Electromagnetic induction is the physical phenomenon by which a variation of the magnetic flux through an electrical circuit creates an electromotive force (induced voltage) in that circuit. Discovered by Faraday in 1831, it is the principle behind all electrical generators.
Discover FizziQ
How to measure it in class
With the FizziQ app, induction can be demonstrated indirectly.
Steps:
- Use the magnetometer to observe magnetic field variations
- Move a magnet near a coil connected to a voltmeter or an oscilloscope
- Observe that the induced voltage only appears during movement
- Check that the direction of the current changes with the direction of movement
- Rotate a magnet near the smartphone and observe the field oscillations
- Connect to everyday phenomena (dynamo, wireless charging)
Scientific activities on this topic
- Biot-Savart law: coil
- Speed measurement with magnetic markers
Possible extensions with FizziQ: how an alternator works and induction braking.
Learn more
Faraday’s law:
The induced electromotive force is equal to the opposite of the derivative of the magnetic flux:
e = -dΦ/dt
The minus sign reflects Lenz’s law: the induced current opposes the cause that produced it.
Conditions for induction:
A variation of the flux is required, which can result from:
- Movement of the magnet or of the coil
- Rotation in a magnetic field
- Variation of the field intensity
- Variation of the coil area
Applications:
| Application | Principle |
|---|---|
| Alternator | Rotation of a magnet in a coil |
| Bicycle dynamo | Rotation of a magnet |
| Transformer | Current variation in a coil |
| Eddy current braking | Induction in a conductive piece |
| Wireless charging | Induction between two coils |
Formula
Faraday-Lenz law: e = -dΦ/dt
Magnetic flux: Φ = B × S × cos(θ)
Electromotive force for a coil with N turns: e = -N × dΦ/dt
where:
- e: induced electromotive force (V)
- Φ: magnetic flux (Wb = T·m²)
- B: magnetic field (T)
- S: area of the loop (m²)
- θ: angle between B and the normal to the surface
Application examples
- A power plant produces electricity by rotating coils in a magnetic field
- Induction cooktops heat through eddy currents in the bottom of the pan
- Metal detectors create induced currents in metallic objects
- Wireless smartphone charging uses induction between two coils
FAQ
Q: Can induction be detected with FizziQ? A: Indirectly. The magnetometer shows field variations, but to see the induced voltage, you need a voltmeter connected to a coil.
Q: Why is movement necessary? A: It is the variation of the flux that creates the voltage, not the flux itself. A constant field does not produce induction.
Q: What is Lenz’s law? A: The induced current creates a magnetic field that opposes the change in flux. It is a form of energy conservation.
Q: Why do we say that induction “slows things down”? A: Induced currents create a force that opposes the motion (magnetic braking). This is the principle of eddy current braking.
Related concepts
Magnetic Field of a Magnet