A loop crossing a strip of field.

A square or circular loop sliding through a strip of field, with flux and emf plotted against position as it enters, crosses and leaves.

Pause the loop where it is wholly inside the field and read zero emf; show the square gives a constant emf while entering but the circle does not.

Expecting a current while a loop sits inside a uniform field.

• Lenz's law (1834): the induced emf has the polarity that would drive a current opposing the change in flux that produced it. This is the minus sign in Faraday's law.
• An N pole pushed towards a coil raises the flux, so the induced current makes the near face of the coil an N pole, which repels the magnet. Seen from the magnet's side the current runs anticlockwise.
• Pulling the N pole away lowers the flux, so the current reverses (clockwise from the magnet's side) and the near face becomes an S pole, which pulls back on the retreating magnet.
• With an open circuit no current flows, but an emf still appears across the open ends, and Lenz's law still gives its polarity.
• If the induced current aided the change, a small push would make the magnet speed up by itself, giving energy for nothing. Lenz's law is what energy conservation demands.
• In the real case you must push against the repulsion; the work you do appears as Joule heat from the induced current.
• Example 6.4: a loop entering a field region gains flux and one leaving it loses flux, so the currents circulate in opposite senses. A loop wholly inside or wholly outside a uniform field region has no induced current.
• Example 6.5: a still loop between fixed magnets, however strong, has no current, since the flux is not changing. A loop moving through a steady electric field gets no current either. A rectangular loop leaving a field region at steady speed has a constant emf; a circular one does not, as its area inside the field changes at a varying rate.

Teaching this idea? Put the simulation on your own page or LMS with this code.