SAT Subject Physics Practice Question 599: Answer and Explanation

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Question: 599

10. A battery and a variable resistor are in series with a small fan. A switch is closed, and the fan runs. When the variable resistor is moved slightly to the left, the fan slows down a little. When the variable resistor is moved slightly to the right, the fan speeds up a little. The following question is about the operation of the circuit described. If the variable resistor in the circuit is set all the way to the right, what happens in the circuit?

I. The fan runs faster.

II. The voltage decreases.

III. The current increases.

A. I only
B. II only
C. I and III only
D. II and III only
E. I, II, and III

Correct Answer: C


The correct answer is (C). When the variable resistor is moved toward the right, more of the resistance value of the variable resistor is bypassed which means there is less resistance in the circuit. A decrease in the resistance of a circuit allows more current to flow (Ohm's Law). Since there is more current flowing in the circuit the fan has more current flowing through it too, and the fan runs faster.

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