The smallest time interval in which the magnetic field can be turned on or off to induced the emf is 47.5 s.
The emf induced in the coil is calculated as follows;
emf = dФ/dt
where;
0.12 = 5.7/dt
dt = 5.7/0.12
dt = 47.5 s
Thus, the smallest time interval in which the magnetic field can be turned on or off to induced the emf is 47.5 s.
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