A conducting bar slides without friction on two parallel horizontal rails 50 cm apart. The resistance of the bar and the rails i
s assumed to be constant and equal to 0.1 Ω. A uniform magnetic field is perpendicular to the plane of the rails. A force F = 0.08 N is required to keep the bar moving at a constant speed of 0.5 m/s. What is the magnitude of the magnetic field?