Electricity and magnetism · GCSE Physics
Magnetism and electromagnetism
Magnetic fields, electromagnets, the motor effect, Fleming’s left-hand rule, induced potential and transformers on Physics routes.
Field lines go from north to south. A current at right angles to a magnetic field feels a force — that is the motor. A changing field through a coil induces a potential difference — that is the generator and the transformer.
The important bits
What you need to know
- 1
Like poles repel, unlike poles attract. Magnetic field lines go from north to south, never cross, and are densest where the field is strongest. The Earth has a magnetic field; a plotting compass aligns with it.
- 2
A current-carrying wire has a magnetic field (right-hand grip: thumb in current direction, fingers show the field). A solenoid’s field looks like a bar magnet. An iron core makes a stronger electromagnet.
- 3
The motor effect: a wire carrying current in a magnetic field experiences a force if the current and field are not parallel. F = BIl when they are at right angles (Higher). Fleming’s left-hand rule gives the direction.
- 4
A coil in a magnetic field, supplied with current, becomes a motor. A split-ring commutator reverses the current each half-turn so the coil keeps rotating in d.c. motors.
- 5
A changing magnetic field through a coil induces a potential difference (generator effect). Faster movement, a stronger magnet or more turns increases the induced p.d. This is GCSE Physics rather than Combined on some specifications.
- 6
A transformer uses two coils on an iron core and a.c. to change potential difference. V₁ / V₂ = n₁ / n₂. For an ideal transformer, V₁I₁ = V₂I₂. Step-up increases V and decreases I.
- 7
Loudspeakers use the motor effect: an alternating current in a coil in a magnetic field makes the cone vibrate and produce sound. Microphones reverse the idea (generator effect).
Go deeper
Electromagnets are magnets you can switch
A coil of wire is a solenoid. Its field is like a bar magnet while the current is on, and it vanishes when the current is off. More turns, more current, and an iron core all increase the strength. That is why scrapyard cranes, relays and circuit breakers use electromagnets: they can drop the load. Draw the field through the centre of the solenoid as straight lines, looping round the outside, and mark north and south with the right-hand grip. If the field through a coil changes, you have left Combined-only territory on some boards and entered induced p.d.: the coil behaves like a tiny generator. Moving the magnet faster gives a bigger reading on the voltmeter.
Go deeper
Motors and transformers share one idea: a field and a current
Motor: current already flowing, field already there, force appears, coil turns. Generator: coil turning in a field, induced p.d. appears, current can flow if the circuit is closed. Transformers need a.c. because only a changing current in the primary makes a changing field in the core, which induces a p.d. in the secondary. The turns ratio sets whether V goes up or down. Power is conserved in the ideal case, so if V rises, I falls — which is why the grid uses them. Fleming’s left-hand rule: thuMb = Motion (force), First finger = Field (N to S), seCond finger = Current. Practise it on paper until the three directions are automatic.
See the idea in action
A wire of length 0.050 m carries 3.0 A at right angles to a 0.20 T field. Force F = BIl = 0.20 × 3.0 × 0.050 = 0.030 N. Direction from Fleming’s left hand: if the field is left to right and the current is into the page, the force is up or down according to the finger orientation you set — mark the diagram rather than guessing. If this wire is one side of a coil, that force provides a moment and the coil rotates until the commutator reverses the current.
Exam technique
Turn knowledge into marks
Label field direction N to S, current direction, and force direction. For Combined, electromagnets and the motor effect are the core. For Physics, add induced p.d. and the transformer equation, and say why a.c. is required. Increase induced p.d. with speed, field strength or number of turns.
Common mistakes
Do not give these marks away
- 01
Drawing field lines that cross, or running them south to north.
- 02
Using the right-hand rule for the motor effect (left hand is force on a current in a field).
- 03
Saying a transformer works with d.c., or mixing up step-up with an increase in current.
What happens to the magnetic field of a solenoid when the current is switched off?
AIt reverses direction and stays on
BIt disappears
CIt becomes a permanent field like steel
DIt only remains if the coil is copper
Show the answer
It disappears. An electromagnet’s field is caused by the current. No current means no field, which is why electromagnets can be switched. A steel core might retain some magnetism; the coil itself does not.
Quick questions
If this is the bit you searched
What is Fleming’s left-hand rule?
A way to find the direction of the force on a current-carrying conductor in a magnetic field: thumb = motion, first finger = field, second finger = current.
Why do transformers need a.c.?
A changing current in the primary is needed to produce a changing magnetic field in the core, which induces a potential difference in the secondary. Direct current would not keep changing.