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Re: [Phys-l] Thermal heating in a resistor?




On May 15, 2010, at 10:25 AM, Herbert Schulz wrote:


On May 15, 2010, at 10:12 AM, Tony wrote:

Why is the formula P=I^2V used to describe thermal power loss in resistor
instead of P=IV or P=V^2/R? Is this just the textbook author's choice of
formula?
-Tony

Howdy,

Assuming an ``ohmic'' material you have V=IR then the three variations P=I^2R (not P=I^2V), P=IV and P=V^2/R are identical. Which one you use will depend upon which one of the three variables (V, I and R) is being held constant. E.g., if you hold R constant then if V varies so will I so it is not obvious what happens if you write P=IV it is NOT obvious what happens with P while writing it as P=I^2R (thinking of varying I) or P=V^2/R (thinking of varying V) makes it clear that it is a quadratic dependence.


Howdy,

Too many ``not obvious'' in that sentence. it should be:

E.g., if you hold R constant then if V varies so will I so it is not obvious what happens when you write P=IV while writing it as P=I^2R (thinking of varying I) or P=V^2/R (thinking of varying V) makes it clear that it is a quadratic dependence.

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)