# Power supplied by a Capacitor and Inductor

## Homework Statement:

the power supplied to a 33 microfarad capacitor from a 120 volt 60 hertz source will be: 1)zero 2)179W 3)179VAR 4)-179VAR

## Relevant Equations:

Xc=1/2pifC
Xl=2pifL
P=V^2/Z
Using above eq Xc=80.38Ohm and
P=179VAR but text book says that the ans is zero. Can any one explain please

## Answers and Replies

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cnh1995
Homework Helper
Gold Member
I think your answer is correct in terms of magnitude. The sign will depend on which power flow convention your course material is using. AFAIK, S=VI* is used almost everywhere, where an inductor "aborbs" lagging VARs.
P Q=179VAR
Also, P is used for "real" power (which is 0 in this case), and Q is used for "reactive" power (which you calculated).

TSny
Homework Helper
Gold Member
When AC voltage is applied to an ideal capacitor, the capacitor takes in energy for part of a cycle of voltage and the capacitor delivers energy back to the circuit for the remaining part of the cycle. The net energy delivered to the capacitor in one complete cycle is zero. So, over many cycles, there is no net energy supplied to the capacitor.

From the point of view of formulas, the average power supplied is not ##P_{\rm avg} = \large \frac{V^2}{Z}##. The formula should contain an additional factor called the "power factor". Is this something you have covered?

When AC voltage is applied to an ideal capacitor, the capacitor takes in energy for part of a cycle of voltage and the capacitor delivers energy back to the circuit for the remaining part of the cycle. The net energy delivered to the capacitor in one complete cycle is zero. So, over many cycles, there is no net energy supplied to the capacitor.

From the point of view of formulas, the average power supplied is not ##P_{\rm avg} = \large \frac{V^2}{Z}##. The formula should contain an additional factor called the "power factor". Is this something you have covered?
How to find power factor in this case as P. F=COS@=R/Z

TSny
Homework Helper
Gold Member
For an ideal capacitor, what is the value of R?

For an ideal capacitor, what is the value of R?
It should be zero if i am not wrong and cos0 becomes 1 so P. F of pure capacitive circuit become unity

TSny
Homework Helper
Gold Member
It should be zero if i am not wrong
Yes, R = 0.
and cos0 becomes 1
No, the formula says ##\cos \theta = R/Z##.
That is, power factor =## R/Z##.

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