Motionele emf

Consider a conducting rod of length L moving with a constant velocity v in the magnetic field B that is pointing in the plane of the screen.

The magnetic force acting on a positive charge within a conducting rod is upward, resulting in opposite charges accumulating on either ends of the rod. This creates an electric field, in the downward direction.

The charges keep accumulating until the electric force is large enough to balance the magnetic force. The potential difference across the rod is present as long as it moves.

If this rod forms a closed conducting loop with a stationary U-shaped conductor, the free charges from the rod redistribute along the loop. This generates a current in the closed loop, and the moving rod generates a motional electromotive force known as a motional emf.

Here, the magnetic flux passing through the closed loop is equal to the magnetic field times the area of the loop. According to Faraday's law, the rate of change of magnetic flux is equal to the magnitude of motional emf.