Chapter 27, Problem 27.72P

CP A uniform bar has mass 0.0120 kg and is 30.0 cm long. It pivots without friction about an axis perpendicular to the bar at point a (Fig. P27.72). The gravitational force on the bar acts in the −y-direction. The bar is in a uniform magnetic field that is directed into the page and has magnitude B = 0.150 T. (a) What must be the current I in the bar for the bar to be in rotational equilibrium when it is at an angle θ = 30.0° above the horizontal? Use your result from Problem 27.70. (b) For the bar to be in rotational equilibrium, should I be in the direction from a to b or b to a?

Figure P27.72