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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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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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 Winter '08
 Zieve
 Physics, Energy, Electric charge, real battery

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