practice_midterm - Physics 7D Midterm Spring 2005 Seat...

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Physics 7D Midterm Name: Spring 2005 Student ID: Seat number Signature: DO NOT OPEN THIS EXAM BOOKLET UNTIL INSTRUCTED. 1. Leave all backpacks, cell phones, pagers, and other belongings at the front of the room. 2. This exam is 1 hour 20 minutes long, closed book, and closed notes. 3. You may use 1 side of an 8.5” x 11” sheet of handwritten notes and an approved calculator. 4. For Multiple choice problems, t ransfer all answers to the table below . Credit will be given only for answers in this table. No partial credit will be given. 5. Free response: Partial credit will be given for neatly written, substantial progress if the grader can follow your reasoning and there is an absence of irrelevant or wrong work. 6. The problems vary in difficulty. For free response questions, part (A) is not necessarily required for part (B), etc. Attempt easier problems first. Multiple Choice (2 points) Answer Free Response (8 points) Score 1 d c 2 b c 3 e c 4 c a 5 b b 6 b a 7 c e 8 a c 9 e b 10 a a 11 c a 12 c e 13 b c 14 b e Total 4 MC total 1 2 3
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Multiple Choice Problems-put correct answer on front page 1) Consider a parallel plate capacitor connected to a battery that maintains the plates at a potential difference Δ V and a slab of dielectric with the same area and with thickness equal to the gap between the plates. Which of the following is a true statement regarding the situation as the dielectric is inserted into the capacitor at small constant velocity by some external force? a) The absolute magnitude of the work done by the battery is equal to the absolute magnitude of the work done by the force that is moving the dielectric in at constant velocity. No, since total work is not zero, and the work or battery is opposite in sign to that of the external force. b) The energy stored in the battery decreases during the process of inserting the dielectric. Yes, it does work. c) The work done by the external force is negative. Yes, the plate is pulled in by electric force, and the external force is against the displacement. d) Exactly 2 of the above statements 2) Two conducting plates of area A are parallel to each other and separated by a distance d, with A>>d 2 , so edge effects can be ignored. If each plate has a charge Q on it, what is the magnitude of the force one plate exerts on the other? a) Q 2 /A ε 0 b) Q 2 /(2A ε 0 ) Field of one sheet at the other is Q/2A ε 0 , charge is Q c) Q 2 /(4 πε 0 d 2 ) d) none of the above 3) Consider two charges, each of magnitude Q, at (x,y) = (1,0), and (0,1). Which of the following charges, when added at the indicated position, would result in a field at the origin equal to zero? a) 2Q at (-1 , -1) b) 2Q at (- 2, - 2) c) Q at (-1,-1) d) –Q at (-1,-1) e) none of the above Magnitude of original E is kQ/(1 2 +1 2 ), none of these cancels that field. 4) Assuming that 3 charges (Q,Q,-2Q) start very far from each other, how much work must be done by an external force to make a configuration with Q fixed at (a,0,0), Q at (0,a,0) and -2Q at (0,0,a)?
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This note was uploaded on 08/22/2009 for the course PHYS 7A taught by Professor Guerra during the Spring '08 term at UC Irvine.

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practice_midterm - Physics 7D Midterm Spring 2005 Seat...

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