A circular wire loop is simultaneously subjected to two changes: its radius is…
PHYSICS · Single correct · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A circular wire loop is simultaneously subjected to two changes: its radius is doubled while the external uniform magnetic field passing through it is reduced to one-half of its original value. The plane of the loop does not rotate. Which statement best describes the induced emf during this process?
(a) No emf is induced because the fractional change in area exactly cancels the fractional change in magnetic field.
(b) An emf is induced that drives current so as to reinforce the increasing flux, because the expanding area dominates over the field decrease.
(c) The direction of the induced emf cannot be predicted without additional information about how quickly the radius and the field change.
(d) The induced emf is determined only by −AdB/dt, so it has the direction associated with a decreasing magnetic field.