a current sits in a magnetic field. The forces on the two sides of the coil act in opposite directions, so they turn the coil. A device called a split-ring commutator reverses the current every half turn, so the coil keeps turning the same way. This turns electrical energy into kinetic energy.
Examples in context
Example 1. The electric bell and the crane magnet. A scrapyard crane uses a powerful electromagnet to lift steel: switch the current on to grab the load, and off to drop it where wanted. The same on/off control is used in electric bells, relays and door locks, all relying on an electromagnet that works only while current flows.
Example 2. Motors everywhere. The motor effect drives the electric motors in fans, washing machines, electric cars and hard drives. In each, a current in a coil sitting in a magnetic field produces a turning force, converting electrical energy into useful movement.
Try this
Cue. State two ways to make an electromagnet stronger. [2 marks] Increase the current and add more turns to the coil (using a soft iron core also works).
Cue. State what a current-carrying wire experiences when placed at right angles to a magnetic field. [1 mark] It experiences a force (the motor effect), so it is pushed.
Cue. State what happens to the force on the wire if the current direction is reversed. [1 mark] The force acts in the opposite direction.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksA coil of wire is wound around an iron nail and connected to a battery, making an electromagnet. (a) Explain why it becomes magnetic. (b) State two ways to make the electromagnet stronger. (c) State one advantage of an electromagnet over a permanent magnet.Show worked answer →
(a) When current flows in the coil, it produces a magnetic field. The iron core becomes magnetised and greatly strengthens this field, so the whole device acts as a magnet.
(b) Any two: increase the current, add more turns to the coil, or use a (larger) iron core.
(c) It can be switched on and off (or its strength can be changed) by controlling the current, which a permanent magnet cannot.
What markers reward: current producing a field magnetising the iron core, two valid ways to strengthen it, and the on/off (or variable) advantage.
Original4 marksA straight wire carrying a current is placed between the poles of a magnet, at right angles to the field. (a) State what the wire experiences. (b) State two ways to increase the size of this effect. (c) State what happens if the current is reversed.Show worked answer →
(a) The wire experiences a force (it is pushed), known as the motor effect.
(b) Any two: increase the current, use a stronger magnet (stronger field), or use a longer length of wire in the field.
(c) If the current is reversed, the force acts in the opposite direction, so the wire is pushed the other way.
What markers reward: a force on the wire (motor effect), two valid ways to increase it, and the force reversing when the current reverses.
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