Electromagnetic induction — Physics, 14–17 years
Changing the magnetic field through a coil can create a voltage, even without a battery. The faster the change and the more turns the coil has, the greater the induced voltage tends to be.
Changing fields create voltage
Electromagnetic induction happens when the magnetic flux through a conductor changes. That change separates charges and creates an induced voltage; if the circuit is closed, a current flows. Moving a magnet into or out of a coil works because the field through the coil is changing, not because the magnet is simply nearby.
Why induction was needed
Once electricity and magnetism were linked, scientists asked whether magnetism could produce electricity as electricity can produce magnetism. Experiments showed that a steady magnetic field is not enough: the field must change relative to the conductor. This solved the practical problem of generating electricity without a chemical battery.
A simple induced voltage
Suppose a coil has 200 turns and its magnetic flux per turn changes by 0.006 Wb in 0.20 s. The average induced voltage is E = NΔΦ/Δt. Substitute: E = 200 × 0.006 ÷ 0.20 = 6 V. Reversing the motion reverses the voltage direction, even though its size can be the same.
The common trap
A common mistake is to say that a magnet near a coil must induce a current. It is reasonable because the coil is inside the magnet's field, but induction responds to change, not presence. A magnet held still gives no continuous induced current; moving it, moving the coil, or changing the field does.
Where it is used
Generators rotate coils or magnets so the magnetic flux changes and electricity is produced. Transformers use changing current in one coil to induce a different voltage in another, while induction hobs heat suitable pans through changing fields. These devices require changing fields and conductors arranged for the intended result.
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