AQA GCSE Physics 8463: Magnetism and electromagnetism – Study Guide

This study guide teaches 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 (1 hour 45 minutes, 100 marks, 50% of the GCSE) with Forces, Waves and Space physics. Both tiers sit it, but much of it is marked “(HT only)” in the specification: from Fleming’s left-hand rule onwards, and the whole of section 4.7.3, is Higher tier only. Exams run every May/June.

Use it with the revision notes and the practice questions for this topic. The course hub is AQA GCSE Physics and the printable checklist lists every statement. Find your gaps with a free diagnostic.

What this topic covers

Spec	What you must be able to do	Tier
4.7.1.1	Describe attraction and repulsion between poles; permanent vs induced magnets	Both
4.7.1.2	Magnetic fields and field lines; plot a field with a compass; evidence that the Earth’s core is magnetic	Both
4.7.2.1	Magnetic field of a current-carrying wire and a solenoid; electromagnets; interpret diagrams of electromagnetic devices	Both
4.7.2.2–4.7.2.4	Motor effect, Fleming’s left-hand rule, F = BIl; motors; loudspeakers	Higher tier only
4.7.3.1–4.7.3.4	Generator effect; alternators and dynamos; microphones; transformers and transmission at high p.d.	Higher tier only

This topic has no required practical. The equations F = BIl, Vp/Vs = np/ns and VsIs = VpIp are all on the Physics equation sheet (Higher tier).

Poles and magnets (4.7.1.1)

The poles of a magnet are where the magnetic forces are strongest. When two magnets are brought close together they exert a force on each other:

• like poles repel (N–N or S–S)
• unlike poles attract (N–S).

This is a non-contact force.

A permanent magnet produces its own magnetic field. An induced magnet is a material that becomes a magnet when placed in a magnetic field.

Permanent magnet	Induced magnet
Has its own field all the time	Magnetic only while in another field
Can attract or repel another magnet	Always attracted
Keeps its magnetism	Loses most or all of it quickly when removed

Magnetic fields (4.7.1.2)

The magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel). The force between a magnet and a magnetic material is always attraction.

• The field is strongest at the poles and weakens with distance.
• The direction of the field at a point is the direction of the force on a north pole placed there. Field lines run from the north (seeking) pole to the south (seeking) pole.
• Closer field lines mean a stronger field.

Plotting a field with a compass. Draw round the magnet on paper. Put a plotting compass near the N pole and mark where the needle points. Move the compass so its tail is on that mark, and mark again. Repeat until you reach the S pole, then join the dots and add arrows (N to S). Repeat from other starting points.

The Earth’s field. A compass contains a small bar magnet, and it points in the direction of the Earth’s magnetic field wherever it is used. This shows the Earth has a magnetic field, which is evidence that the Earth’s core must be magnetic.

Electromagnetism (4.7.2.1)

A current in a wire produces a magnetic field around the wire. The field is a set of concentric circles centred on the wire. It is stronger with a larger current and weaker further from the wire (the circles are drawn further apart).

To find the direction, use the right-hand grip rule: grip the wire with your right thumb pointing along the conventional current; your fingers curl in the direction of the field. Reversing the current reverses the field.

Demonstrating the effect. Put a compass next to a wire and switch on: the needle deflects. Reverse the current: it deflects the other way.

Solenoids. Shaping the wire into a coil (a solenoid) increases the field strength, because the fields from each turn add together. Inside the solenoid the field is strong and uniform (parallel, equally spaced lines). Outside, the field has the same shape as a bar magnet’s.

An electromagnet is a solenoid with an iron core. The iron core increases the field strength, and the field can be switched off by switching off the current.

Interpreting devices. For an unfamiliar device, trace the sequence: current in coil → iron core becomes a magnet → attracts an iron part → part moves (closes contacts, strikes a bell) → current off → core loses magnetism → spring pulls the part back. In a relay, a small current in one circuit switches on a large current in another.

The motor effect and F = BIl (4.7.2.2) – Higher tier only

When a conductor carrying a current is placed in a magnetic field, the magnet and the conductor exert a force on each other. This is the motor effect.

Fleming’s left-hand rule. Hold the thumb, first finger and second finger of your left hand at right angles:

• First finger: Field (N to S)
• seCond finger: Current (+ to −)
• thuMb: Motion (force).

Reversing either the current or the field reverses the force. Reversing both leaves it unchanged. If the wire is parallel to the field, there is no force.

For a conductor at right angles to the field:

F = B I l – force (N) = magnetic flux density (T) × current (A) × length (m)

The force is larger with a stronger field, a larger current or a longer length of wire in the field.

Worked example 1. A wire carries 3.0 A at right angles to a field of 0.20 T. The length of wire in the field is 15 cm.

l = 15 cm = 0.15 m
F = B I l = 0.20 × 3.0 × 0.15 = 0.090 N

Worked example 2. A force of 0.036 N acts on 6.0 cm of wire in a 0.12 T field. Find the current.

