Electromagnetic induction
Key idea: An e.m.f. is induced when the magnetic field through a coil changes. A faster change, a stronger magnet or more turns gives a larger e.m.f.; a current flows only if the circuit is complete.
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Guided steps
- Compare entry, rest and withdrawal with the same magnet and coil.
- Connect changing field, induced e.m.f. and the path for current.
- Predict the stationary reading and withdrawal direction before testing.
- Move the magnet and compare e.m.f. and current.
- Compare the meter readings and explain what changes their signs.
- Decide whether moving together changes the field through the coil.
- Check e.m.f. and sustained current separately for the open coil.
- Compare how speed, magnet strength and turn count affect e.m.f.
- Choose two changes and explain why each increases e.m.f.
- Plan a fair comparison that measures voltage.
- Sketch both pulses and compare their height, duration and sign.
- Review field change and current, then use the topic check.
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A current makes a magnetic field. In 1831 Michael Faraday showed that the reverse also works: a magnet can make a current, but only while something is changing. That discovery, electromagnetic induction, is the basis of electrical generators. This lesson shows when an e.m.f. is induced and what makes it larger.
Faraday’s magnet-and-coil experiment
Connect a coil of wire to a sensitive centre-zero galvanometer, which shows a small current in either direction.
- With nothing moving, the galvanometer reads zero.
- Push the north pole of a bar magnet into the coil. The needle flicks to one side.
- Hold the magnet still inside the coil. The needle returns to zero.
- Pull the magnet out. The needle flicks the other way.
- Push the magnet in faster. The needle flicks further.
- Move the coil towards the magnet instead of moving the magnet. The needle flicks again, the same as when the magnet moved in.
Induced e.m.f. during magnet entry and withdrawal
Illustrative e.m.f.–time pulses with opposite signs for entry and withdrawal, separated by a zero-e.m.f. interval while the magnet is held still.
Scroll across the figure to read all labels.
View figure data
| Time (s) | e.m.f. |
|---|---|
| 0 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 0 |
| 6 | 0 |
| 7 | -1 |
| 8 | 0 |
| 10 | 0 |
What the experiment shows
- An e.m.f. is induced in the coil only while the magnetic field through the coil is changing. A strong magnet held still inside the coil induces nothing.
- It does not matter whether the magnet or the coil moves. What matters is the change in the field through the coil.
- Reversing the change, from pushing in to pulling out, reverses the direction of the induced e.m.f. Which way it points is explained by Lenz’s law.
The same thing happens when a straight wire is moved across a magnetic field, cutting through the field lines. Moving the wire along the field lines, without cutting them, induces nothing.
What the change induces is an e.m.f., an electrical energy transfer per unit charge that can drive current. A sustained current also needs a complete conducting path. The galvanometer detects current in this experiment, not e.m.f. independently; with the same circuit resistance, a larger induced e.m.f. gives a larger current.
Check your understanding 1 Predict before moving
A north pole faces the same air-cored coil, connected to a centre-zero galvanometer. Predict the meter reading while the magnet is held still, after any brief settling. Then compare the deflection direction for entry and withdrawal, keeping the leads unchanged.
Show answer
The settled stationary reading is zero. Withdrawal gives the opposite deflection to entry. There is no universal left/right sign without specifying the meter connections; the relevant prediction is the reversal.
Test the motion prediction
Move the public model’s Bar magnet handle from outside the coil to its centre and back, using drag or the keyboard arrow keys after focusing it. If reduced motion pauses the model, select Play. Compare Induced e.m.f. and Current in the meter, and wait for the readings to settle after holding the magnet still. This model has a fixed complete meter circuit; it does not simulate an open coil. The static graph above states its own illustrative voltage scale and chosen measurement-lead sign.
A bar magnet, north pole facing a 200-turn coil connected to a galvanometer, has its centre 16 centimetres from the end of the coil. The induced e.m.f. is 0 millivolts.
