Lesson 1 of 5
Producing an induced voltage
I can deduce induction from changing magnetic fields, the opposing direction and factors affecting induced e.m.f. from experiments.Syllabus
Syllabus K323, 19(a). Deduce induction from changing magnetic fields, the opposing direction and factors affecting induced e.m.f. from experiments.
Make a guess. You are not marked.
A strong magnet rests inside a coil that is connected to a sensitive voltmeter. What does the voltmeter read?
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Zero, because the field through the coil is not changing.
An e.m.f. is induced only while the magnetic field through the coil changes. A magnet at rest gives a strong but steady field, so there is no induced e.m.f.
A changing magnetic field through a coil can induce an e.m.f. across its ends. A strong field that remains unchanged through the coil gives no sustained induced e.m.f.
E.m.f. is energy supplied per unit charge, measured in volts. Current is the rate of flow of charge, measured in amperes. They describe different quantities.
Move a magnet relative to a coil
Connect a coil to a suitable sensitive voltmeter and move a bar magnet along the coil's axis. As a pole approaches, the field through the coil changes and the meter can register a voltage. Stop and hold the magnet still: after the transient settles, the reading returns to zero. Withdraw the same pole and the induced polarity reverses.
One winding, fixed meter connections
The sensitive voltmeter reads VA - VB: its positive lead stays at A. The display shows polarity, with no numerical calibration. Blue arrows show magnet motion.
1. The north pole approaches
The magnetic field through the turns is changing. This winding and these connections give a positive reading during the approach.
2. The magnet is held still
Once the motion has stopped and the transient has passed, this unchanged arrangement gives no sustained induced e.m.f.
3. The north pole withdraws
Withdrawal reverses the magnetic change and the meter polarity. Swapping the meter leads would reverse both signs.
| Change made | Voltage observation |
|---|---|
| Bring N towards the coil | A reading of one polarity; use this as the reference. |
| Hold N stationary | Zero after the transient settles. |
| Withdraw the same N pole | The opposite polarity to the reference approach. |
| Bring S towards the coil instead | The opposite polarity to the N-pole approach. |
| Make the same approach faster | A greater peak magnitude over a shorter event, if the meter can resolve it. |
An e.m.f. does not guarantee a current
An induced e.m.f. can exist across the ends of an open coil. A continuing induced current also needs a closed conducting path. A high-resistance voltmeter draws little current and measures the terminal voltage with little loading.
A sensitive galvanometer in a closed coil circuit shows induced current instead. Its deflection is evidence of induction, but its current scale is not a voltage scale. The current depends on the induced e.m.f. and the resistance of the complete circuit.
Identify what changes through the loop
- Move the coil: a coil moving relative to a stationary magnet can experience a changing field through its loops.
- Change a nearby current: varying the current in an electromagnet changes its field, even when the coil and electromagnet stay still.
- Rotate the coil: changing its orientation in a steady external field changes the field passing through its loops.
Moving the magnet and coil together while keeping their relative arrangement unchanged does not by itself produce induction. Ask what changes through the coil, rather than only whether something moves.
The words your school mark scheme uses
Many mark schemes describe induction with magnetic flux. Magnetic flux measures how much magnetic field passes through a coil. Picture it as the number of field lines passing through the loops. When a wire moves across a magnetic field, we say it cuts the field lines.
- Faraday's law
- The size of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linking the coil. In other words, it depends on how fast the coil cuts magnetic field lines.
Faraday's law explains the three factors listed below: a faster change, a larger change in field, and more turns all raise the rate at which flux linkage changes. Your syllabus does not ask you to name the law, but school mark schemes often write "by Faraday's law". Name it and give the full statement, including "rate of change".
Compare the magnitude fairly
- Faster change
- Move the same magnet along the same path more quickly. The more rapid field change gives a greater induced e.m.f.
- A larger field change
- A stronger magnet can give a greater induced e.m.f. when it produces a larger field change over the same path and time.
- More linked turns
- More turns experiencing the same changing field increase the total induced e.m.f. Keep the turns' position and orientation comparable.
For practical work: meter signs and a controlled investigation (optional)
The labels positive and negative depend on the winding and which meter lead is connected to each coil end. Swapping the meter leads reverses the displayed sign; it does not change the physical cause of induction.
More turns can also increase circuit resistance. A change in galvanometer current alone does not establish the same ratio of induced e.m.f.s. For a voltage comparison, use suitable voltage measurements and account for loading.
Investigate induction with controlled observations
- Secure the coil and connect a voltage sensor or meter able to show both polarities. Select a voltage range that includes the expected peak with useful resolution.
- Mark the magnet's path and endpoints. Record which pole faces the coil and keep the winding and meter connections fixed.
- Record an approach, a stationary interval and a withdrawal. Compare the polarities and identify when the magnetic situation is changing.
- Change one factor at a time. For a speed comparison, use the same magnet, coil and path. For different magnets or turn counts, keep the motion and geometry comparable.
- Repeat the comparison and inspect the recorded peaks and timing for variation.
A brief peak needs a sufficiently fast recording method: a slow display may miss its maximum. Faster sampling does not correct inconsistent magnet motion. Use the same marked travel and comparable duration when motion is meant to be controlled, and keep the magnet aligned without striking the coil.
Put these in order
A bar magnet is pushed into a coil that is connected to a sensitive meter, and the meter needle flicks. Put the explanation in order.
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1. The magnet moves towards the coil. As the magnet approaches, more of its field passes through the coil.
2. The magnetic field through the coil changes: field lines are cut by the coil. A changing magnetic field through a coil induces an e.m.f. across it.
3. An e.m.f. is induced across the coil. An e.m.f. can drive a current only if there is a complete circuit for it.
4. The coil and meter form a complete circuit, so an induced current flows and the needle deflects.
Check your understanding
A coil and a permanent magnet are kept still. Current in a nearby electromagnet is then increased, changing the magnetic field through the coil. Which statement is correct?
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An e.m.f. is induced during the change; a current also needs a closed circuit.
The cause is the changing magnetic field through the coil. Induced e.m.f. and induced current are different quantities: voltage can exist across open coil ends, while current requires a complete conducting circuit.
Practise an exam question
Write your answer first, then tick the mark points it makes.
Exam-style question
A bar magnet is pushed into a coil connected to a sensitive ammeter. Explain why the ammeter shows a reading, and why the reading is larger when the magnet is pushed in faster. [3]
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