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

Question

A square loop of wire with a small resistance is moved with constant speed from a field free region into a region of uniform B field (B is constant in time) and then back into a field free region to the left.

For each statement select T True, F False. (If the first is T and the rest F, enter TFFFF. )

A) When entering the field the coil experiences a magnetic force to the right.
B) While the loop is entirely in the field, the emf in the loop is zero.
C) When leaving the field the coil experiences a magnetic force to the left.
D) Upon leaving the field, a counterclockwise current flows in the loop.
E) Upon entering the field, a counterclockwise current flows in the loop.

Explanation / Answer

A) True
Current directions in the top and the bottom sides of the loop are in opposite directions. Both are in the same magnetic field. Therefore, forces on them are equal and opposite and cancel each other. The right side of the loop is outside the magnetic field and, therefore, experiences no magnetic force.
So, the net force on the loop is the force on the left side. The current in the loop is clockwise. For this, the current in the left side is from bottom to top.
And magnetic field is out of the plane of figure. The direction of force is in the direction of cross product of direction of current and direction of field. This direction is towards right.

B) True
When the loop is entrirely in the field, then there is no change in magnetic flux through it. Therefore, emf is zero.
______________________________________...
Read Faraday's law of induction and Lenz's law.

C) False
When inside the field, then magnetic flux through the coil is out of plane of figure. When leaving the field, the flux decreases. This means that change in flux is into the plane of figure.
Therefore, the current in the loop is such that the flux produced by it is out of the plane of figure. For that the magnetic field produced by the current should be out of the plane of figure. This is possible when the current is anticlockwise.

D)

TRUE

E)True

Before entering the field, the magnetic flux through the coil is zero. Upon entering the field, the flux is out of the plane of figure because magnetic field is out of the plane of figure. So, the change in flux is out of the plane of figure.
Therefore, the current in the loop is such that the flux produced by it is into the plane of figure. For that the magnetic field produced by the current should be into the plane of figure. This is possible when the current is clockwise.

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