- Current, viewed from magnet
- zero
- Motion
- held still
- Force on 1 m of wire
- 0 mN
- B at compass 1
- 7.02 μT
- Compass 1 from the wire
- 11.4 cm
- B inside the coil
- 0.133 mT
- Induced e.m.f.
- 0 mV
- Current in the meter
- 0 mA
- Magnet speed
- 0.00 m/s
Try this
0 of 5 donePredict each direction, then play Approach, Hold still and Withdraw with the same pole facing the coil. (not done yet)
Approach and withdrawal give opposite currents. Hold gives zero current despite non-zero magnetic flux. Directions are viewed from the magnet’s side; self-inductance is neglected.
Place a compass at a neutral point, where the magnetic fields cancel. (not done yet)
At a neutral point the fields are equal and opposite, so the compass has no field to line up with. Field lines never cross there.
Reverse the current in the straight wire and watch the compasses. (not done yet)
Reversing the current reverses the field. Right-hand grip rule: thumb along the current, fingers curl the way the field goes.
Make the right-hand end of the coil a north pole. (not done yet)
Seen from an end, anticlockwise current makes that end a north pole. More current or more turns make the field stronger.
Push the magnet into the coil fast enough to swing the needle past half-scale. (not done yet)
A current flows only while the field through the coil changes. Moving faster, using a stronger magnet or more turns gives a bigger current; the needle swings the other way when the magnet comes out.
You’ve tried every task.
Check your understanding 2 Compare entry, rest and withdrawal
Use the Bar magnet handle to move from outside the coil towards its centre, hold it still, then return along the same path. Compare Induced e.m.f. and Current in the meter. Explain the settled readings and how the signs compare at the same magnet position during entry and withdrawal.
Show answer
Both readings vary while the linked field changes and settle to zero when the magnet is held still. At the same position, reversing motion reverses the e.m.f. and current signs because the field change reverses. This model keeps the meter circuit complete, so current follows the induced e.m.f.; it does not show the open-coil case. Check the change in field as well as the direction of motion.
Check your understanding 3 Which change actually induces?
A strong magnet and coil move together at constant speed, with their separation and orientation unchanged, through a region with no changing external field. Is an e.m.f. induced by their motion together? Choose what determines your answer.
Show answer
No e.m.f. is induced by this common motion: the coil experiences the same linked magnetic field. Moving only the magnet or only the coil can change that field. Movement matters through the change it creates, not by itself.
Guided practice 1
An open circuit
Problem
A magnet is pushed into a coil whose ends are not connected to anything. Is an e.m.f. induced? Does a current flow? Answer each question separately.
Check the change and the conducting path separately
Hints
Hint 1
the e.m.f.
Is the field through the coil changing?
Hint 2
the current
What does a current need, apart from an e.m.f.?
Show solution step by step
The e.m.f.
Method
Yes, an e.m.f. is induced.
Reason
Moving the magnet changes the field through the coil, whether or not the coil is connected.
Working
Answer
The field changes, so an e.m.f. is induced.
The current
Method
No current flows.
Reason
A current needs a complete circuit, and the coil’s ends are not connected.
Working
Answer
E.m.f. induced, but no current.
Quick check
A voltmeter across the ends of the coil would show the e.m.f., because it draws almost no current.
The size of the induced e.m.f.
The faster the field through the coil changes, the larger the induced e.m.f. So the e.m.f. is increased by:
- moving the magnet or coil faster, because the same change happens in less time;
- using a stronger magnet, because there is a bigger change in field for the same movement;
- using more turns on the coil, because an e.m.f. is induced in every turn, and the turns are in series, so their e.m.f.s add up.
Spot the mistake 1
Only a stronger magnet?
Learner response
A learner says: “The only way to get a bigger induced e.m.f. is to use a stronger magnet.” Correct the claim by giving two other changes, and explain each.
Choose two changes and the reason for each
Show solution step by step
Move faster
Method
Move the magnet or the coil faster.
Reason
The field through the coil then changes faster, and the size of the e.m.f. depends on how fast it changes.
