Some of you thought that the energy in the capacitor is proportional to the current. If this were the case, the energy in the capacitor when it was fully charged would be zero, since the current is zero then. You have to use the formula for energy in a capacitor. Substituting the time dependence for q, you can solve for t. A 20-turn circular coil 20 cm in diameter lies in the xy plane.
T passes through the coil.
Lots of silly, careless mistakes made on this problem. Some of you confused the diameter with the radius. Others forgot to take into account that the 20 turns amplifies the magnetic moment. Recognizing that the coil has the area of a circle, this value comes out to be 4.775 A-m 2 . You are now ready to express u as a vector, but you have to remember the right-hand rule.