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Faraday's law
Intermediate
Induced emf equals the rate of change of flux.
ε = −dΦ/dtA changing flux through a circuit induces an EMF: ε = −N dΦ/dt. The minus sign is Lenz's law. Faster changes give more EMF.
A coil of 100 turns with flux dropping by 1 mWb in 0.1 s: 1 V.
EMF arises from change of flux, not flux itself.
In more detail
Faraday's law states that a changing magnetic flux through a loop induces an emf, ε = −dΦ/dt. The induced emf drives current in any closed conducting loop.
Real-world uses
- Generators produce electricity by changing flux through coils.
- Induction cookers use changing fields to heat pans directly.
- Engineers designing wireless chargers and induction hobs rely on Faraday's law to create voltages from changing flux.
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Frequently asked questions
What causes the change in flux?
Moving a magnet, changing the field strength, or changing the coil area or orientation.
Why does the minus sign matter?
It expresses Lenz's law: the induced emf opposes the change.
What makes the emf change?
A change in flux, whether caused by a moving magnet, a changing current or a rotating coil.
Related topics in Magnetism & Induction
Electromagnets · Permanent magnets · Magnetic flux · Lenz's law · Motional emf · Eddy currents