A square loop of wire with a small resistance is moved withconstant speed from a

Question

A square loop of wire with a small resistance is moved withconstant speed from a field free region into a region of uniform Bfield (B is constant in time) and then back into a field freeregion to the left. The self inductance of the loop isnegligible. Select all correct answers: A) While the loop is entirely in the field, the emf in the loop iszero. B) Upon leaving the field, a clockwise current flows in theloop. C) When entering the field the coil experiences a magnetic force tothe right. D) When leaving the field the coil experiences a magnetic force tothe left. E) Upon entering the field, a clockwise current flows in the loop.

Explanation / Answer

Let's look at it from Faraday and Lenz's Law As the loop moves in, it becomes filled with a B field. Constant area, so flux increases. This is similar to aN pole moving into coil from behind the page To oppose the change, the coil would set up a N pole pointing intopage Use right hand rule, current flows clockwise while goinginto B field E) is true By opposite reasoning, current flows counter-clockwisewhile going out of B field B) is false By energy conservation, the coil won't induce acurrent by itself as it moves in or out of the B field, so a forcehas to make this happen. The magnetic force is opposite towhatever direction the applied force goes. Into the B field --> Magnetic force is to the right Out of B field --> Same C) is true D) is false Finally, A) is true because if it's entirely in the B field, the flux is not changing,so no induced EMF

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