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Unformatted text preview: Reading : Electrodynamics, Inductance Key concepts: Faradays law; Lenzs law; eddy currents; induced felds. Sel-inductance; back em; inductors; energy in the B-feld; L-R circuits. 1. The cylindrical bar magnet shown is moving to the right. Poles of the magnet are indicated, with Feld lines emanating from the north pole and returning at the south pole. Points a and b are slightly below the symmetry axis. a. What is the direction of the induced E-Feld at a ? b. Repeat for b . [Imagine circular conducting loops around the symmetry axis, passing through the points in question. What would be the directions of the induced currents?] 2*. Shown in cross-section is a long solenoid of radius R with a B-Feld into the page inside its windings, and negligible outside. A positive charge Q is at rest at a point outside the solenoid. The solenoid current is increasing at rate dI / dt . a. What direction is the induced E-Feld at the location of the charge? b. What is the magnitude of the force on the charge ? Ans : F = 1 2 nR 2 dI dt Q r . c. How would your answers change if r < R ? Ans : F = 1 2 n dI dt Qr . 3. A bar magnet is near a circular loop of copper wire which is suspended at rest by a thread as shown. a. How does the loop move if the magnet is moved toward it? b. Away from it? c. How would your answers change if the poles of the magnet were reversed? a b S N r Q B N S PHY 54 Assignment 6 Summer 2011 4. In the situation shown from above, a bar magnet is rotating counter-clockwise as shown at a small distance above a conducting disk, initially at rest, which is free to rotate about a vertical axle through its center. Does the disk move? If so how? Explain your answers. 5. A small disk magnet is dropped from rest vertically down three equally long empty cylindrical tubes, in succession. Tube A is made of a non-conductor, tube B of aluminum, and tube C of copper. It takes time T for the magnet to emerge from the bottom of the tube. Comment on the following statements....
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