I = F / (B l) = 0.036 / (0.12 × 0.060) = 5.0 A

Electric motors (4.7.2.3) – Higher tier only

A coil carrying a current in a magnetic field tends to rotate. In the two sides of the coil that are at right angles to the field, the current flows in opposite directions. By Fleming’s left-hand rule, one side is pushed up and the other down. These forces make the coil turn. A split-ring commutator reverses the current in the coil every half turn so the coil keeps rotating in the same direction.

A larger current or stronger field makes it turn faster; reversing the current or field reverses the rotation.

Loudspeakers and headphones (4.7.2.4) – Higher tier only

A moving-coil loudspeaker, or headphone, uses the motor effect to convert variations in current into pressure variations in sound waves.

1. An alternating current flows in a coil attached to a paper cone.
2. The coil sits in the field of a permanent magnet, so a force acts on it.
3. As the current changes direction, the force changes direction, so the coil and cone move in and out.
4. The vibrating cone produces pressure variations in the air: a sound wave with the same frequency as the current.

The generator effect (4.7.3.1) – Higher tier only

If a conductor moves relative to a magnetic field, or the magnetic field around a conductor changes, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced. This is the generator effect.

The induced current generates its own magnetic field that opposes the original change. Push a magnet’s N pole into a coil in a complete circuit and the near end of the coil becomes an N pole, repelling the magnet. Pull it out and that end becomes an S pole, attracting it.

Size of induced p.d. increases with	Direction reverses if
faster movement (faster rate of change)	the direction of movement reverses
stronger magnetic field	the magnetic field (poles) is reversed
more turns on the coil

No movement means no induced p.d., even with a strong magnet inside the coil.

Alternators and dynamos (4.7.3.2) – Higher tier only

Both rotate a coil in a magnetic field (or a magnet near a coil).

• An alternator generates ac. The coil connects to the circuit through slip rings, so the output p.d. reverses every half turn. Its p.d.–time graph is a wave that goes positive and negative.
• A dynamo generates dc. A split-ring commutator swaps the connections every half turn, so the output never reverses. Its graph is a series of humps, all on the same side of the axis.

Spinning either one faster gives a larger peak p.d. and more cycles per second (shorter time for each cycle on the graph).

Microphones (4.7.3.3) – Higher tier only

A moving-coil microphone uses the generator effect, the reverse of a loudspeaker. Sound waves make a diaphragm vibrate. The diaphragm moves a coil within the field of a permanent magnet. The moving coil has a p.d. induced across it, so the current in the circuit varies in the same pattern as the sound’s pressure variations.

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

A basic transformer has a primary coil and a secondary coil wound on an iron core. Iron is used because it is easily magnetised.

How it works. An alternating current in the primary coil produces a changing magnetic field in the iron core. This changing field passes through the secondary coil and induces an alternating p.d. across it. A transformer needs ac: a steady dc gives no change in field, so nothing is induced.

Vp / Vs = np / ns – step-up: Vs > Vp (more turns on the secondary); step-down: Vs < Vp.

If a transformer is 100% efficient, power in = power out:

Vs × Is = Vp × Ip

Worked example 3. A phone charger steps 230 V down to 12 V. The primary has 1150 turns. Find the secondary turns.

ns = np × Vs / Vp = 1150 × 12 / 230 = 60 turns

Worked example 4. The charger supplies 2.3 A at 12 V. Find the current drawn from the mains, assuming 100% efficiency.

power out = Vs × Is = 12 × 2.3 = 27.6 W
Ip = 27.6 / 230 = 0.12 A

Why transmit at high p.d.? For a given power, P = V × I, so raising the p.d. lowers the current. The heating in the cables depends on I²R, so a lower current wastes much less energy.

Worked example 5. 1.0 MW is sent through cables of total resistance 5.0 Ω.

at 25 kV:  I = 1.0 × 10⁶ / 25 000 = 40 A;   loss = 40² × 5.0 = 8000 W
at 400 kV: I = 1.0 × 10⁶ / 400 000 = 2.5 A; loss = 2.5² × 5.0 = 31.25 W

Step-up transformers raise the p.d. for transmission; step-down transformers lower it to a safer value for homes.

Common errors

• Drawing field lines from S to N, or letting them cross.
• Saying a magnet “repels” iron. Magnetic materials are always attracted.
• Using the right hand for the motor effect.
• Leaving length in cm in F = BIl.
• Saying a stationary magnet inside a coil induces a p.d. It must move or the field must change.
• Saying transformers work on dc.
• Mixing up which coil is primary in Vp/Vs = np/ns: keep p with p and s with s.
• Stating that high p.d. “reduces resistance”. It reduces current, and so the I²R loss.

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.