7. One day, you try to make an inductor by winding a wire of length / and radius a in the shape of a helix whose circular cross-section has radius r. a. Show that its self-inductance is given by: 4a What would be the length of the inductor and how many turns will it have if the windings are to be close together as possible without overlapping? b. If it carries current /, calculate the energy stored in the inductor