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A small, circular washer of radius a 0.750 cm is held directly below a long, str

ID: 1786960 • Letter: A

Question

A small, circular washer of radius a 0.750 cm is held directly below a long, straight wire carrying a current of I = 14.5 A. The washer is located h = 0.515 m above the top of a table (see figure below). Assume the magnetic field is nearly constant over the area of the washer and equal to the magnetic field at the center of the washer. (a) If the washer is dropped from rest, what is the magnitude of the average induced emf in the washer over the time interval between its release and the moment it hits the tabletop? nV (b) What is the direction of the induced current in the washer? clockwise O counterclockwise

Explanation / Answer

Flux = A*B = A*(u0i/2pi r)

time taken to drop t = sqrt(2h/g) = sqrt(2*0.515/9.8) = 0.3241 s

Average emf = A*(B2-B1)/t = pi*0.75e-2^2*2e-7*14.5*(1/0.75e-2 -1/(0.515+0.75e-2))/0.3241

= 2.078*10^-7

= 207.8 nV answer

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