In: Physics
A loop of wire has a radius of 2.0 cm and is in a constant magnetic field of 0.88 T directed into the plane of the paper. If the loop collapses so the radius of the circle shrinks to zero in a time of 0.35 s, a) find the magnitude of the average induced emf in the loop. b) If the loop is part of a larger circuit, in which direction does the induced current flow as the loop closes.
induced emf = - d/dt (mag flux)
mag flux = Br2
emf = d/dt ( Br2) = - 2r x dr/dt
we will find dr/dt
at t=0 -> r=2 cm
at t=0.35 -> r=0
dr/dt = change in radius / time = -2/0.35 cm/s = -2/35 m/s
r=2cm = 0.02 m
B=0.88 T
putting these values in the equation of emf,
we get emf = - 2r x dr/dt
= - 2 x 3.14 x 0.02 x 2/35 V = 6.3 mV
the direction of magnetic field willl be clockwise as shown in figure. by lenz law, induced current must produce a magnetic field which is in the same direction of initial B-field to resist the decrease in flux