How does an electric current create a magnetic field?
Electromagnetism: the magnetic field around a current-carrying wire, the field of a solenoid, and how electromagnets are made and used.
A focused answer to AQA GCSE Physics 4.7.2, covering the magnetic field around a current-carrying wire, the stronger uniform field of a solenoid, how an electromagnet is built and strengthened, and the advantages of electromagnets.
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What this dot point is asking
AQA wants you to describe the magnetic field around a straight current-carrying wire, describe the field of a solenoid, explain how an electromagnet is made and how to make it stronger, and state the advantages of electromagnets. This is part of topic 4.7.2 of the AQA GCSE Physics (8463) specification.
The field around a wire
The discovery that an electric current produces a magnetic field, made by Oersted, is the foundation of all electromagnetism. It links electricity and magnetism into a single subject and underpins motors, generators, transformers, loudspeakers and many other devices. The strength of the field at any point depends on both the size of the current and how far the point is from the wire, so the field lines (the circles) are drawn closer together near the wire to show a stronger field there.
The solenoid
The reason a coil produces such a strong field is that the circular fields from each individual turn of wire add together inside the coil, reinforcing one another to give a strong, uniform field running along the centre of the solenoid. Outside the solenoid the field spreads out and looks exactly like that of a bar magnet, which is why a solenoid has a definite north and south pole. Reversing the current swaps the poles.
Electromagnets
This control is the central advantage that AQA wants you to be able to state and apply. A permanent magnet is always magnetic and cannot be turned off, whereas an electromagnet is magnetic only while current flows, so it can be switched on and off and its strength varied by changing the current. This makes electromagnets useful in devices such as scrapyard cranes (which must pick up and then release scrap metal), electric bells, relays (where a small current switches a larger circuit) and loudspeakers. Adding a soft iron core greatly strengthens the field because the iron becomes an induced magnet that adds its own field to that of the coil, and soft iron is used rather than steel because it loses its magnetism quickly when the current is switched off, which is exactly what a switchable device needs.
Try this
Q1. Describe the shape of the magnetic field around a straight current-carrying wire. [1 mark]
- Cue. Concentric circles around the wire.
Q2. Give two ways to increase the strength of an electromagnet. [2 marks]
- Cue. Increase the current, add more turns to the coil, or add an iron core.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksDescribe the magnetic field produced around a long straight wire when a current flows through it, and describe how the field changes if the current is increased and if the direction of the current is reversed.Show worked answer →
The magnetic field around a long straight current-carrying wire forms concentric circles (rings) centred on the wire, lying in planes at right angles to the wire (1 mark). The field is strongest close to the wire and gets weaker with distance, shown by the circles being further apart further out (1 mark). Increasing the current makes the field stronger at every point (1 mark). Reversing the direction of the current reverses the direction of the magnetic field, so the field lines point the opposite way around the wire (1 mark). Markers reward the circular shape, the weakening with distance, and the effects of changing the current size and direction.
AQA 20214 marksExplain why electromagnets, rather than permanent magnets, are used in devices such as scrapyard cranes, and describe two ways the strength of an electromagnet can be increased.Show worked answer →
An electromagnet is only magnetic while a current flows through it, so it can be switched on and off (1 mark), which is essential for a scrapyard crane that needs to pick up scrap metal and then release it by switching the current off; a permanent magnet could not release the load (1 mark). The strength of an electromagnet can be increased by increasing the current through the coil, by adding more turns to the coil, or by adding or using an iron core (2 marks for any two correct methods). Markers reward the switchable advantage linked to the application, and two valid strengthening methods.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)