K323 / 2027
Electromagnetic induction overview

Topic 1 of 5

Producing an induced voltage

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

1. The north pole approachesThe north pole on the right-hand end of the magnet moves right towards the fixed coil. The coil winding and meter connections are held fixed. Terminal A is at the near end of this winding and is connected to the positive meter input; B is at its far end and connects to the negative input. The high-resistance voltmeter reports a positive potential at A relative to B during this approach. Pale curved lines are the hidden parts of the same continuous air-core winding. The meter has a high resistance, so this is an e.m.f. demonstration; no arrow claims a substantial coil current.MotionSNABFixed coilAir coreVA - VB+-PositiveSensitive voltmeterHigh resistance; polarity only

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

2. The magnet is held stillThe same magnet, coil, terminal connections and voltmeter remain in place. The magnet is stationary and no other magnetic change is present. After transients, the voltmeter reads zero. This panel contains no motion arrow. Pale curved lines are the hidden parts of the same continuous air-core winding. The meter has a high resistance, so this is an e.m.f. demonstration; no arrow claims a substantial coil current.StationarySNABFixed coilAir coreVA - VB+-ZeroSensitive voltmeterHigh resistance; polarity only

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

3. The north pole withdrawsThe north pole still faces the fixed coil, but the magnet now moves left away from it. The winding and positive-at-A meter connections have not been exchanged. The potential at A relative to B is negative during withdrawal, the opposite polarity to the approach. Pale curved lines are the hidden parts of the same continuous air-core winding. The meter has a high resistance, so this is an e.m.f. demonstration; no arrow claims a substantial coil current.MotionSNABFixed coilAir coreVA - VB+-NegativeSensitive voltmeterHigh resistance; polarity only

Withdrawal reverses the magnetic change and the meter polarity. Swapping the meter leads would reverse both signs.

The same coil and meter connections are used for approach, rest and withdrawal. The induced polarity reverses when the motion reverses; a stationary magnetic arrangement gives no sustained reading.
Compare observations while keeping the meter leads fixed.
Change madeVoltage observation
Bring N towards the coilA reading of one polarity; use this as the reference.
Hold N stationaryZero after the transient settles.
Withdraw the same N poleThe opposite polarity to the reference approach.
Bring S towards the coil insteadThe opposite polarity to the N-pole approach.
Make the same approach fasterA greater peak magnitude over a shorter event, if the meter can resolve it.

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.

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.

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.

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

  1. 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.
  2. Mark the magnet's path and endpoints. Record which pole faces the coil and keep the winding and meter connections fixed.
  3. Record an approach, a stationary interval and a withdrawal. Compare the polarities and identify when the magnetic situation is changing.
  4. 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.
  5. 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.

Optional check 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?
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?