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A bar magnet lies in front of a coil. The line joining the centre of the magnet to the centre of the coil (the central axis) is perpendicular to the plane of the coil. Which of the following motions of the magnet will NOT induce a current in the coil?
- Translational motion back and forth along the central axis.
- Translational motion up and down (perpendicular to the central axis).
- The magnet's axis is tilted to the central axis and the magnet spins about the central axis.
- The magnet's axis swings back and forth about the central axis within the plane of the paper.
Correct answer: The magnet's axis is tilted to the central axis and the magnet spins about the central axis.
Solution
EMF is induced only when the flux through the coil changes. If the magnet is tilted at a fixed angle to the central axis and merely spins about that axis, the geometry relative to the coil (and hence the flux through it) stays constant in time, so no EMF and no current. Back-and-forth translation, up-and-down translation, and swinging all change the flux, so they induce current.
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