Chapter 5 of 8

All 8 chapters
  1. 1Magnetic fields, readunderstood
  2. 2Fields around wires and coils, readunderstood
  3. 3The motor effect, readunderstood
  4. 4Electric motors, readunderstood
  5. 5Electromagnetic induction, readunderstood
  6. 6Generators and dynamos, readunderstood
  7. 7Transformers, readunderstood
  8. 8Speakers and microphones, readunderstood

S4.13.5 Chapter 5

Electromagnetic induction

A circuit has no cell in it, only a coil and a meter. Yet the needle moves when a magnet is pushed into the coil. Where does the current come from?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain why moving a magnet in a coil makes a current
  • Explain what makes the induced current bigger

In the motor effect, a current in a magnetic field makes a force. Now turn the idea round. Move a wire or a magnet, so that the wire is in a changing field. A voltage appears in the wire. This is electromagnetic induction, and the voltage is induced.

If the wire is part of a complete circuit, the induced voltage makes a current flow. It is an induced current. No cell is needed. The energy comes from whoever moves the magnet.

Join a coil to a meter with its zero in the middle. Push a magnet in, hold it still, and pull it out. Watch the needle each time.

Draw it first

Push a magnet into a coil, hold it, pull it out

  1. Three pictures of a coil joined to a centre zero meter, with a bar magnet labelled S and N beside the coil, N end nearest the coil. Step 1: the magnet is moving in, with an arrow towards the coil, and the needle swings right. Step 2: the magnet is still, and the needle is at zero. Step 3: the magnet is moving out, with an arrow away from the coil, and the needle swings left. Labels N, S, coil, magnet and meter.SNmagnetcoilmeter

    1 Push the N pole into the coil. The needle swings to one side, say the right, while the magnet is moving.

  2. Three pictures of a coil joined to a centre zero meter, with a bar magnet labelled S and N beside the coil, N end nearest the coil. Step 1: the magnet is moving in, with an arrow towards the coil, and the needle swings right. Step 2: the magnet is still, and the needle is at zero. Step 3: the magnet is moving out, with an arrow away from the coil, and the needle swings left. Labels N, S, coil, magnet and meter.SNmagnetcoilmeter

    2 Hold the magnet still. The needle goes back to zero, even though the magnet is next to the coil. A steady field makes no voltage.

  3. Three pictures of a coil joined to a centre zero meter, with a bar magnet labelled S and N beside the coil, N end nearest the coil. Step 1: the magnet is moving in, with an arrow towards the coil, and the needle swings right. Step 2: the magnet is still, and the needle is at zero. Step 3: the magnet is moving out, with an arrow away from the coil, and the needle swings left. Labels N, S, coil, magnet and meter.SNmagnetcoilmeter

    3 Pull the magnet out. The needle swings the other way, to the left. The direction of the current has reversed.

Three facts come from this. The current flows only while the magnet moves, so it is the change that matters and not the magnet itself. Pulling out reverses the current. Turning the magnet round, so the S pole goes in, also reverses it.

The explanation is in the field lines. As the magnet moves, the number of field lines that pass through the coil changes. A changing field through the coil, or a wire that cuts across field lines, induces a voltage. If nothing changes, there is none.

It does not matter whether the magnet moves or the coil does. What counts is that one moves relative to the other. A coil pushed over a still magnet gives the same effect. So does a change in the current of a nearby coil. You meet that again in the chapter on transformers.

What makes the induced voltage bigger? Move the magnet faster, so the field changes more quickly. Use a stronger magnet, so more field lines pass through the coil. Use more turns of wire, so each turn adds its own voltage. The needle swing shows each of these.

A coil joined to a centre zero meter, with a bar magnet labelled S and N, N end nearest the coil, moving towards the coil. The needle swings far to the right. Labels N, S, coil, magnet and meter.SNmagnetcoilmeter
Push the magnet in fast and the needle swings further than in the slow push.
A coil of 12 turns joined to a centre zero meter, with a bar magnet labelled S and N, N end nearest the coil, moving towards the coil. The needle swings far to the right. Labels N, S, coil, magnet and meter.SNmagnetcoilmeter
The first push of the strip again, at the same speed, but with twice as many turns of wire. The needle swings about twice as far.

The bars show a learner’s voltages for a magnet pushed into the same coil at three speeds. Twice the speed gives twice the voltage, and three times the speed gives three times. In an ideal coil, the induced voltage is also in proportion to the number of turns.

