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

These revision notes condense OxfordAQA International GCSE Physics (9203) specification sections 3.6.1 (Generating electricity) and 3.6.4 (The motor effect), version 5.2, for exams May/June 2018 onwards. The qualification is untiered. For full explanations and longer worked examples, read the study guide, then test yourself with the practice questions. The course hub is at OxfordAQA International GCSE Physics, the checklist tracks your progress, and the free diagnostics show which topics need work first.

The big idea

Generators and motors are the same physics run in opposite directions.

	Generator effect (3.6.1)	Motor effect (3.6.4)
What you supply	Movement of a conductor or magnet	A current in a conductor
What you need in place	A magnetic field	A magnetic field
What you get	An induced pd, and a current in a complete circuit	A force on the conductor
Used in	Alternators, dynamos, power station generators	Electric motors

Generator effect: key facts

• A pd is induced across a conductor when it moves relative to a magnetic field, or sits in a changing magnetic field.
• The conductor must cut across field lines. Moving along the lines induces nothing.
• No relative movement means no pd, however strong the magnet.
• A complete circuit lets the pd drive an induced current. An open circuit has the pd only.
• Moving the magnet into a coil, out of a coil, or moving the coil relative to the magnet all induce a pd.

The opposing field

The induced current has its own magnetic field, and this field opposes the field of the permanent magnet. In practice this means:

Movement	Pole formed at the near end of the coil	Effect
North pole approaches	North	Repulsion, resists the approach
North pole leaves	South	Attraction, resists the departure
South pole approaches	South	Repulsion, resists the approach
South pole leaves	North	Attraction, resists the departure

The work done against this opposition is the energy that ends up in the circuit.

Reversal

Turn the movement round, or turn the magnet end for end, and the induced pd and any current reverse. Reverse both and the original direction returns.

Factors that increase the induced pd

1. Faster movement.
2. Stronger magnetic field.
3. More turns on the coil.
4. Larger area of the coil.

Alternator and dynamo

Feature	Alternator	Dynamo
Output	ac, the direction reverses	dc, the direction stays the same
pd-time graph	Smooth wave above and below the axis	Series of humps, all on one side of the axis
Why	Each side sweeps up, then down, through the field	The connection to the circuit swaps every half turn
Zero pd	Plane of coil at right angles to the field lines	Plane of coil at right angles to the field lines
Maximum pd	Plane of coil parallel to the field lines	Plane of coil parallel to the field lines

You do not need detailed knowledge of slip rings or split rings.

Method: reading frequency from a graph

1. Find the time for one complete cycle (peak to the next peak, or zero to the next zero going the same way). This is the period.
2. Frequency = 1 ÷ period.
3. For a simple coil turning once per cycle, rotations per second equal the frequency. Multiply by 60 for rotations per minute.
4. If the coil turns faster, the period shortens, the frequency rises and the peak pd rises.

Worked reminder: a trace has a period of 0.10 s. Frequency = 1 ÷ 0.10 = 10 Hz, which is 600 rotations per minute.

Power station generators

• A fuel, or moving water or wind, supplies energy to a turbine.
• In a fuel-burning station, the fuel heats water to make steam, and the steam turns the turbine.
• The turbine turns wire coils between magnets, so the coil cuts field lines and a pd is induced.
• Energy chain: stored or moving energy, then kinetic energy of the turbine, then electrical energy.

Motor effect: key facts

• A current-carrying conductor has its own magnetic field.
• Placed in an external magnetic field so that it cuts the field lines, the conductor and the magnet push on each other.
• If the conductor is parallel to the field, there is no force.
• The force is largest when the wire is across the field, at 90 degrees.

Increasing the force

1. Stronger magnetic field.
2. Larger current.
3. Longer length of conductor in the field.

Reversing the force

The direction of the force reverses if either the current or the field is reversed. Reverse both and the force direction is unchanged.

Fleming’s left-hand rule

Left hand, thumb and first two fingers at right angles.

Digit	Quantity	Memory
First finger	Magnetic field, north to south	Field
Second finger	Current, positive to negative	Current
Thumb	Force or motion	Motion

Method in steps:

1. Draw the field direction and the current direction on your diagram.
2. Point the first finger along the field.
3. Rotate your hand until the second finger points along the current.
4. Read the thumb. That is the force direction.

Worked reminder: field from right to left, current up the page. First finger points left, second finger up the page, and the thumb points out of the page towards you.

The electric motor

• A coil with a current in it, placed in a magnetic field, tends to rotate.
• The two long sides carry currents in opposite directions, so the forces on them are opposite and give a turning effect.
• The turning effect is zero once the coil lies square-on to the field, because the two forces then lie along the coil’s own plane. A practical motor swaps the current connections at this point (a split ring does this) to keep the turn going the same way.
• A bigger current, a stronger magnet or more turns increase the turning effect.

Must-know distinctions

• Induced pd versus current: a pd is induced whenever the conductor cuts the field. A current flows only if the circuit is complete.
• Generator versus motor effect: in a generator, movement produces a pd. In a motor, a current in a field produces movement.
• Alternator versus dynamo: the coil and field are the same. What differs is the wiring between coil and circuit, which decides whether the output reverses.
• Field versus force direction: in Fleming’s rule these are at right angles, so the force is never along the field.

Quick self-test

1. State two ways of inducing a pd across the ends of a straight wire.
2. A magnet sits motionless in the middle of a coil. What is the induced pd?
3. Name the four factors that increase the induced pd.
4. A trace from a simple alternator has a period of 0.020 s. Calculate the frequency.
5. A coil in a simple generator turns at 900 rotations per minute. Calculate the frequency of the output, and its period to 2 significant figures.
6. Describe how the pd-time graph of a dynamo differs from that of an alternator.
7. A wire carries current up the page in a field that points from right to left. Give the direction of the force.
8. A wire carries current down the page in a field that points into the page. Give the direction of the force.
9. State three ways of increasing the force on a current-carrying wire in a magnetic field.
10. A wire carries a current parallel to a magnetic field. What is the force on it?
11. The current and the magnetic field are both reversed. What happens to the force?
12. Why does a coil carrying a current in a magnetic field turn?

Answers

1. Move the wire so it cuts across a magnetic field, or place it in a changing magnetic field (for example by moving a magnet nearby).
2. Zero, because nothing is changing.
3. Faster movement, stronger field, more turns, larger coil area.
4. 1 ÷ 0.020 = 50 Hz.
5. 900 ÷ 60 = 15 Hz. Period = 1 ÷ 15 = 0.067 s.
6. The alternator graph goes above and below the axis. The dynamo graph stays on one side, in a series of humps.
7. Out of the page, towards you.
8. To the right.
9. Stronger field, larger current, longer length of conductor in the field.
10. Zero.
11. It stays in the same direction.
12. The forces on the two sides of the coil act in opposite directions, which gives a turning effect.

Where marks are usually lost

• Writing that a magnet inside a coil “induces a current” without saying it must be moving.
• Saying the induced current “stops” the magnet. It opposes the motion, so you must do work.
• Giving “a bigger magnet” instead of “a stronger magnetic field”.
• Giving “a longer wire” without saying that this means more turns on a coil.
• Drawing a dynamo graph that goes negative.
• Reading the period wrongly from a graph by taking half a cycle.
• Using the right hand for Fleming’s rule, or swapping the first and second fingers.
• Forgetting that the force reverses when only one of current and field is reversed.

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.