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 right. The self inductance of the loop is negligible.

True False When leaving the field the coil experiences a magnetic force to the left.
True False  Upon entering the field, a clockwise current flows in the loop.
True False  When entering the field the coil experiences a magnetic force to the left.
True False  While the loop is entirely in the field, the emf in the loop is zero

(If you can't see the figure, the loop is going toward the right and the field that it is going into is pointing out of the page.)

Explanation / Answer

1) True
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.
We found in (2) that clockwise current produces magnetic force towards right. By similar reasoning, anticlockwise current will produce magnetic force to the left.

2)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.

2) False
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.


4) True
When the loop is entrirely in the field, then there is no change in magnetic flux through it. Therefore, emf is zero.

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