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Unformatted text preview: dang (rtd356) HW 08 li (59050) 1 This print-out should have 33 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 3) 10.0 points A rectangular loop of copper wire of resis- tance R has width a and length b . The loop is stationary in a constant, the magnetic field B at time t = 0 seconds is directed into the page as shown below. The uniform magnetic field varies with time t according to the rela- tionship B = B cos t , where and B are positive constants and B is positive when the field is directed into the page. B B B B b a n turns The direction of the induced current in the loop when t = 2 , after the magnetic field begins to oscillate is 1. undetermined, since the current is zero. 2. counter-clockwise. 3. clockwise. correct Explanation: When t = 2 , B = B cos 2 = 0; i.e. , the field has been decreasing, and is about to change direction. The induced current will be in a direction to oppose this change; i.e. , clockwise. 002 (part 2 of 3) 10.0 points What is the expression for the magnitude of the induced current in the loop as a function of time in terms of a , b , B , , R , t , and fundamental constants. 1. I = abB R sin t 2. I = abB R sin t 3. I = ab B R sin t correct 4. I = ab B R sin t 5. I = RB ab sin t 6. I = R B ab sin t Explanation: Calculating the flux, = abB cos t. Calculating the emf , E =- d dt (negative sign not required) = ab B sin t. Using Ohms Law I = E R = ab B R sin t. 003 (part 3 of 3) 10.0 points Select a sketch a graph of the induced cur- rent I vs t , taking clockwise current to be positive. 1. t I 2 3 2 2 5 2 3 dang (rtd356) HW 08 li (59050) 2 2. t I 2 3 2 2 5 2 3 correct 3. t I 2 3 2 2 5 2 3 4. t I 2 3 2 2 5 2 3 5. t I 2 3 2 2 5 2 3 6. t I 2 3 2 2 5 2 3 Explanation: The graph is a sine wave with period 2 . t I 2 3 2 2 5 2 3 004 10.0 points A circular coil enclosing an area of 96 . 8 cm 2 is made of 170 turns of copper wire as shown in the figure. A 7 . 8 resistor is inserted in the copper wire. Initially, a uniform magnetic field of magnitude 3 . 34 T points horizontally from left-to-right through the perpendicular plane of the coil. When viewed from the right the coil is wound counter-clockwise. R Magnetic Field B ( t ) During a 1 . 3 minute time interval the field uniformly changes at a constant rate, until a reversed field is reached equal in magnitude to the initial field. How much charge flows through the coil?...
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This note was uploaded on 06/15/2010 for the course PHY 303L taught by Professor Turner during the Fall '08 term at University of Texas at Austin.
- Fall '08