In Faraday's classic experiment of moving a bar magnet through a wire coil, when is a current actually induced in the coil?
AOnly when the magnet is stationary inside the coil
BOnly when the magnet moves relative to the coil
CWhenever the coil is made of iron
DOnly when the coil is disconnected from any circuit
Answer & Solution
Correct answer: B. Only when the magnet moves relative to the coil
1. Faraday's law of induction depends on a changing magnetic flux through the coil, not merely the presence of a magnet.
2. The text states: "current is induced only when the magnet moves with respect to the coil. When the magnet is motionless with respect to the coil, no current is induced in the coil."
3. A stationary magnet inside the coil produces a constant (unchanging) flux, so no current is induced -- motion is essential.
4. So the deciding factor is relative motion between magnet and coil, not the coil's material or whether it is connected elsewhere.
_Source: OpenStax Physics Ch 20 "Magnetism", p.693 §20.3 Electromagnetic Induction_
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