THE TRANSIENT MAGNETIC-FIELD OUTSIDE AN INFINITE SOLENOID

The electromotive force (emf) in a loop outside an infinite solenoid w ith changing current is usually calculated using the vector potential because the magnetic field outside an infinite solenoid is supposed to be zero. However, the magnetic field will only be zero for steady cur rents. A change in the applied voltage will give rise to a change in t he current, which will propagate along the solenoid in the same way as a wave on a transmission line. This gives rise to a transient magneti c field outside the solenoid. It is quite possible to calculate this t ransient magnetic field and use it in Faraday's law to calculate the e mf directly without using the vector potential. In practice, it is usu ally simpler to use the vector potential. However, care should be take n to ensure that students are not given the impression that there is n o magnetic field and that it is the vector potential that acts on char ges in the loop. We give examples of the magnetic field configuration outside an infinite solenoid for a steplike change in driving voltage and for an ac driving voltage. (C) 1996 American Teachers.