Example

Question:

Figure 6.7 shows planar loops of different shapes moving out of or into a region of a magnetic field which is directed normal to the plane of the loop away from the reader. Determine the direction of induced current in each loop using Lenz’s law.

Solution:

(i) Magnetic flux through rectangular loop abcd increases (loop moves into field region). Induced current must flow along path bcdadab (i.e. circular: b→c→d→a→b), so as to oppose increasing flux.
(ii) Due to outward motion, flux through triangular loop abc decreases. Induced current flows along bacb (b→a→c→b), to oppose loss of flux.
(iii) As magnetic flux decreases when irregular loop abcd leaves field region, induced current flows along cdabc (c→d→a→b→c), to oppose the flux reduction.
Note: No induced current while loops are entirely inside or outside the magnetic field region.