fall_07_exam3_sol_phy2049

fall_07_exam3_sol_phy2049 - PHY2049 Fall 2007 Prof. Yasu...

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PHY2049 Fall 2007 1 Prof. Yasu Takano P r o f . P a u l A v e r y Nov. 19, 2007 Exam 3 Solutions 1. ( Prob. 30.60 ) A coil with inductance 1.9 H and a 10 Ω resistor are connected in series to an ideal battery of voltage 95 V. At 0.10 sec after the connection is made, what is current in the cir- cuit (in amps)? (1) 3.9 (2) 5.6 (3) 8.1 (4) 0 (5) 1.4 The current in a LR circuit is given by ( ) / 1 tR L V R ie =− . Plugging in values for t, R, L and V gives I = 3.9 A. 2. ( F2005 exam ) A tightly-wound circular wire loop of 6 cm radius and 8 turns rotates around its diameter at 55 Hz inside a solenoid of radius 18 cm and 4.2 T B field. If the wire loop has a re- sistance of 0.067 Ω , what is the rms current induced in the loop in amps? (1) 1,390 (2) 330 (3) 173 (4) 12,500 (5) 560 The induced emf is given by Faraday’s law as / B Nd dt = −Φ E , where 2 cos B rB t π ω Φ= . Tak- ing the derivative, and using = 2 f, we get 2 2s i n 2 N r fB ft ππ = , so the rms emf is 22 rms 2 Nr f B = , which includes a factor of 1/ 2 to convert from peak to rms. Using the values given, we get rms 92.8 V = and rms rms / 1390 I RA = = .

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PHY2049 Fall 2007 2 3. A series RLC circuit is driven by an AC emf source of a fixed frequency. In order to make this circuit more inductive you need to decrease: (1) its natural frequency (2) its resistance (3) its inductance (4) its capacitance (5) the amplitude of the emf The natural (angular) frequency is given by 1/ LC ω = . Decreasing it is equivalent to increas-
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This test prep was uploaded on 04/17/2008 for the course PHY 2054 taught by Professor Avery during the Spring '08 term at University of Florida.

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fall_07_exam3_sol_phy2049 - PHY2049 Fall 2007 Prof. Yasu...

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