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FinalExamSolution - PHYSICS 9C FINAL EXAM March 15, 2011 I...

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PHYSICS 9C FINAL EXAM March 15, 2011 I certify by my signature below that I will abide by the UC Davis Code of Academic Conduct. This includes not copying from anyone else’s exam not letting any other student copy from my exam not discussing this exam with any student who has not yet taken it, nor providing any information, written or oral, that might get to a student who has not yet taken it. Anyone suspected of cheating will be automatically reported to Student Judicial Affairs. LAST NAME: FIRST NAME: STUDENT ID: (P R I N T) (P R I N T) (LAST 4 DIGITS) Signature: Problem 1. (2 points) Which of the following graphs depicts the current versus voltage relationship for a device that obeys Ohm's Law? A ) B) C ) D ) E ) Solution: Ohm law states that i=V/R therefore Answer B Problem 2. (2 points) In a RC series circuit with E = 8.0 V, R = 100 k Ω , and C = 25 μ F , calculate how long it takes for the capacitor to become charged to half of its final value. A) 2.50 s. B) 1.73 s. C) 0.5 s. D) 0.4 s. E) 0.6 s. Solution: Since q(t)=EC(1-exp(-t/RC)), its final value Q=EC. To find the time, we have q(t)/Q=1/2=1-exp(-t/RC), or exp(-t/RC)=0.5, t/(RC)=-ln(0.5)=ln(2) t=RCln(2)=10 5 *25*10 -6 ln(2)=2.5ln(2)=1.73 s. Answer B i V i V i V i V i V + - C R E
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Problem 3. (2 points) Electrons are going around a circle in a counterclockwise direction as shown. At the center of the circle they produce a magnetic field that is: A) to the left B) out of the page C) into the page D) to the right E) zero Solution: Since these are the electrons, the current (as the flow of positive charge) goes clockwise and produces magnetic field into the page. Answer C. Problem 4. (2 points) When does a constant magnetic field exert a force on a charged particle? A) always B) never C) when the particle is at rest D) when the particle is moving across magnetic field lines E) when the particle is moving parallel to the magnetic field lines Solution: Since the magnetic force is determined by the cross product F =q[ vB ], only answer D is correct. Problem 5. (2 points) Two straight, parallel wires that are each 1 m long carry currents of 30 A and 50 A in the same direction. If the perpendicular distance d between the wires is 20 cm, what is the force between them? A. 1.5 mN, repulsion B. 1.5 mN, attraction C. 3.0 mN, repulsion D. 3.0 mN, attraction E. 3.0 N, repulsion Solution: The force is attractive whose value F= μ 0 /2 π * i 1 * i 2 L / d =2*10 -7 *30*50*1/0.2=1.5 mN, Answer B. i 1 i 2 d e -
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x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x xxxxxxxx B frame F push Problem 6 (two points). Two circular coils are arranged as on the figure. There is a counterclockwise current in the top coil (shown by solid line), which increases with time. What is the direction of the induced current in the bottom coil? A.
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FinalExamSolution - PHYSICS 9C FINAL EXAM March 15, 2011 I...

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