A conducting bar slides without friction on two parallel horizontal rails that are $50$cm apart and connected by a wire at one end. the resistance of the bar and the rails is constant and equal to $0\mathrm{.}1$ ohms. a uniform magnetic field is perpendicular to the plane of the rails. a $0\mathrm{.}08$N force parallel to the rails is required to keep the bar moving at a constant speed of $0\mathrm{.}5$m$/$s $.$ what is the magnitude of the magnetic field?