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Midterm 2A Solutions

# Midterm 2A Solutions - A PHYSICS 9C MIDTERM 1 LAST NAME...

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1 PHYSICS 9C MIDTERM 1 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 bottom of stairs. B. At the top of stairs. C. At all points for which the potential jumps. D. At all points for which the potential remains flat. 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 C. Problem 2 (2 points). If you stretch a copper wire by 5 times what happens to its resistance? A Increases 5 times B Increases 25 times C. Decreases 5 times D. Decreases 25 times E. Does not change Solution: Since R= ρ *L/A, the length L increases 5 times, and at the same time A decreases 5 times (to keep the volume of the wire V=LA constant), the resistance will increase 25 times. Answer B. Problem 3 (2 points). A charge is brought onto parallel plate capacitor, after which the distance between the plates is doubled. 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 doubled, C new = ε 0 *A/d new = 0 *A/(2d)=0.5 C, i.e. the capacitance is halved. As a result the voltage is doubled. Answer A. A

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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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Midterm 2A Solutions - A PHYSICS 9C MIDTERM 1 LAST NAME...

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