FinalSolutions

FinalSolutions - PHYSICS 9C FINAL EXAM March 21, 2008 I...

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Q Q Q Q Q PHYSICS 9C FINAL EXAM March 21, 2008 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. (two points) Five identical charges are arranged as shown in the Figure. What is the value of the electric flux through a spherical Gaussian surface surrounding the central charge? A. 0 B. Q/ ε 0 C. 2Q/ ε 0 D. 3Q/ ε 0 E. 5Q/ ε 0 Solution. The Gauss law states that the flux of the electric field thru the surface is equal to the charge enclosed by the surface. Answer B.
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A B C D E V(x) Problem 2. (two points) The figure shows the dependence of electric potential as a function of coordinate. In which region the value of the electric field is greatest? Solution. Since the electric field is given by the derivative of the potential with respect to coordinate, () dV Ex dx =− it is the region B where the derivative is largest. Problem 3. (two points) Two wires are made of the same material and have the same length but different radii. They are joined end-to-end and a potential difference is maintained across the combination. Of the following, the quantity that is the same for both wires is: A) potential difference across each wire B) current inside each wire C) current density inside each wire D) resistance of each wire E) electric field inside each wire Solution. Since the radii of both wires are different, hence the resistances are different. Since the combination looks like two serially connected resistors, the current in the circuit should be the same. The potential difference across each wire is different because resistances are different while current is the same. Electric field inside each wire is also different because potential differences are different while wires have the same length. Current densities inside each wire are different because cross sections of the wires are different. The only correct answer is B.
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ε 1 ε 2 R 1 R 2 I V A V B + + - - Problem 4. (two points) In the fragment of a circuit shown ε 1 =5 V, ε 2 =10 V, R 1 =3 Ohm, R 2 =5 Ohm and the current is 1 A. What is the potential difference between the points A and B: V A -V B = A. +5 V B. – 3V C. +1 V D. +3 V E. -12 V Solution. Starting from point A and walking towards point B we have: V A -E 1 -R 1 *I+E 2 -R 2 *I=V B . This gives V A -5- 3*1+10-5*1=V B or V A -3=V B , or V A -V B =+3 Volts. Problem 5. (two points) Consider an RL circuit shown in the figure. While switch is in position A the capacitor is being charged. Then switch is thrown in position B. What happens to the current in the LC part of the circuit?
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This note was uploaded on 04/07/2008 for the course PHY 9C taught by Professor Zieve during the Winter '08 term at UC Davis.

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FinalSolutions - PHYSICS 9C FINAL EXAM March 21, 2008 I...

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