A bar chart called voltage (mV) for the same coil and magnet pushed in at three speeds. The bar for 0.1 m/s reaches 20. The bar for 0.2 m/s reaches 40. The bar for 0.3 m/s reaches 60. The scale goes from 0 to 60 in steps of 20.02040600.1 m/s0.2 m/s0.3 m/svoltage (mV)
An example. Real results are close to this pattern, not exactly on it.

Worked example

A coil of 200 turns gets an induced voltage of 0.6 V when a magnet is pushed in. The magnet and the speed stay the same. What is the voltage with a coil of 500 turns?

  1. The voltage is in proportion to the number of turns.
  2. 500 turns is 500 ÷ 200 = 2.5 times as many.
  3. So the voltage is 0.6 V × 2.5 = 1.5 V.

Answer: 1.5 V

Worked example

A magnet pushed in at 0.2 m/s induces 0.15 V in a coil. What voltage does it induce at 0.6 m/s, in the same coil?

  1. In an ideal case the voltage is in proportion to the speed.
  2. 0.6 ÷ 0.2 = 3, so the speed is 3 times as big.
  3. The voltage is 3 times as big: 0.15 V × 3 = 0.45 V.

Answer: 0.45 V

Which way does the induced current flow? Lenz’s law says that the induced current is always in the direction whose magnetic field opposes the change that causes it. Push a N pole into the coil. The end of the coil nearest it becomes a N pole too, and so pushes back.

This must be so. If the coil pulled the magnet in, the magnet would speed up and make more current, and so on. That would make energy from nothing. Instead you must push against the force, and the work you do becomes the energy of the current.

For a straight wire moving across a field there is a quick rule, using the right hand. Fleming’s right hand rule: First finger Field, seCond finger Current, thuMb Motion. The thumb is the way the wire moves. The second finger then gives the way the induced current flows.

A right hand drawn with the thumb pointing up, the first finger pointing away from you and the second finger pointing to the left, all at right angles. The first finger ends in a small circle with a cross, meaning into the page. Labels: Thumb, Motion. First finger, Field, into the page. Second finger, Current. Right hand.ThumbMotionFirst fingerFieldinto the pageSecond fingerCurrentRight hand
The right hand for induction, the left hand for the motor effect. The fingers stand for the same things in both.

Here is a way to remember the pair. The motor effect starts with a current and makes motion, so use the left hand. Induction starts with motion and makes a current, so use the right. Both hands have the same fingers: Field, Current and Motion.

Try it

Hear a current made by a magnet

You need: about 3 m of thin insulated wire, a tube a little wider than your magnet, such as a pill bottle or a roll of card, a small magnet, such as one from an old speaker, an old pair of earphones with the plug cut off and the wire ends bared, by an adult, sandpaper or a nail file.

Be safe: Ask an adult to cut off the plug and bare the ends. Clean the wire ends by rubbing them with sandpaper or a nail file, not a knife. Hold the earphone against your ear only gently. This makes only a tiny voltage, but never connect it to any other equipment. Keep magnets away from phones and bank cards, and watch your fingers when magnets snap together.

  1. Wind the wire into a coil of about 100 turns round the tube. Leave a little wire free at each end. Rub the wire ends clean with sandpaper or a nail file.
  2. Join the two ends of the coil to the two bare ends of one earphone wire. Hold the earphone gently against your ear.
  3. Push the magnet quickly into the coil, then hold it still. Then pull it out quickly. If your magnet will not go in, move it quickly up to the end of the coil and away.
  4. Try again with the magnet moved faster. Then try again with a coil of fewer turns.

What you will see: You may hear a faint click when the magnet moves in. You may hear another when it moves out. You hear nothing when it is still. A faster move gives a louder click, and a coil with more turns does too. If your earphone is silent, wind more turns, move the magnet faster, or use a stronger magnet.

If you cannot try it, this is what happens.
A table with the columns Magnet and Earphone. Rows: moving in, click. still, silent. moving out, click.MagnetEarphonemoving inclickstillsilentmoving outclick

A wrong idea many people have

A magnet inside a coil makes a current, because of its magnetism.

Why it feels right: We know magnets and coils are linked, so a magnet near a coil seems to be enough.

The test that breaks it: Push a magnet into a coil joined to a meter, then leave it there. The needle swings as it moves and returns to zero when it stops. The magnet is still there and there is no current.

What is true: A current is induced only while the magnetic field through the coil is changing. A still magnet gives none.

A wrong idea many people have

Induction gives you more energy out than you put in.

Why it feels right: No cell is used, so the current seems to come from nowhere.

