A conducting bar moves on rails to the right with constant velocity v as shown,

Question

A conducting bar moves on rails to the right with constant velocity v as shown, and the uniform constant magnetic field B is directed into the page. This results in a current in the circuit and energy being dissipated in the resistor. The energy being dissipated in the resistor comes from: (a) the uniform, constant magnetic field B; (b) the external agent that is pushing the bar; (c) the kinetic energy of the moving bar; (d) a non conservative electric field; or (e) the inertia of the conduction electrons in the resistor. Explain why.

Explanation / Answer

A motional emf occurs when a conductor is moved through a magnetic field. When we slide the bar to the right at speed v, an induced emf is set up across the bar, and an induced current will flow.

The bar acts like a battery, so the current flows counter-clockwise. But now the current in the moving bar feels a force from the magnetic field. By right hand rule, this force is directed to the left. The force opposes the direction of motion of the rod. The rod will stop, unless a larger force keeps pulling it to the right. As the rod slides, energy is dissipated in the resistor. The energy comes from the force pushing the rod to the right.

Answer: (b) the external agent that is pushing the bar

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