# Faraday's and Lenz's Laws - assignments a Change the loop...

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Dr. Chang Magnetic Flux Magnetic Flux through a wire loop is determined by the product of the loop plane area and the external B field component perpendicular to the loop plane. Φ B = B perpendicular A loop Another expression for the magnetic flux is Φ B = B . A = B A cos , where is the angle between the B vector and the normal Ѳ Ѳ to the surface of the loop. Φ B is positive if both B and A normal point in the same direction or < 90 Ѳ o . If >90 Ѳ o , Φ B will be negative. The above expressions assume the magnetic field is uniform over the loop plane. If not, then Φ B = Faraday’s Law of Induction says that a change in Φ B induces an average emf E in the loop. This emf then drives an “induced” current around the loop. The magnitude of E is given by l E l = N where N is the number of loops. There are several ways to change the flux. Since Φ B = B perpendicular A loop , any change in either B or A will lead to a change in flux . Here are some typical examples you would encounter in solving problems in this area (Look for these in your HW

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Unformatted text preview: assignments): a) Change the loop plane orientation with respect to the B field b) Expand or shrink the loop plane area c) Change the B field strength d) Flip or rotate the B field direction e) Move the loop into or out of a uniform B field region The faster the change in flux, the larger is the average induced emf E . The induced current I is then given by Ohm’s Law , I = E /R, where R is the resistance of the wire loop. Lenz’s Law says an induced current will flow in the loop in such a direction (CW or CCW) that it creates a B field of its own to oppose any change in magnetic flux through the loop plane. Therefore use this to find the direction in which the induced current will flow in the loop when the flux in the loop changes . Once you know the direction of I , then it is easy to set the polarity of E if you remember I leaves from the high or + side of E ....
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Faraday's and Lenz's Laws - assignments a Change the loop...

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