Midterm 2B Solutions

Midterm 2B Solutions - PHYSICS 9C MIDTERM 2 May 15, 2006...

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1 PHYSICS 9C MIDTERM 2 May 15, 2006 LAST NAME: FIRST NAME: SECTION: Problem 1 (2 points). If the potential profile has a form of stairs, at which points the electric field due to such profile reaches its largest values? A. At the top of stairs. B. At the bottom of stairs. C. At all points for which the potential remains flat. D. At all points for which the potential jumps. E. Electric field due to such profile will be constant and therefore does not reach its maximum anywhere. Solution: Since E(x)=-dV/dx, the electric field will go to infinity at each point where the potential jumps. Answer D. Problem 2 (2 points). If you stretch a copper wire by 3 times what happens to its resistance? A Increases 3 times B Increases 9 times C. Decreases 3 times D. Decreases 9 times E. Does not change Solution: Since R= ρ *L/A, the length L increases 3 times, and at the same time A decreases 3 times (to keep the volume of the wire V=LA constant), the resistance will increase 9 times. Answer B. Problem 3 (2 points). A charge is brought onto parallel plate capacitor, after which the distance between the plates is halved. The voltage across the capacitor is A. Doubled B. Halved C. Does not change D. Increases 4 times E. Decreases 4 times. Solution: The voltage across the capacitor V=Q/C. If the distance between the plates is halved, C new = ε 0 *A/d new = 0 *A/(d/2)=2C, i.e. the capacitance is doubled. As a result the voltage is halved. Answer B. B
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2 Problem 4 (2 points). A real battery is working in a circuit. Is it possible that the voltage across it becomes larger than its EMF?
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This note was uploaded on 11/11/2011 for the course PHY 9C taught by Professor Zieve during the Spring '08 term at UC Davis.

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Midterm 2B Solutions - PHYSICS 9C MIDTERM 2 May 15, 2006...

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