bengaltiger14
New member
How do we know when to use either of those equations? For example, one problem below uses V^2/R in it's solution while the next problem uses I^2 x R.
Question about using V^2/R vs I^2 x R when calculating power
- Thread starter bengaltiger14
- Start date
Help Support Professional Engineer & PE Exam Forum:
The first problem you're given the total current that is going through the core, so you don't know how much of it is going through the core resistance and the core inductance but the voltage across both is the same and is equal to the input voltage Vtest. So naturally it's easier to use what is given to solve the problem, which is V2. Since you are given the power is watts (real power) is only the power through the resistor. Since it's an open circuit and no current is flowing through the Coil, the Coil R and Coil X can be ignored. At the end of the first problem, you can see that they used the current to solve for the total impedance of the core (taking into account both the resistance and reactance) using the I2 * Z formula.
The second problem the voltage drops across R copper and X copper so you would have to figure out that voltage in order to use a V2 R equation but we know that the same current is going through both of the Resistor and Inductor, so it's easier to use I2 * R to solve for the answer. Again the power is given in watts (so that's only the power through the Resistor. If the power was given in VA, it's the power through both, of the power was given in VAr, then it's the power only going through the Inductor.
Basically we're using the information that we are given to determine if we can use either equation.
