A conducting rod of length 0.20 m makes slides without friction on conducting metal rails, as shown in the sketch. The apparatus is in a uniform magnetic field that has magnitude B = 0.400 T and that is directed into the page. The resistance of the circuit is a constant R = 5.00 Ω.
What magnitude and direction (to the left or to the right) of the external force must be applied to the bar to keep it moving to the right at a constant speed of 12.0 m/s?