Working
Answer
Faster movement, faster change, larger e.m.f.
Add turns
Method
Use a coil with more turns.
Reason
An e.m.f. is induced in each turn, and the turns are joined in series, so the e.m.f.s add.
Working
Answer
More turns, larger total e.m.f.
Common mistake
Magnet strength is one factor. Movement speed and the number of turns are separate factors; compare each with the others and the path held fixed.
Check your understanding 4 Test the magnitude fairly
Plan how to test whether more turns increase induced e.m.f. without mistaking increased current for a direct e.m.f. measurement. Specify the instrument, fixed conditions and comparisons. Work on paper or here, then compare the complete method below; a note is optional.
Show answer
Compare coils of the same geometry with different series turn counts, using the same magnet, starting and finishing positions, orientation and controlled motion speed. Measure peak voltage with a sufficiently high-resistance voltmeter or voltage sensor that can resolve the pulse, rather than treating galvanometer current alone as e.m.f. Repeat matched trials. More turns also change wire resistance, so a current-only comparison confounds induced e.m.f. with circuit resistance. Check the measured quantity, one changed factor, fixed path/speed and repeated observations. Repeat the same motion for each coil so the peak voltages can be compared fairly.
Try it yourself 1
A faster push on the graph
Problem
The graph near the start of this lesson shows the e.m.f. as a magnet is pushed into a coil. The same magnet is now pushed along the same entry path in half the time, with the motion profile scaled to twice the speed. The coil and measurement leads are unchanged. Predict the changes in entry-pulse height, width and sign, and sketch both pulses.
The same change happens in half the time
Sketch both pulses on paper and label time and e.m.f. axes. Explain height, width and sign, then compare the full reasoning and graph below. Use the note only if it helps; you need not transcribe the sketch.
Hints
Hint 1
height
What happens to the size of the e.m.f. when the field changes faster?
Hint 2
width
How long does the movement now take?
Show solution step by step
The height
Method
The pulse is taller.
Reason
The same change in field happens in half the time, so the field changes faster and the e.m.f. is larger.
Working
Answer
Twice as fast: about twice the peak e.m.f.
The width
Method
The pulse is narrower.
Reason
The movement, and so the change, is over in half the time.
Working
Answer
Twice as fast: the pulse lasts about half as long.
Quick check
The pulse keeps its entry sign because the pole, motion direction and leads are unchanged. In this speed-scaling model the peak doubles, duration halves and area is unchanged: the same total linked-field change occurs. Check both axis labels, both pulse widths and heights, and the same sign before comparing with the plotted model.
Same entry path, faster motion
A slower 1 V pulse lasts 2 seconds; a faster 2 V pulse lasts 1 second, for the same start-to-end change in linked field.
Scroll across the figure to read all labels.
View figure data
| Series | Time (s) | Induced e.m.f. (V) |
|---|---|---|
| Slower entry | 0.0 | 0 |
| Slower entry | 2.0 | 0 |
| Slower entry | 3.0 | 1 |
| Slower entry | 4.0 | 0 |
| Slower entry | 5.0 | 0 |
| Faster entry | 0.0 | 0 |
| Faster entry | 2.0 | 0 |
| Faster entry | 2.5 | 2 |
| Faster entry | 3.0 | 0 |
| Faster entry | 5.0 | 0 |
Common mistakes
- Saying a strong magnet inside a coil induces an e.m.f. Nothing is induced unless the field through the coil is changing.
- Saying a current is always induced. An e.m.f. is induced; a current flows only in a complete circuit.
- Thinking only the magnet can move. Moving the coil has the same effect.
- Expecting any movement to induce an e.m.f. A wire moving along the field lines, without cutting them, has no e.m.f. induced in it.
- Giving “a bigger magnet” as a factor. The factor is a stronger magnet.
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Syllabus and review details
- SEC G3 Physics 2027 · 2027
Content Structure, PDF page 9; Subject Content, PDF pages 10–28
Last reviewed: