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fall_05_exam3_sol_phy2049

fall_05_exam3_sol_phy2049 - PHY2049 Fall 2005 Prof Andrew...

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PHY2049 Fall 2005 1 Prof. Andrew Rinzler Prof. Paul Avery Nov. 21, 2005 PHY2049, Fall 2005 Exam 3 Solutions 1. An RL circuit with resistance 100 is connected in series to a battery with EMF 12V. If the current builds up to 1/3 of its final steady-state value in 6.0 sec, what is the inductance L in Hen- rys? (1) 1480 (2) 550 (3) 600 (4) 6.8 (5) 18 In an RL circuit with initial current zero, the current vs time relation is ( ) / max 1 tR L i i e = . Thus when max /3 i i = , we get ( ) / ln 3/ 2 tR L = or ( ) / ln 3/ 2 1480 L tR = = H. 2. A loop lies in a magnetic field with the loop°s normal vector perpendicular to a magnetic field, B, as shown in the diagram. An external source (not shown) causes a current to begin to flow in the loop. What does the loop do? (1) Rotates around vertical axis (2) Rotates around horizonal axis (3) Rotates around axis perpendicular to page (4) Moves left (5) Moves up From the right hand rule, we see that the right side of the loop is pushed down and the left side is pushed up. Thus the loop can be seen to rotate around a vertical axis. B
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PHY2049 Fall 2005 2 3. A long wire is bent 12 times around in the shape of a circle of radius 8 cm. If it spins 8 times a second around its diameter in a magnetic field of 3.0 T, what is the amplitude (in volts) of the emf generated in the loop?
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