A conducting bar is free to slide horizontally on the rails of a conducting fram

Question

A conducting bar is free to slide horizontally on the rails of a conducting frame, as shown in the figure below. A light bulb is attached to the right end of the rails. A spatially uniform magnetic field is oriented vertically (perpendicular to the horizontal plane of the rails and bar). Initially the bar is stationary. The strength of the magnetic field begins increasing in time at a constant rate, which induces a current through the bar, frame, and light bulb. The bar begins to move due to the magnetic force exerted on it. Once the bar begins to move, the brightness of the light bulb

a. suddenly drops to zero.
b. decreases.
c. increases.
d. stays the same.

Explanation / Answer

decrease, because when the bar move, it tend to make the area become smaller, so that the rate of change of magnetic flux is smaller (F=B*S). so that it ll decrease.

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