PHYSICS · Subjective · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
Two identical coaxial circular coils A and B, each of N turns and radius r, are separated by a fixed distance d along their common axis. Coil A is connected to a battery through a key, and coil B is connected to a sensitive galvanometer.
(i) Immediately after the key in coil A is pressed to establish a steady current I0, what is the direction of the first deflection of the galvanometer connected to B? Explain your answer.
(ii) After the current in A attains the steady value I0, the key is opened and the current in A falls according to I(t)=I0e−t/τ. Derive an expression for the magnitude of the induced emf in coil B as a function of time.
(iii) During the decay of current in A, determine whether the force between the two coils is attractive or repulsive. Justify your conclusion.
(iv) Prove quantitatively that the mechanical work required (or released) in part (iii) per unit charge flowing in coil B is equal in magnitude to the induced emf you obtained in part (ii).