Physics Power formulas

[wrigley09]

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Hey guys.

I get confused with the P=iV vs P=i^2R equations. Are they interchangeable, or do you use one over the other under certain conditions?

 

[Isoprop]

Depends on the situation. For example, it both of your equations, it says that power is proportional to current and power is proportional to the square of the current. So if you double the current, what happens to the power?

The answer is it depends.

If voltage is kept constant, then P is proportional to I. But if the resistance is kept constant, then P is proportional to I^2.

With equations, you can solve for 1 variable if you know all others. If the MCAT asks about relationships between 2 variables (eg double x, what happens to y?), all the other variables need to be held constant during the change.

 

[wrigley09]

Oh, I figured it was something relatively simple that I was missing 🙂 Thanks!

 

[ahamdi2]

just Remember P=IV and if u know Ohm's law V=IR you can substitute IR for V in the power formula to get P=I*IR = I^2R

 

[GRod18]

for electrical power you would use those.

For mechanical power you would use P = Work/Time P = Force x change in velocity.

However units are all the same Watts