Electromagnetic Induction

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 6.15 — Induced voltage when a conductor moves through a field or the field through it changes, and the factors affecting its size
• 6.16 — Generating electricity by rotating a magnet in a coil or a coil in a field, and the factors affecting the induced voltage

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Ørsted showed that a current makes a magnetic field. For the next eleven years the obvious question sat unanswered: can a magnetic field make a current? Faraday found the answer in 1831, and it is not quite the answer anyone expected. A magnetic field on its own does nothing whatever. What produces a voltage is a CHANGE.

Take a coil of wire connected to a sensitive voltmeter and push a bar magnet into it. The needle kicks. Hold the magnet still inside the coil — and the needle drops back to zero, even though the magnet is right there and the field through the coil is as strong as it ever was. Pull the magnet out and the needle kicks the OTHER way. This is ELECTROMAGNETIC INDUCTION, and the rule is that a voltage is induced whenever the magnetic field passing through a circuit is CHANGING.

Think of it like a till receipt rather than the money in the drawer

A shop's till records a transaction every time money goes in or comes out. It records nothing at all about how much money is sitting in the drawer — you could have a thousand pounds in there, and if nobody buys anything the till prints nothing. A coil is that till. It does not respond to how much field is passing through it; it responds only to the field CHANGING. Hold a colossally strong magnet motionless inside a coil and you get nothing, exactly as a full drawer prints no receipts. Move a feeble magnet quickly and you get a reading, because something happened.

The three ways to induce a voltage, and what they have in common

1. MOVE A MAGNET into or out of a coil. The field through the coil changes as it moves.
2. MOVE A CONDUCTOR across a magnetic field, cutting through the field lines rather than sliding along them.
3. CHANGE THE CURRENT in a nearby coil, which changes the field it produces, which changes the field through the first coil — with nothing moving at all.
4. The common feature is not motion. It is CHANGE. The third method proves it: a transformer works with both coils bolted down.
5. If nothing changes — a stationary magnet, or a conductor moving ALONG the field lines rather than across them — the induced voltage is zero.

The factors that decide how big the induced voltage is all say the same thing in different words: make the change bigger or faster. A STRONGER magnet gives a bigger voltage. MORE TURNS on the coil gives a bigger voltage, because each turn contributes. MOVING FASTER gives a bigger voltage, because the same change happens in less time. And moving a conductor at right angles to the field, so that it cuts the maximum number of field lines, gives more than moving it at a shallow angle.

The DIRECTION of the induced current follows a rule that sounds perverse and is really conservation of energy in disguise: the induced current always flows in the direction that OPPOSES the change producing it. Push a north pole into a coil and the coil's near end becomes a north pole, pushing back. Pull the magnet out and the near end becomes a south pole, trying to hold it in. This is why a generator is hard to turn when it is supplying a large current — and it must be, because otherwise you would get electrical energy for nothing.

A GENERATOR is this effect arranged so the change never stops. Rotate a coil in a magnetic field — or rotate a magnet inside a coil, which is easier to build for large machines — and the field through the coil changes continuously as it turns. The induced voltage rises to a maximum when the coil is cutting field lines fastest, falls to zero when it is momentarily moving along them, then rises again the opposite way as the coil's sides swap over. The output is therefore an ALTERNATING voltage, and it alternates because rotation is inherently a to-and-fro business as far as the coil is concerned.

Reading a generator output when something is changed

A generator produces a peak voltage of 12 V at 50 Hz. The rotation speed is doubled and nothing else is altered. What is the new peak voltage and the new frequency?

1. Turning faster means the field through the coil changes faster, so the induced voltage is larger.
2. Doubling the rotation rate doubles the rate of change, so the peak voltage doubles: 12 × 2 = 24 V.
3. The coil also completes twice as many rotations each second, so the frequency doubles: 50 × 2 = 100 Hz.
4. Note what does NOT happen: adding turns or a stronger magnet would raise the voltage while leaving the frequency alone. Only the speed changes both.

Answer: 24 V peak, at 100 Hz — rotation speed is the one change that alters both.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Electromagnetic induction

The production of a voltage across a conductor when the magnetic field passing through the circuit changes — either because the conductor moves through the field, or because the field itself changes.

The three ways to induce a voltage

• MOVE A MAGNET into or out of a coil
• MOVE A CONDUCTOR so that it cuts across magnetic field lines
• CHANGE THE CURRENT in a nearby coil, so the field it produces changes — nothing needs to move
• In every case what matters is CHANGE, not the strength of a steady field

What makes the induced voltage larger

• A STRONGER magnetic field
• MORE TURNS on the coil
• MOVING FASTER — or, for a generator, ROTATING FASTER
• Moving at RIGHT ANGLES to the field, so the maximum number of field lines is cut
• For a generator, faster rotation raises the FREQUENCY as well as the voltage; more turns and a stronger magnet raise only the voltage

Showing induction with a coil and a magnet

1. Connect a coil of many turns to a sensitive centre-zero voltmeter.
2. Push a bar magnet into the coil and note the deflection.
3. Hold the magnet still inside the coil: the reading returns to zero.
4. Withdraw the magnet and note that the deflection is in the OPPOSITE direction.
5. Repeat the insertion more quickly, and note the larger deflection for the same magnet.

