Stored energy in a system of inductors. An infinite solenoid with n turns per unit length and radius a carries a current I. A shorter solenoid, of length d and radius b, is placed over the first as shown in Figure 9.70a. The two solenoids do not actually touch. Figure 9.70b shows the relation of the two solenoids from a side view. The second solenoid also carries a current I and has a total of N turns. Assume both solenoids to be long (i.e. infinite for calculation purposes) and that the flux densities of the solenoids are in the same direction. Calculate the change in the energy in the system when the second solenoid is slipped over the first.
