OxfordAQA International GCSE Physics: Electromagnetic induction, generators and the motor effect (9203)

This guide is aligned to the OxfordAQA International GCSE Physics (9203) specification, version 5.2, for exams May/June 2018 onwards. It covers specification sections 3.6.1 (Generating electricity) and 3.6.4 (The motor effect). The qualification is untiered, so the same content applies to every candidate. Use it with the revision notes and the practice questions. The course hub is at OxfordAQA International GCSE Physics, the printable checklist lists every statement, and the free diagnostics show you where to start. For general exam technique, see the exam preparation guide.

What this unit covers

Section	What you must be able to do
3.6.1 a	Describe the generator effect, a potential difference induced when a conductor moves relative to a magnetic field or sits in a changing field. Suggest what affects its size
3.6.1 b	Describe the pd induced in a coil when a magnet is pushed in or pulled out, or when the coil is moved past a magnet
3.6.1 c	Explain that an induced current has a magnetic field that opposes the field of the permanent magnet
3.6.1 d	Say what happens to the induced pd and current when the motion or the magnet polarity is reversed
3.6.1 e	Name the four factors that increase the induced pd. Explain how an alternator gives ac and a dynamo gives dc, including pd-time graphs
3.6.1 f	Describe how power stations use turbines to turn coils between magnets
3.6.4 a	Describe the motor effect, including the case where there is no force
3.6.4 b	State three ways to increase the force
3.6.4 c	Reverse the force direction, and find it with Fleming’s left-hand rule
3.6.4 d	Explain why a coil carrying a current in a field tends to rotate, which is the basis of an electric motor

The specification says detailed knowledge of slip rings and split rings is not required, so this guide mentions them only briefly.

The generator effect (3.6.1 a)

A wire sitting still in a steady magnetic field has no pd across its ends. A pd appears only when the wire and the field are changing relative to each other. There are two ways this can happen.

• The conductor moves relative to the field, so it cuts through magnetic field lines.
• The conductor sits in a field that is itself changing, for example the field of a magnet that is being moved nearby.

This is the generator effect, and the pd is called an induced pd. When the conductor forms part of a complete circuit, the pd drives an induced current. An open circuit has the pd but no current.

Moving the wire along the field lines does not cut them, so no pd is induced. Only movement that cuts across the lines counts.

A magnet and a coil (3.6.1 b)

A coil gives a bigger effect than a single wire because every turn adds to the pd. The set-up is a coil joined to a sensitive centre-zero meter.

Action	Meter reading
Magnet pushed into the coil	Deflects one way
Magnet held still, inside or outside	Zero
Magnet pulled out	Deflects the other way
Coil moved towards a fixed magnet	Deflects, as if the magnet had moved towards the coil

Only relative movement matters, so it makes no difference whether you move the magnet or the coil.

The opposing field (3.6.1 c)

When the induced current flows, it produces a magnetic field of its own. This field opposes the field of the permanent magnet, so the coil resists the movement that created the current.

Push the north pole of a magnet towards a closed coil. The end of the coil facing the magnet becomes a north pole, and the two like poles repel. You feel a push back. Pull the magnet away and the induced current reverses. The nearer end of the coil becomes a south pole, which pulls on the departing north pole and resists you again.

This is also an energy argument. The work you do against the repulsion is the energy that appears in the circuit. If the induced field helped the movement, energy would be created from nothing.

Worked example 1: induced polarity

The north pole of a bar magnet is lowered towards the top of a vertical closed coil, then lifted away again. State the pole formed at the top of the coil in each movement.

1. Lowering: the induced field opposes the magnet, so the top becomes a north pole and pushes back on the incoming north pole.
2. Lifting: the current reverses, the top becomes a south pole, and it pulls on the north pole as it leaves.

North while lowering, south while lifting.

Reversing the pd and current (3.6.1 d)

Reverse the direction of motion and the polarity of the induced pd reverses, so any current flows the opposite way. Reverse the magnet instead, so the other pole leads, and the same thing happens. If you reverse both, the pd goes back to its original direction.

Factors that increase the induced pd (3.6.1 e)

Factor	Why it increases the pd
Faster movement	The wire cuts field lines at a greater rate
Stronger magnetic field	There are more field lines to cut
More turns on the coil	Each turn contributes, so the contributions add
Larger area of coil	The coil cuts through more of the field

Worked example 2: reading experimental data

A magnet is dropped through coils of different lengths of the same wire, all wound with the same area. A data logger records the peak pd each time.

Number of turns	Peak pd (mV)
100	0.9
200	1.8
300	2.7
400	3.6

1. Peak pd rises as the number of turns rises.
2. Each 100 turns adds 0.9 mV, so doubling the turns doubles the pd. The two quantities are in direct proportion.
3. Prediction for 500 turns: 5 × 0.9 = 4.5 mV.
4. Variables to control: the same magnet, the same release height (so the same speed through the coil), and the same coil area.

