AQA GCSE Physics 8463: Magnetism and electromagnetism – Revision Notes

Condensed recall for the final weeks. For full explanations and worked examples, use the Magnetism and electromagnetism study guide.

These notes cover Topic 7, Magnetism and electromagnetism (sections 4.7.1 to 4.7.3), of the AQA GCSE Physics (8463) specification, for first teaching 2016 with exams from June 2018 (version 1.1). The topic is examined on Paper 2 at Foundation and Higher tier every May/June. Sections 4.7.1 and 4.7.2.1 are for both tiers; everything from 4.7.2.2 onwards is Higher tier only.

Test yourself with the practice questions. The course hub is AQA GCSE Physics, the printable checklist lists every statement, and the free diagnostics show where to focus.

Key definitions (4.7.1)

Term	Definition
Pole	Place where the magnetic forces are strongest
Permanent magnet	Produces its own magnetic field
Induced magnet	Becomes a magnet when placed in a magnetic field; loses most/all magnetism quickly when removed
Magnetic field	Region around a magnet where a force acts on another magnet or a magnetic material
Magnetic materials	Iron, steel, cobalt, nickel
Field direction	Direction of the force on a north pole placed at that point: N to S
Solenoid	Coil of wire; strong, uniform field inside
Electromagnet	Solenoid with an iron core

Rules to recall:

• Like poles repel; unlike poles attract. Both are non-contact forces.
• Magnet and magnetic material: always attraction. Induced magnetism always causes attraction.
• Field strongest at the poles; weaker further away; closer lines = stronger field.
• A compass points along the Earth’s magnetic field, which is evidence that the Earth’s core is magnetic.

Method in steps: plotting a field with a compass

1. Draw round the magnet on paper.
2. Put a plotting compass by the N pole; mark where the needle points.
3. Move the compass so its tail is on the mark; mark again.
4. Repeat until you reach the S pole; join the marks.
5. Add arrows N → S. Repeat from new starting points.

Current and magnetic fields (4.7.2.1)

• Straight wire: concentric circles round the wire; stronger with larger current; weaker with distance.
• Right-hand grip rule: thumb along current, fingers show field direction.
• Solenoid: field adds from each turn; strong and uniform inside; bar-magnet shape outside.
• Adding an iron core makes the field stronger.
• Demonstrate: compass beside a wire deflects when the current flows, and the other way when it is reversed.
• Devices (relay, bell): current on → core magnetised → iron armature attracted → contacts move. Current off → core loses magnetism → spring returns armature.

Equations – Higher tier only (all on the equation sheet)

Equation	Symbols and units
F = B I l	F force (N), B magnetic flux density (T), I current (A), l length (m)
Vp / Vs = np / ns	V in volts; n = number of turns
Vs × Is = Vp × Ip	power out = power in (100% efficient transformer), W

Also useful from Electricity: P = V I and P = I² R.

Motor effect – Higher tier only (4.7.2.2–4.7.2.4)

• Motor effect: a current-carrying conductor in a magnetic field and the magnet exert a force on each other.
• Fleming’s left-hand rule: First finger = Field; seCond finger = Current; thuMb = Motion (force).
• Force is zero if the wire is parallel to the field.
• Size of force depends on: flux density B, current I, length of wire in the field l.
• Motor: current flows in opposite directions in the two sides of the coil → forces in opposite directions → coil rotates. Split-ring commutator keeps it turning one way.
• Loudspeaker/headphones: ac in coil in a permanent magnet’s field → force reverses as current reverses → cone vibrates → pressure variations = sound.

Worked reminder: 25 cm of wire, 1.2 A, 0.50 T, at right angles → F = 0.50 × 1.2 × 0.25 = 0.15 N.

Generator effect – Higher tier only (4.7.3)

• A conductor moving relative to a field, or a changing field around a conductor, induces a p.d.; in a complete circuit this drives an induced current.
• The induced current’s field opposes the change that caused it.

Increase size of induced p.d.	Reverse direction
Move faster	Reverse the movement
Stronger magnet	Reverse the poles
More turns on the coil

• Alternator: slip rings → ac; graph crosses the axis.
• Dynamo: split-ring commutator → dc; graph stays one side of the axis.
• Faster rotation → higher peaks and more cycles per second.
• Microphone: sound moves diaphragm and coil in a magnet’s field → p.d. induced → current varies like the sound.

