A copper bar of length L and negligible resistance slides at constant speed v along metal rails. The bar is moving through a region in which there is a uniform magnetic field B directed out of the page. A voltmeter is connected across a resistor of resistance R. The resistor is connected to the metal rails as indicated in the diagram.

Determine the magnitude of the magnetic force acting on the bar and indicate the direction of this force on the diagram. Your answer should be in terms of the given variables and known constants.