P = iv & v = ir

[Fletcher Moore]

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MCAT questions like to ask about proportions. Suppose you get a purely verbal question (no calculation) that uses the concepts: voltage, current, power, & resistance. In the power formula, the current is inversely proportional to the current; in the resistor formula, the voltage is proportional the current. How do you know which formula is relevant?

I assume these are two different currents and/or voltages, measured in different ways, but I do not really understand. (And I was a physics major ; ; )

 

[addo]

They are different equations.
P=IV will give you power
V=IR will give you voltage
Since I=V/R, you can also rearrange the power equation to P=V^2/R, or P=I^2R.
If a question give you voltage and current, and asks for power just use the first equation. If the question asks what is the power dissipated by a resistor use P=V^2/R or P=I^2R

 

[Rabolisk]

Proportionality assumes that something is constant. V and I are proportional if R is held constant. I and V are inversely proportional if P is held constant.

For example, if you double the current through a resistor, then the voltage across the resistor must double since we are talking about the same resistor. This in turn quadruples the power dissipated by the said resistor.