A conducting bar slides without friction on two parallel horizontal rails that are 25.0 cm apart and connected by a wire at one end. The resistance of the bar and the rails is constant and equal to 0.100 Ω. A uniform magnetic field is perpendicular to the plane of the rails. A 0.0450-N force parallel to the rails is required to keep the bar moving at a constant speed of 0.750 m/s. What is the magnitude of the magnetic field?