A square loop of wire with a small resistance is moved with constant speed from
ID: 1536928 • Letter: A
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. When leaving the field the coil experiences a magnetic force to the right. Upon entering the field, a clockwise current flows in the loop. Upon leaving the field, a counterclockwise current flows in the loop. When entering the field the coil experiences a magnetic force to the right.Explanation / Answer
(i) True.
When inside the field, then magnetic flux through the coil is into the plane of figure. When leaving the field, the flux increases. This means that change in flux is outside 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.
(ii) False.
Before entering the field, the magnetic flux through the coil is zero. Upon entering the field, the flux is into the plane of figure because magnetic field is into the plane of figure. So, the change in flux is into 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 outside the plane of figure. This is possible when the current is anti-clockwise.
(iii) False.
As explained above.
(iv) 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 right side. The current in the loop is anti-clockwise. For this, the current in the right side is from top to bottom.
And magnetic field is into 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 left.
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