Electromagnetic induction is the generation of an electromotive force in a conductor or coil by a change in the magnetic flux linked with it. The induced voltage is proportional to the rate of change of flux and to the number of turns.
Learning objectives
- State Faraday law of induction
- Calculate an induced EMF from a flux change and a number of turns
- Apply Lenz law to determine the direction of the induced effect
- Calculate the motional EMF of a conductor moving in a field
- Explain induction in generators, transformers and motors
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
Generators use this effect: a rotating magnet or coil creates a changing flux, and the induced voltage drives current. Transformers use it to transfer energy between coils that are not electrically connected.
Intermediate How it behaves in a circuit
The induced voltage depends on the rate of change, not on the absolute value of the flux. A steady flux, however large, induces no voltage.
Advanced Limits, assumptions, engineering detail
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Magnetic flux | Φ | weber (Wb) |
| Flux density | B | tesla (T) |
| Induced EMF | e | volt (V) |
| Flux linkage | λ | weber (Wb) |
Worked examples
Coil in a changing field
Problem. A 200-turn coil links a flux that rises from 0 to 0.02 Wb in 0.1 s. What is the average induced EMF?
Solution. e = N × ΔΦ/Δt = 200 × 0.02/0.1.
Conductor moving in a field
Problem. A 0.5 m conductor moves at 10 m/s perpendicular to a 0.8 T field. Find the induced EMF.
Solution. e = B × L × v = 0.8 × 0.5 × 10.
Flux change in a short pulse
Problem. A 100-turn coil experiences a flux change of 0.005 Wb in 20 ms. What average EMF is induced?
Solution. e = 100 × 0.005/0.020.
Practical applications
- Generators and alternators
- Transformers
- Induction motors
- Wireless charging and power transfer
- Metal detectors and proximity sensors
- Induction heating and braking
Automotive applications
The alternator induces three-phase voltage in its stator, the ignition coil induces a high-voltage pulse, and crankshaft and wheel sensors use induction or Hall effect to detect movement. Wireless charging of electric vehicles transfers energy by induction, and regenerative braking converts motion back into electrical energy.
Industrial applications
Induction motors, transformers, induction furnaces and non-destructive testing all rely on electromagnetic induction. Induction heating exploits eddy currents deliberately, while in motors and transformers the same currents are a source of loss to be minimised.
Measurement procedure
- Observe induced pulses with an oscilloscope; a changing magnetic field can couple into the probe leads and distort the measurement.
- Use a search coil and a known flux change to measure flux density.
- Keep the measurement loop area small to reduce induced pickup.
- Remember that the induced voltage depends on the rate of change, so the waveform shape matters.
Common mistakes
- Thinking that a large steady flux induces a voltage; only a change does.
- Ignoring Lenz law when predicting the direction of the induced current.
- Forgetting the number of turns in the EMF calculation.
- Confusing the average EMF over an interval with the instantaneous EMF.
Safety considerations
- Interrupting a current through an inductive winding produces a high induced voltage; use suppression and rated switching devices.
- Rotating machines and transformers store magnetic energy; follow isolation and discharge procedures.
- High-voltage induction systems, such as wireless chargers, can create strong fields; respect clearances and warnings.