Course: 🔷 Separate Physics only | Tier: Higher | Linked: Motors and the generator effect notes
✏️ You need: your exercise book and a calculator. Write your answers before you open them.
📌 KEY DEFINITIONS – learn them word for word
The generator effect: when a wire (or coil) moves relative to a magnetic field, or the magnetic field around it changes, a potential difference is induced across the ends of the wire.
If the wire is part of a complete circuit, a current is induced.
The induced current makes its own magnetic field, which opposes the change that made it.
🎬 Watch first: Free Science Lessons video
Watch this video, then read the notes below. Pause it and draw the magnet-and-coil experiment in your book.
📖 Notes
Making electricity with a magnet 🧲⚡
Connect a coil of wire to a sensitive ammeter (or voltmeter). Now move a bar magnet in and out of the coil. The needle moves – you have made electricity!
| What you do | What the meter shows |
|---|---|
| ➡️ Push the magnet into the coil | Needle moves one way (e.g. right) |
| ✋ Hold the magnet still inside the coil | Needle goes back to 0 – nothing is moving! |
| ⬅️ Pull the magnet out of the coil | Needle moves the other way (e.g. left) |
| 🔄 Push the magnet in faster | Needle moves further – a bigger reading |
🔑 The golden rule: something must be moving (or changing). No movement = no electricity. It works either way round – you can move the magnet or move the coil.
How to get a bigger potential difference 📈
- 🏃 Move the magnet (or coil) faster
- 🧲 Use a stronger magnet
- 🌀 Use more turns of wire on the coil
To reverse the direction: move the magnet the other way, or turn the magnet round (swap north and south).
Motor effect vs generator effect 🔁
They are opposites – don’t mix them up!
| You put in… | You get out… | Example | |
|---|---|---|---|
| Motor effect | ⚡ current | 🔄 movement | electric fan |
| Generator effect | 🔄 movement | ⚡ current | wind turbine |
Generators: the alternator and the dynamo ⚙️
A generator spins a coil inside a magnetic field. The coil keeps cutting through the field, so a potential difference is induced all the time.
| Alternator | Dynamo | |
|---|---|---|
| Connects to the circuit using | slip rings and brushes | a split-ring commutator and brushes |
| Makes | a.c. (alternating current) | d.c. (direct current) |
| Graph of p.d. against time | 〰️ a wave that goes above and below zero | ⌒⌒ bumps that stay above zero |
| Used in | power stations, wind turbines, car engines | old bike lights |
⏱️ One full turn of the coil = one full wave. So if the coil turns 50 times every second, the frequency is 50 Hz – just like UK mains electricity. Spinning the coil faster gives a higher frequency and a bigger peak p.d.
🧮 Explain what the meter shows – 3 questions to ask
1. Is anything moving or changing? No → reading is 0. Yes → a p.d. is induced.
2. Which direction? Opposite movement → opposite reading.
3. How fast / how strong / how many turns? More → bigger reading.
Frequency calculations: frequency (Hz) = number of turns each second · period (s) = 1 ÷ frequency
✅ Worked example
A bar magnet is pushed into a coil and the ammeter needle moves right. What happens when the magnet is pulled out quickly?
1. Moving? Yes → a current is induced.
2. Direction? Opposite movement → the needle moves left.
3. How fast? Quickly → a bigger reading.
Answer: the needle moves further to the left.
🔍 See the magnet-and-coil diagram · See alternator and dynamo graphs
✏️ Questions
Level 1: What is it?
Q1 What is the name of the effect when moving a magnet in a coil makes a potential difference?
Show answer
The generator effect.
Q2 ✋ A magnet is held still inside a coil. What does the ammeter read? Why?
Show answer
0. Nothing is moving, so no potential difference is induced.
Q3 💨 A wind turbine uses the generator effect. Which energy store is transferred into electrical energy?
Show answer
The kinetic energy store (of the moving blades and coil) is transferred electrically.
Level 2: What happens if…?
Q4 Pushing a magnet into a coil makes the needle move right. What happens when the magnet is pulled out?
Show answer
The needle moves the other way (left) – the current is reversed.
Q5 Give three ways to make the induced potential difference bigger.
Show answer
1. Move the magnet faster
2. Use a stronger magnet
3. Use more turns of wire on the coil
Q6 🔄 The magnet is turned round so its south pole goes into the coil first. What happens to the reading?
Show answer
The needle moves the opposite way – reversing the magnet reverses the induced current.
Level 3: Generators
Q7 ⚙️ An alternator uses slip rings. What type of current does it make?
Show answer
Alternating current (a.c.).
Q8 🚲 A dynamo uses a split-ring commutator. What type of current does it make? Describe the shape of its graph.
Show answer
Direct current (d.c.). The graph is a series of bumps that all stay above zero – the current never changes direction.
Q9 🧲 Why is it harder to push a magnet into a coil when the coil is connected in a complete circuit?
Show answer
The induced current makes its own magnetic field, which opposes the change – it pushes back against the magnet.
Level 4: Calculations – frequency
Remember: one full turn = one full wave. Frequency = turns each second. Period = 1 ÷ frequency.
Q10 🏭 The coil in a power station alternator turns 50 times every second. What is the frequency of the a.c. it makes?
Show answer
50 Hz – the same as UK mains electricity.
Q11 Calculate the period of a 50 Hz a.c. supply – the time for one full wave.
Show answer
Period = 1 ÷ frequency = 1 ÷ 50 = 0.02 s
Q12 ⏩ A small alternator’s coil turns 10 times each second. It is then spun twice as fast.
(a) What is the new frequency?
(b) What happens to the peak potential difference?
Show answer
(a) 10 × 2 = 20 turns each second, so 20 Hz
(b) The peak p.d. gets bigger, because the coil cuts the magnetic field faster.
⚠️ Where students lose marks
- Mixing up the motor effect (current → movement) and the generator effect (movement → current).
- Saying a magnet held still in a coil makes a current. It doesn’t – something must be moving or changing.
- Mixing up slip rings (alternator, a.c.) and the split-ring commutator (dynamo, d.c.).
- Saying “more wire” or “bigger coil”. Say more turns on the coil.
Next: Motors and the generator effect notes · Transformers and the National Grid · Magnetism topic page
