Interactions Over Small and Large Distances: Magnetism and Electromagnetism — AQA Combined Science: Synergy
This topic covers magnets, magnetic fields, the magnetic effect of a current, and the motor effect.
Magnets
- A permanent magnet produces its own magnetic field, with a north and a south pole.
- An induced magnet becomes magnetic in a magnetic field and loses most magnetism when removed; induced magnetism always causes attraction.
- Like poles repel, unlike poles attract — non-contact forces.
- Magnetic materials are iron, steel, cobalt and nickel.
Magnetic fields
A magnetic field is the region where a force acts on a magnet or magnetic material. Field lines run from north to south and are closest (strongest) at the poles. A plotting compass shows the field direction.
The Earth's magnetism
A compass needle (a small magnet) points north because the Earth has its own magnetic field, generated by its core — evidence that the core is magnetic.
The magnetic effect of an electric current
A current in a wire creates a magnetic field of concentric circles around it. The field is stronger with a larger current and closer to the wire.
Shaping the wire into a solenoid (coil) concentrates the field, producing a strong, uniform field like a bar magnet. Adding an iron core makes an electromagnet, which can be switched on/off and varied — used in cranes, bells and relays.
The motor effect (Higher Tier)
A current-carrying conductor in a magnetic field experiences a force (the motor effect), because the two fields interact.
- The force is greatest when the wire is at 90° to the field, zero when parallel.
- Direction is found with Fleming's left-hand rule.
- Reversing the current or the field reverses the force.
- Size of force: $F = B , I , L$ (flux density × current × length).
Electric motors (Higher Tier)
In an electric motor, the two sides of a current-carrying coil in a magnetic field experience forces in opposite directions, making the coil rotate. A split-ring commutator reverses the current each half turn so it keeps spinning the same way.
Exam tips
- Distinguish permanent and induced magnets (induced is temporary, always attractive).
- Describe field lines: N to S, strongest at the poles.
- Strengthen an electromagnet with more current, more turns and an iron core.
- For the motor effect, force is greatest at 90°; use Fleming's left-hand rule.