A long straight wire carries a current I 0. At distances a and b (a+b) from it, there are two other wires each having resistance lamda per unit, parallel to the former one, which are interconnected by a resistance R (Fig). A connector slides without friction along the wires with a constant velocity v. Assuming the resistances of the wires, the conductor, the sliding contacts, and the self-inductance of the frame to be negligable, calculate force F required to maintain velocity of the rod constant as function of time t .