The test that breaks it: Push a magnet into a coil that is joined into a complete circuit. With a big coil and a strong magnet the push back is large enough to feel. You meet it on a bicycle too: with the lamp on, pedalling is harder. With the circuit broken, the push back disappears. The work you do against the push is the energy that appears in the current.

What is true: By Lenz’s law the induced current opposes the change. You must do work, and that work is the energy of the current.

A wrong idea many people have

Use the left hand rule for induction as well as for the motor effect.

Why it feels right: Both rules use Field, Current and Motion on three fingers, so one hand seems to do for both.

The test that breaks it: Use the left hand for a wire pushed up across a field to the right. You find a current out of the page. Now use the right hand for the same wire moved up. You find a current into the page. The two cases are opposite.

What is true: The left hand gives the force when a current is given. The right hand gives the current when a motion is given. The fingers stand for the same quantities, but the roles swap.

Key words

electromagnetic induction
making a voltage in a conductor by changing the magnetic field through it.
induced current
a current made by an induced voltage in a complete circuit.
Lenz’s law
the induced current is in the direction whose field opposes the change that makes it.
Fleming’s right hand rule
first finger Field, seCond finger Current, thuMb Motion, with the right hand. For a wire moving across a field.

Back to your guess

The answer is C: A changing magnetic field through a coil makes a voltage, and a current flows in a complete circuit.

A magnet stores no current. The voltage comes from the change. As the magnet moves, the magnetic field through the coil changes. That makes a voltage in the wire. This is electromagnetic induction. If the magnet stops, the voltage stops.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Why does a current flow in a coil when a magnet is pushed into it?

    Tap your answer.

    1. Not this one. A magnet stores no current. The voltage comes from the change in the field through the coil. Try another.

    2. Not this one. Magnetism does not flow along a wire. A changing field through the coil makes a voltage in the wire. Try another.

    3. Not this one. A magnet held still induces nothing. Only a changing field through the coil induces a voltage. Try another.

    4. Yes, that is right. A changing magnetic field through a coil induces a voltage in it. If the circuit is complete, a current flows.

    See the answer

    The magnetic field through the coil changes, which induces a voltage.

    A changing magnetic field through a coil induces a voltage in it. If the circuit is complete, a current flows.

    • If you chose A: A magnet stores no current. The voltage comes from the change in the field through the coil.
    • If you chose B: Magnetism does not flow along a wire. A changing field through the coil makes a voltage in the wire.
    • If you chose C: A magnet held still induces nothing. Only a changing field through the coil induces a voltage.

  2. A magnet is held still inside a coil joined to a meter. What does the meter show?

    Tap your answer.

    1. Not this one. A steady field does not induce a voltage, however strong it is. The field has to change. Try another.

    2. Not this one. There is no change in the field, so there is no induced current at all. Try another.

    3. Yes, that is right. A voltage is induced only while the field through the coil is changing. A still magnet gives no change, so the current is zero.

    4. Not this one. A reversed current needs a change in the opposite direction, such as pulling the magnet out. Try another.

    See the answer

    Zero.

    A voltage is induced only while the field through the coil is changing. A still magnet gives no change, so the current is zero.

    • If you chose A: A steady field does not induce a voltage, however strong it is. The field has to change.
    • If you chose B: There is no change in the field, so there is no induced current at all.
    • If you chose D: A reversed current needs a change in the opposite direction, such as pulling the magnet out.

  3. Which of these makes the induced voltage in a coil bigger?

    Tap your answer.

    1. Not this one. A slower change gives a smaller voltage. Move it faster to get a bigger one. Try another.

    2. Not this one. Fewer turns give a smaller voltage. More turns give a bigger one. Try another.

    3. Not this one. A weaker magnet gives a smaller voltage. A stronger one gives a bigger voltage. Try another.

    4. Yes, that is right. A faster change of the field gives a bigger voltage. A stronger magnet and more turns also give a bigger voltage.

    See the answer

    Pushing the magnet in faster.

    A faster change of the field gives a bigger voltage. A stronger magnet and more turns also give a bigger voltage.

    • If you chose A: A slower change gives a smaller voltage. Move it faster to get a bigger one.
    • If you chose B: Fewer turns give a smaller voltage. More turns give a bigger one.
    • If you chose C: A weaker magnet gives a smaller voltage. A stronger one gives a bigger voltage.

  4. The N pole of a magnet is pushed into a coil, and the needle swings right. Then the N pole is pulled out. What does the needle do?

    Tap your answer.

    1. Not this one. Pulling out is the opposite change, so the current reverses and the needle swings the other way. Try another.