Alternators and dynamos (3.6.1 e)

Both machines spin a coil between the poles of a magnet. As the coil turns, its sides cut field lines and a pd is induced.

Alternator (produces ac)

The coil’s connection to the outside circuit does not swap over. In one half of each turn a given side of the coil travels upwards and in the other half it travels downwards. The induced pd therefore changes polarity every half turn. That is alternating current.

The pd also changes in size.

Coil position	Sides of the coil	Induced pd
Plane of coil parallel to the field lines	Cutting across the lines at the fastest rate	Maximum
Plane of coil at right angles to the field lines	Moving along the lines, not cutting them	Zero
Past the zero position	Cutting the lines the other way	Maximum, reversed

The graph of pd against time is a smooth wave. It rises from zero to a positive peak, falls through zero to a negative peak of the same size, and returns to zero. One full wave is one rotation.

Dynamo (produces dc)

A dynamo has the same coil and field, but the connections to the outside circuit swap every half turn (the job of a split ring, which you do not need to describe in detail). Each time the induced pd would reverse, the connections reverse too, so the output keeps the same direction. The pd-time graph is a row of humps that never cross the axis, dropping to zero between them. Its size still varies, but the direction does not change.

Worked example 3: reading an alternator graph

The trace from an alternator shows a peak pd of 8.0 V. One complete cycle takes 0.050 s. The coil makes one rotation per cycle.

1. Frequency = 1 ÷ period = 1 ÷ 0.050 = 20 Hz.
2. Rotations per second = 20, so per minute: 20 × 60 = 1200 rotations per minute.
3. The coil is now turned twice as fast. The period halves to 0.025 s and the frequency doubles to 40 Hz. The sides cut the lines faster, so the peak pd increases above 8.0 V.

Power station generators (3.6.1 f)

A power station turns a wire coil between magnets using a turbine. A fuel, or a flow of water or wind, supplies the energy that spins the turbine. In a fuel-burning station, the fuel heats water to make steam, and the steam pushes the turbine blades. The turbine is on the same shaft as the generator, so the coil turns, its sides cut the magnetic field lines, and a pd is induced.

The motor effect (3.6.4 a)

A wire carrying a current has a magnetic field around it. Put it in the field of a magnet so that it cuts across the field lines, and the two fields interact. Each of the magnet and the wire pushes on the other with a force. This is the motor effect.

If the wire is parallel to the magnetic field, it does not cut the lines and there is no force. The force is largest when the wire lies across the field at 90 degrees.

Increasing the force (3.6.4 b)

• Increase the strength of the magnetic field.
• Increase the size of the current.
• Increase the length of the conductor that lies in the field.

Direction and Fleming’s left-hand rule (3.6.4 c)

The force reverses if the current reverses, and it reverses if the magnetic field reverses. Reverse both and the force is back where it started.

Fleming’s left-hand rule: with your left hand, spread the thumb, first finger and second finger so that each is at 90 degrees to the other two.

Digit	Represents
First finger	Direction of the magnetic Field, north to south
Second finger	Direction of the Current, positive to negative
Thumb	Direction of the force, or Motion

Worked example 4: direction of the force

A straight wire carries current into the page. It lies in a magnetic field directed towards the right-hand edge of the page. Find the direction of the force.

1. First finger points right (field).
2. Second finger points into the page (current).
3. The thumb points down the page, which is the direction of the force.

Reverse the current, or the field, and the force points up the page.

The electric motor (3.6.4 d)

Take a rectangular coil in a field pointing from left to right, seen end-on. The left side carries current into the page and the right side carries it out of the page, because the current goes round the loop.

• The left side has current into the page, so the force is down (worked example 4).
• The right side has current out of the page, so the force is up.

The two forces are opposite, so they turn the coil. When the coil lies square-on to the field, the two forces act along the coil’s own plane and give no turning effect. In a practical motor, the connections to the coil swap at this point (a split ring does this), so the forces keep turning it the same way. As with the generator, more current, a stronger field or more turns increase the turning force.

Common errors

• Writing that a stationary magnet inside a coil induces a pd. It does not, because nothing is changing.
• Saying the induced field “stops” the motion. It opposes it, so the movement continues but is harder.
• Mixing up the two fingers in Fleming’s rule. Second finger is current, first finger is field.
• Treating a dynamo as a generator that produces a steady pd. The output is dc but still varies in size.

Official syllabus

OxfordAQA International GCSE Physics (9203), version 5.2, for exams May/June 2018 onwards, published by OxfordAQA, sections 3.6.1 and 3.6.4.