region, there will no longer be a changing flux, because no matter where it is within the region, the same
number of field lines will always be passing through the loop. Without a changing flux, there will be no
induced EMF, so the current will stop. This is shown in Figure 20.11c .
Figure 20.11c Loop of wire with no current flowing, because it is not experiencing a changing
magnetic flux.To solve a problem that involves Lenz’s Law, use this method:
• Point your right thumb in the initial direction of the magnetic field.
• Ask yourself, “Is the flux increasing or decreasing?”
• If the flux is decreasing, then just curl your fingers (with your thumb still pointed in the direction of the
magnetic field). Your fingers show the direction of the induced current.
• If flux is increasing in the direction you’re pointing, then flux is decreasing in the other direction. So,
point your thumb in the opposite direction of the magnetic field, and curl your fingers. Your fingers
show the direction of the induced current.
Induced EMF in a Rectangular Wire
Consider the example in Figures 20.11a –c with the circular wire being pulled through the uniform
magnetic field. It can be shown that if instead we pull a rectangular wire into or out of a uniform field B
at constant speed v , then the induced EMF in the wire is found by
Here, L represents the length of the side of the rectangle that is NOT entering or exiting the field, as
shown below in Figure 20.12 .
Figure 20.12 Rectangular wire moving through a uniform magnetic field.