Transformers and the National Grid – Higher tier only (4.7.3.4)

• Primary coil and secondary coil on an iron core (easily magnetised).
• ac in primary → changing field in core → changing field through secondary → alternating p.d. induced in secondary.
• Step-up: Vs > Vp, ns > np. Step-down: Vs < Vp, ns < np.
• High p.d. transmission: same power at higher V means lower I, so less I²R heating in cables and less energy wasted.

Method in steps: transformer problems

1. Write Vp/Vs = np/ns; substitute the three known values.
2. Rearrange for the unknown.
3. For currents: Vs × Is = Vp × Ip (assume 100% efficient).
4. Check: step-up raises V and lowers I; step-down does the opposite.

Worked reminder: np = 400, ns = 8000, Vp = 11 kV → Vs = 11 000 × 8000 / 400 = 220 kV (step-up). Sending 2.0 MW at 220 kV needs I = 2.0 × 10⁶ / 220 000 = 9.1 A, far less than at 11 kV (182 A), so the cables waste much less energy as heat.

Must-know distinctions

• Permanent vs induced magnet: own field always vs only in another field; repel or attract vs always attract.
• Motor effect vs generator effect: current + field → force vs movement + field → induced p.d.
• Loudspeaker vs microphone: motor effect (current → sound) vs generator effect (sound → current).
• Alternator vs dynamo: slip rings, ac vs split-ring commutator, dc.
• Step-up vs step-down: more turns on secondary vs fewer turns on secondary.
• Fleming’s left-hand rule vs right-hand grip rule: force on a wire vs direction of the field round a wire.

Quick self-test

1. What is the force between a magnet and an unmagnetised iron nail?
2. In which direction do magnetic field lines point?
3. What is an electromagnet?
4. (Higher tier only) A 0.30 m wire carries 2.5 A at right angles to a 0.40 T field. Calculate the force.
5. (Higher tier only) A force of 0.12 N acts on a wire carrying 2.0 A at right angles to a 0.15 T field. Calculate the length of wire in the field.
6. (Higher tier only) A transformer has 200 primary turns and 50 secondary turns. Vp = 240 V. Calculate Vs and state the type of transformer.
7. (Higher tier only) A 100% efficient transformer gives 6.0 V and 3.0 A from a 230 V supply. Calculate the primary current.
8. (Higher tier only) What type of current does an alternator produce?
9. (Higher tier only) Which effect does a loudspeaker use, and which does a microphone use?
10. (Higher tier only) Why is the core of a transformer made of iron?
11. (Higher tier only) State two ways to increase the p.d. induced by moving a magnet in a coil.
12. Explain how a compass gives evidence that the Earth’s core is magnetic.

Answers

1. Attraction (always).
2. From the north pole to the south pole.
3. A solenoid with an iron core.
4. F = 0.40 × 2.5 × 0.30 = 0.30 N.
5. l = F / (B I) = 0.12 / (0.15 × 2.0) = 0.40 m.
6. Vs = 240 × 50 / 200 = 60 V; step-down.
7. Ip = (6.0 × 3.0) / 230 = 18 / 230 = 0.078 A (2 s.f.).
8. Alternating current (ac).
9. Loudspeaker: motor effect. Microphone: generator effect.
10. Iron is easily magnetised.
11. Any two: move the magnet faster; use a stronger magnet; use more turns on the coil.
12. A compass needle is a small magnet that lines up with the Earth’s magnetic field wherever it is, so the Earth must have a magnetic field produced by a magnetic core.

Where marks are usually lost

• Field lines drawn S to N, crossing, or without arrows.
• Saying magnetic materials can be repelled by a magnet.
• Forgetting that induced magnets lose their magnetism quickly when removed.
• Using the right hand, or mixing up which finger is current and which is field, in Fleming’s left-hand rule.
• Leaving length in cm or current in mA in F = BIl.
• Motor explanations that say “the coil spins” without forces in opposite directions on the two sides.
• Saying a stationary magnet inside a coil induces a current.
• Transformer answers that omit “alternating” or “changing” magnetic field.
• Saying high p.d. transmission “reduces resistance” instead of “reduces current, so less heating”.
• Dynamo graphs drawn crossing the time axis.

Official syllabus

AQA GCSE Physics (8463) specification, for first teaching 2016, exams from June 2018, version 1.1 (30 September 2019), published by AQA – section 4.7 Magnetism and electromagnetism.