Derive the equation relating the total charge q that flows through a search coil to the magnetic-field magnitude b. the search coil has n turns, each with area a, and the flux through the coil is decreased from its initial maximum value to zero in a time δt. the resistance of the coil is r, and the total charge is q = iδt, where i is the average current induced by the change in flux. (use any variable or symbol stated above as necessary.)

q =

(b) in a credit card reader, the magnetic strip on the back of a credit card is rapidly "swiped" past a coil within the reader. explain, using the same ideas that underlie the operation of a search coil, how the reader can decode the information stored in the pattern of magnetization on the strip.