Discussion
A circular coil expands radially in a region of magnetic field and no electromotive force is produced in the coil. This can be because
Answer: B
No emf is produced when the magnetic field is in the same plane as the coil, whether or not it varies.
An emf is induced only if the magnetic flux through the coil changes. Flux is the field component perpendicular to the coil's plane multiplied by the area. If the field lies entirely in the plane of the coil, no field lines pass through it, so the flux stays zero however much the coil expands or the field changes.
A constant field with a perpendicular component, as in options A and D, would give an increasing flux as the area grows, and so an emf. A perpendicular component that is increasing would add to the change in flux rather than cancel it.
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