AQA GCSE Physics (8463) · Paper 2
🧲 Magnetism & Electromagnetism
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Magnets & fields
Like poles repel, and unlike poles attract; these are non-contact forces, and they are strongest at the poles of a magnet. A permanent magnet produces its own magnetic field, while an induced magnet only becomes magnetic when it is placed inside another magnetic field, and it is always attracted to that field. The magnetic materials are iron, steel, cobalt and nickel.
A magnetic field can be plotted using a compass: at each position, the direction the needle points is marked, the compass is moved a small step in that direction, and the process is repeated; joining the marks together traces out the field lines. A compass points north because the Earth's core generates its own magnetic field, and this is the evidence that the field exists.
Electromagnets & motors
Any current-carrying wire produces a circular magnetic field around it, which is stronger with more current and weaker further from the wire. Coiling the wire into a solenoid concentrates the field, making it strong and uniform inside the coil and shaped like a bar magnet's field outside it. Placing an iron core inside the solenoid turns it into an electromagnet, which is a magnet that can be switched on and off.
Being able to switch the magnetism on and off makes electromagnets very useful. Scrapyard cranes use them to pick up and drop steel on command, relays use a small current to close a switch in a separate high-power circuit, and they are also used in electric bells and locks.
The motor effect (Higher)
A current-carrying wire in a magnetic field feels a force, because the field of the wire and the external field interact. The direction of the force can be found using Fleming's left-hand rule: the thumb points in the direction of motion, the first finger points in the direction of the field, from north pole to south pole, and the second finger points in the direction of the current. In a motor, the two sides of a current-carrying coil are pushed in opposite directions, one up and one down, and this produces rotation; a split-ring commutator flips the current every half-turn so that the coil keeps spinning the same way.
Generators & transformers (Triple, Higher) Separate science only
The generator effect is the motor effect in reverse. If a conductor is moved through a magnetic field, or the field through a coil is changed, a potential difference is induced across it. If the circuit is closed, a current flows, and the field produced by that current always opposes the change that created it.
In a transformer, alternating current in the primary coil produces a constantly changing magnetic field in the iron core, which induces an alternating potential difference in the secondary coil. If the secondary coil has more turns than the primary, the transformer is a step-up transformer; if it has fewer turns, it is a step-down transformer.