TPRH Science Workbook #280

[aspiringdoc09]

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# 280. The resistance of a variable resistor in a circuit is decreased, causing the current in the circuit to increase. Which of the following must be true?

A. The power of the circuit increases
B. The power of the circuit decreases
C. The voltage of the circuit increases
D. The voltage of the circuit decreases

I chose C which is wrong, but my issue is how was I suppose to know that the voltage is fixed and they are asking about power? Is it because V acts as a constant when you manipulate I and R? I guess it makes sense that A is correct, but if you are quickly doing these questions, then C makes sense too.

 

[Buttafuoco]

I think it's one of those things you just have to know for basic physics and the MCAT- that the circuits are ideal circuits with a magic battery with a fixed voltage and with wires with negligible resistance unless stated otherwise.

 

[aspiringdoc09]

I think it's one of those things you just have to know for basic physics and the MCAT- that the circuits are ideal circuits with a magic battery with a fixed voltage and with wires with negligible resistance unless stated otherwise.

Yeah, but it could have easily been C, right? I usually get these questions. I wasn't expecting them to be point at power. Of course, I completed this problwm in less than 20 secs, so that definitely invites room for error.

 

[Buttafuoco]

I guess you kind of have to think about things by the way these problems are set up. In order to use V = IR in the way we do, you have to have a constant voltage from the battery. If we were just given resistance and voltage wasn't fixed there could be a whole host of possible values for V and I.

 

[pfaction]

P=IV=I^2*R=V^2/R

Voltage is tied to the battery inherently, can't increase that. If you use P=I^2*R the answer A is evident.

 

[aspiringdoc09]

Ok. I guess I never really thought about the battery supply a constant voltage. MCAT really makes you think about stuff like that. Thanks guys!