NUST NET Physics · Physics · Magnetism and alternating current. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
An EMF is induced in a circuit whenever the magnetic flux through it changes. It does not matter how the change is made: moving a magnet, moving a wire, turning a coil or changing a current nearby. A steady flux, however large, induces nothing.
Faraday's law gives the size of the EMF: the rate of change of flux linkage. Lenz's law gives its direction: the induced current always opposes the change that causes it, which is conservation of energy at work.
A changing current in a coil induces an EMF in the same coil (self-induction) or in a neighbouring coil (mutual induction). Generators and transformers rely on this, so a transformer works only on AC.
Scaling transformer current the same way as voltage. Power is unchanged in an ideal transformer, so if the voltage is stepped up 5 times the current is stepped down 5 times.
Answer: 20 V, acting in the same direction as the current.
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