A steady field against a changing field

A. A STEADY field induces NOTHING, however strong it is. A coil motionless inside a powerful magnet has no voltage across it.

B. A CHANGING field induces a voltage, however weak the field is. The size depends on how fast it changes.

Model answer [5 marks]

A magnet is pushed into a coil, held still inside it, and then pulled out. Describe and explain the readings on a voltmeter connected to the coil. [5]

As the magnet is pushed in, the magnetic field through the coil is changing, so a voltage is induced and the voltmeter deflects. While the magnet is held still inside the coil the field through the coil is no longer changing, so no voltage is induced and the reading falls to zero, even though the magnet is still there. As the magnet is pulled out the field through the coil is changing again, so a voltage is induced once more, but the change is now in the opposite sense and the voltmeter deflects the other way. Moving the magnet more quickly at either stage would give a larger deflection, because the field would be changing more rapidly.

Model answer [4 marks]

State three changes that would increase the voltage produced by a simple generator, and identify which of them also changes the frequency. [4]

Rotating the coil faster, increasing the number of turns on the coil, and using a stronger magnet would each increase the induced voltage. Of these, only rotating the coil faster also changes the frequency, because the frequency of the output is the number of rotations the coil completes each second. Increasing the turns or the field strength raises the peak voltage while leaving the frequency unchanged.

Not this: A strong magnet held inside a coil will produce a voltage, because the field through the coil is large.

This: It produces nothing at all. Induction responds to the RATE OF CHANGE of the field, not to its size. A weak magnet moved quickly beats a strong magnet held still, every time.

Mark-losing trap. No CHANGE means no induced voltage — a stationary magnet does nothing, however strong.

Mark-losing trap. The induced current always OPPOSES the change causing it. That is conservation of energy, not perversity.

Mark-losing trap. A conductor moving ALONG field lines cuts none of them and induces nothing.

Mark-losing trap. Rotating faster raises the frequency AND the voltage. More turns raises only the voltage.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — A bar magnet is held completely still inside a coil connected to a voltmeter. What does the voltmeter read?
2. Grade 7 · State [3 marks] — State three changes that would increase the voltage induced when a magnet is moved into a coil.
3. Grade 8 · Explain [3 marks] — A generator's coil is made to rotate twice as fast. Explain what happens to the peak voltage and to the frequency of the output.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of the readings observed when a magnet is pushed into a coil, held still, and then withdrawn.
5. 9+ · Evaluate [6 marks] — A student proposes a generator that needs no fuel: the induced current in the coil is fed to an electromagnet arranged to help the coil turn, so that the machine drives itself once started. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who spent eleven years failing before the experiment worked, and the one whose accounting explains why a generator resists being turned. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Michael Faraday — the one who spent eleven years failing before the experiment worked

This page is Faraday's 1831 discovery and the story of how he got there is the lesson. He had been trying since 1821 to make a magnetic field produce a current, and he kept failing because he was doing the sensible thing: setting up a strong steady field and looking for a steady current. There is none. The result came only when he noticed a momentary twitch of the needle at the instant he connected and disconnected the battery — the failures were the discovery, because what they proved is that a steady field induces nothing. Asked what use it was, he is said to have replied that one day the government would tax it.

• “Why did it take you eleven years to find induction?”
• “What did you notice at the moment you connected the battery?”
• “Why does a magnet held still inside a coil produce nothing?”
• “Why does the induced current oppose what caused it?”
• “Did you know what a generator would be worth?”

James Prescott Joule — the one whose accounting explains why a generator resists being turned

The direction rule on this page — that the induced current opposes the change producing it — is not an arbitrary sign convention but Joule's principle enforcing itself. If the induced current helped the motion instead of opposing it, a generator would speed itself up and pour out electrical energy from nothing. Joule spent years establishing, against considerable scepticism, that energy is neither created nor destroyed but converted at a fixed exchange rate, measuring the warming of water stirred by falling weights to a precision his contemporaries thought absurd. He is the right person to ask why turning a generator with the lamps switched on takes more effort than turning it with them off.

• “Why does a generator get harder to turn when it supplies more current?”
• “Where does the electrical energy from a generator actually come from?”
• “What would happen if the induced current helped the motion instead?”
• “How did you measure the exchange rate between work and heat?”
• “Why did people refuse to believe your results at first?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Michael Faraday in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: The Motor Effect
• Next lesson: Transformers
• Magnets and Magnetic Fields — Why only repulsion proves you are holding a magnet, and what a field line is actually a picture of.
• Induced Magnetism and Field Patterns — Why an iron nail sticks to a magnet at all, why it never pushes back, and how to plot a field you cannot see.
• Electromagnets — The four pages of Latin that showed a current makes a field — and why that field goes round the wire rather than along it.
• The Motor Effect — Put a current in a magnetic field and something moves — the effect behind every electric motor and every loudspeaker.
• Transformers — Two coils that never touch, the reason the National Grid runs at 400 000 volts, and why none of it works on a battery.
• All of Physics · Incandio Science