In the аrrаngement shоwn, а cоnducting bar оf negligible resistance slides along horizontal, parallel, frictionless conducting rails connected as shown to a 3.0-Ω resistor. A uniform 2.0-T magnetic field is perpendicular to the plane of the paper. If L = 50 cm, at what rate is thermal energy being generated in the resistor at the instant the speed of the bar is equal to 4.2 m/s?