    2. Yes, that is right. Pulling the magnet out changes the field the opposite way from pushing it in, so the induced current is reversed.

    3. Not this one. The field is changing as the magnet is pulled out, so a voltage is induced and the needle swings. Try another.

    See the answer

    It swings left.

    Pulling the magnet out changes the field the opposite way from pushing it in, so the induced current is reversed.

    • If you chose A: Pulling out is the opposite change, so the current reverses and the needle swings the other way.
    • If you chose C: The field is changing as the magnet is pulled out, so a voltage is induced and the needle swings.

  5. A N pole is pushed towards a coil. By Lenz’s law, what is the end of the coil nearest the magnet?

    Tap your answer.

    1. Yes, that is right. The induced current is in the direction whose field opposes the push. Two N poles repel, so the end nearest the magnet becomes N.

    2. Not this one. That would help the change, and make energy from nothing. Lenz’s law says the current opposes the change. Try another.

    3. Not this one. However small, the induced current makes a field. It acts against the change by Lenz’s law. Try another.

    4. Not this one. The pole stays the same while the magnet is moving in. It opposes the change all the time. Try another.

    See the answer

    A N pole, so that it pushes the magnet back.

    The induced current is in the direction whose field opposes the push. Two N poles repel, so the end nearest the magnet becomes N.

    • If you chose B: That would help the change, and make energy from nothing. Lenz’s law says the current opposes the change.
    • If you chose C: However small, the induced current makes a field. It acts against the change by Lenz’s law.
    • If you chose D: The pole stays the same while the magnet is moving in. It opposes the change all the time.

  6. A wire is moved across a magnetic field. Which hand and rule give the direction of the induced current?

    Tap your answer.

    1. Yes, that is right. For a wire moving across a field, Fleming’s right hand rule gives the induced current. Thumb along the motion, first finger along the field, second finger shows the current.

    2. Not this one. The left hand is for the force on a wire that carries a current. The thumb stands for Motion, not for the current. Try another.

    3. Not this one. The grip rule gives the field round a current. It is not for a moving wire. Try another.

    See the answer

    The right hand rule: first finger Field, second finger Current, thumb Motion.

    For a wire moving across a field, Fleming’s right hand rule gives the induced current. Thumb along the motion, first finger along the field, second finger shows the current.

    • If you chose B: The left hand is for the force on a wire that carries a current. The thumb stands for Motion, not for the current.
    • If you chose C: The grip rule gives the field round a current. It is not for a moving wire.

  7. From an earlier chapter

    What is the turning effect on a motor coil when the coil stands at right angles to the field?

    Tap your answer.

    1. Not this one. The largest turning effect is when the coil lies along the field. Go to S4.13.4. Try another.

    2. Not this one. The turning effect falls to zero at right angles to the field, not to half. Go to S4.13.4. Try another.

    3. Yes, that is right. The forces on the sides act along the line through the axle, so they cannot turn the coil. The split ring reverses the current at that place.

    4. Not this one. The forces are the same size at every angle, but their turning effect changes with the angle. Go to S4.13.4. Try another.

    See the answer

    Zero, so the current is reversed there.

    The forces on the sides act along the line through the axle, so they cannot turn the coil. The split ring reverses the current at that place.

    • If you chose A: The largest turning effect is when the coil lies along the field. Go to S4.13.4.
    • If you chose B: The turning effect falls to zero at right angles to the field, not to half. Go to S4.13.4.
    • If you chose D: The forces are the same size at every angle, but their turning effect changes with the angle. Go to S4.13.4.

  8. A coil of 100 turns gives 0.4 V when a magnet is pushed in. The same magnet is pushed in the same way into a coil of 250 turns. What is the voltage?

    See the answer

    1 V

    250 turns is 250 ÷ 100 = 2.5 times as many, so the voltage is 2.5 times as big: 0.4 × 2.5 = 1 V.

  9. A magnet pushed in at 0.5 m/s induces 0.3 V in a coil. What voltage does it induce at 1.5 m/s in the same coil?

    See the answer

    0.9 V

    1.5 ÷ 0.5 = 3. The speed is 3 times as big, so the voltage is 3 times as big: 0.3 × 3 = 0.9 V.

Keep it

Copy this into your exercise book.

A changing field through a coil induces a voltage. No change, no voltage. Bigger: faster, stronger magnet, more turns. Lenz: the current opposes the change. Right hand: Field, Current, Motion.

Draw: A coil, a meter and a magnet with an arrow, and a needle swung to one side.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (913 KB). It opens on any phone.