GCSE / Physics
Magnetism and Electromagnetism
Learn the key GCSE Physics ideas behind magnetism and electromagnetism, including magnetic poles and fields, permanent and induced magnets, electromagnets, solenoids, the motor effect, Fleming’s left-hand rule, electric motors, loudspeakers, electromagnetic induction, generators, microphones and transformers. This resource also covers the main equations and exam concepts needed for magnetism and electromagnetism questions.
Magnetism is caused by magnetic fields around magnets and current-carrying conductors. Every magnet has a north pole and a south pole; unlike magnetic poles attract while like poles repel, and the magnetic force is strongest near the poles.
A permanent magnet produces its own magnetic field continuously, whereas an induced magnet becomes magnetic only when placed in another magnetic field. Induced magnets usually lose most or all of their magnetism when the external field is removed.
A magnetic field is the region around a magnet or magnetic material where another magnet experiences a force. Magnetic field lines are drawn from the north pole to the south pole, and closer field lines represent a stronger magnetic field.
The direction of a magnetic field at any point is defined as the direction in which the north-seeking pole of a small compass would point. A plotting compass can therefore be moved around a magnet to map the magnetic field pattern.
The Earth behaves as though it has a large magnetic field surrounding it. A compass aligns with this field, which is why the north-seeking end of a compass points approximately towards geographic north.
When electric current flows through a straight conducting wire, it produces a circular magnetic field around the wire. Increasing the current increases the magnetic field strength, while increasing the distance from the wire reduces the field strength.
The direction of the magnetic field around a current-carrying wire depends on the direction of conventional current. Reversing the current reverses the direction of the magnetic field.
A solenoid is a coil of wire containing many turns. When current passes through it, the magnetic fields from the individual turns combine to produce a strong, almost uniform magnetic field inside the solenoid.
The magnetic field of a solenoid can be made stronger by increasing the current, increasing the number of turns per unit length, or placing an iron core inside the coil. A solenoid with an iron core acts as an electromagnet.
The motor effect occurs when a current-carrying conductor is placed in a magnetic field. The magnetic field produced by the current interacts with the external magnetic field and produces a force on the conductor.
For a conductor at right angles to a magnetic field, the force is calculated using , where is force in newtons, is magnetic flux density in tesla, is current in amperes and is the length of conductor in the magnetic field in metres.
Fleming's left-hand rule is used to determine the direction of the force in the motor effect. The first finger represents the magnetic field, the second finger represents conventional current and the thumb shows the direction of force or motion.
An electric motor uses the motor effect to convert electrical energy into rotational kinetic energy. Forces on opposite sides of a current-carrying coil form a turning effect, causing the coil to rotate in a magnetic field.
Loudspeakers and headphones also use the motor effect. A varying current passes through a coil in a magnetic field, producing changing forces that make the cone or diaphragm vibrate and generate sound waves.
Electromagnetic induction occurs when a conductor moves relative to a magnetic field, or when the magnetic field through a conductor changes. This change induces a potential difference, and if the circuit is complete an induced current flows.
The size of an induced potential difference can be increased by moving the conductor or magnet faster, using a stronger magnetic field, or increasing the number of turns in a coil. Reversing the direction of motion or field reverses the induced potential difference.
The induced current produces a magnetic field that opposes the change that caused it. This opposition is the idea behind Lenz's law and is an important feature of the generator effect.
An alternator uses electromagnetic induction to generate alternating current by rotating a coil in a magnetic field or rotating a magnetic field relative to a coil. The induced potential difference changes direction periodically, producing an alternating output.
A dynamo also uses the generator effect but is designed to produce direct current. Both dynamos and alternators convert kinetic energy into electrical energy using electromagnetic induction.
A moving-coil microphone works by electromagnetic induction. Sound waves make a diaphragm and attached coil vibrate in a magnetic field, inducing a varying potential difference that represents the original sound signal.
A transformer contains a primary coil and a secondary coil wound around an iron core. An alternating current in the primary coil produces a changing magnetic field in the core, which induces an alternating potential difference in the secondary coil.
The transformer voltage relationship is , where and are the primary and secondary potential differences and and are the numbers of turns on the primary and secondary coils.
A step-up transformer has more turns on the secondary coil than the primary coil and therefore increases potential difference. A step-down transformer has fewer secondary turns and reduces potential difference.
For an ideal transformer, electrical power input equals electrical power output, so . Increasing transmission potential difference therefore reduces current for the same power, helping reduce energy losses in power cables.