# Calculating Active and Reactive powers from voltage

Discussion in 'Homework Help' started by Tys, Jan 23, 2015.

1. ### Tys Thread Starter New Member

Jan 23, 2015
1
0
Hi,
It's probably quite easy but I just cant see it!
For an ac current (frequency 3hz) in a circuit with a resistor (of value 10kohms) and a capacitor, V1 (across the generator) Vc and Vr was measured. We're meant to calculate the reactive/active power just from these values, but the formula I know, and the powers triangle, needs way more information than that.
Am I missing something obvious? Just a pointer in the right direction would help a lot.
Thanks.

2. ### Tainara New Member

Sep 10, 2014
6
2
Do you have the capacitance value? If not you can calculate it. Since you have the frequency (f) you can find the angular frequency (ω) by 2π f and then replace the result of the angular frequency(ω) in another angular frequency formula(ω) that will only let the capacitance as variable for you. With the capacitance found you can calculate the capacitive reactance and in consequence Ic and the total impedance of the circuit. With those values found I think you can use the active and reactive power formulas then.

3. ### crutschow Expert

Mar 14, 2008
12,977
3,220
It's a matter of looking at the data given you and determining how to calculate the rest. Don't be limited by trying to fit the given info into a cookbook formula.
You can calculate the current from knowing Vr and the value of R.
From that, the frequency, and Vc, you can calculate Xc and thus the value of C.
The rest should be standard calculations to get the reactive/active power.

4. ### WBahn Moderator

Mar 31, 2012
17,715
4,788
You need to look at what you need (based on the formulas and relationships you know), identify the things you don't know, and then see how you can use what you DO know in order to find the things that you DON'T know but that you NEED.

For instance, if I have two resistors in series and I want to know the power dissipated by the second resistor, then I need to know the voltage across the second resistor and the current through it. Perhaps all I'm given is the voltage across the chain of resistors, the voltage across the other resistor, and the value of the other resistor. At first glance it would appear that I don't have ANY of the information about the resistor I'm supposed to find the power for. But I can find the voltage across it because I have the voltage across the two resistor and the voltage across the other resistor and I know that voltages add across components in series. So that's done. Since they are in series I know that the current in the one I'm interested in is the same as the current in the other one. So now I have a new problem of finding the current in the other resistor, but that's easy because I'm given the voltage across it and the value of the resistance.