Unformatted text preview: pt] Answer these five multiplechoice questions by circling the correct letter.
Showing your work is not required, but partial credit is possible if you justify your answer. (a) [3pt] In circuit shown, three identical light bulbs are connected. Initially switch S is closed.
When switch S is opened, bulb 3 turns off. As for bulbs 1 and 2, which statement is true?
(Assume the bulbs’ resistance stays approximately constant.) A. Bulb 1 gets brighter while bulb 2 gets dimmer.
B. Both bulbs 1 and 2 get brighter.
C. Both bulbs 1 and 2 get dimmer.
D. Bulb 1 gets dimmer while bulb 2 gets brighter.
E. Bulb 1 stays the same brightness while bulb 2 gets brighter. Opening the switch increases the total resistance so less current flows through bulb 1,
and therefore less power goes to it, making it dimmer. The voltage drop across bulb 1
therefore also decreases, so the voltage across bulb 2 increases, increasing its power
consumption, making it brighter. The assumption about the resistances is not needed
for the solution. It was included only to make it easier to think about the problem. The
resistance of bulb 1 will increase as it heats up, but this only happens if it receives more
current, so this won’t change the conclusions. 6 (b) [3pt] Which of the following has the largest current flowing through it?
A. a 1m long copper wire of diameter 1 mm connected to a 10V battery
B. a 0.5m long copper wire of diameter 0.5 mm connected to a 5V battery
C. a 2m long copper wire of diameter 2 mm connected to a 20V battery
D. a 1m long copper wire of diameter 0.5 mm connected to a 5V battery
E. All of the choices have the same current. Let be the voltage, the resistance, the crosssectional area,
length, and the resistivity of copper. Then where (in u...
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 Spring '12
 ScottYost
 Physics, Resistance

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