eq 1. P = I^2R
eq 2. P = V^2 / R

V = 250V
I = 10A
P = 2kW

I get different answers using eq.1 and eq.2. when making R the subject:

eq 1. R = P / I^2
= 20 ohms.

eq 2. R = V^2 / P
= 31 ohms.


How do I know which equation to use, when they are suppose to give the
same answer?

Steve

 

Your problem is your numbers:

Remember P = VI

Therefore P = 250 x 10 = 2500W

Whereas you have P = 2000W

If P = 2500W then R = 25 Ohms

This can be verified by: R = V/I = 250/10 = 10 Ohms

Dave