A conducting bar is free to slide horizontally on the rails of a conducting frame,as shown in the figure.A lightbulb 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 lightbulb.The bar begins to move due to the magnetic force exerted on it.Once the bar begins to move,the brightness of the lightbulb
A)stays the same.
B)increases.
C)decreases.
D)suddenly drops to zero.

Question

A conducting bar is free to slide horizontally on the rails of a conducting frame,as shown in the figure.A lightbulb 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 lightbulb.The bar begins to move due to the magnetic force exerted on it.Once the bar begins to move,the brightness of the lightbulb 
A)stays the same. 
B)increases. 
C)decreases. 
D)suddenly drops to zero.

Quizplus · Accepted Answer

As the conducting bar moves, the magnetic field passing through the circuit changes, causing the induced current and the corresponding magnetic force to also change. This change in current and force results in a change in the speed of the bar, causing the brightness of the lightbulb to decrease. Therefore, the correct choice is C - decreases.

A Conducting Bar Is Free to Slide Horizontally on the Rails