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Freshmen EE circuit. Resistance, inductivity, capacitance are given and effective value of voltage. No phase angles are given.

When calculating complex power, may I assume the phase angle of the voltage source? Current should adjust accordingly and give me the same power no matter which angle I assume.

Am I right? (I know there are formulas with effective U)

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  • \$\begingroup\$ You may assume either current or voltage as the reference zero degrees. Current is the better choice for series connected components, and voltage for parallel. The choice sometimes makes the maths a bit easier. \$\endgroup\$ – Chu Aug 31 '15 at 12:34
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Yes, it's typical to assume a phase angle of zero for the source, but it really doesn't matter- you should get the same results if the math is done right.

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No. The phase angle between voltage and current has a great deal to do with the power. This is the difference between VA (Volt-Amps) and Watts, and is what power factor is all about.

When the voltage and current are in phase, then power is simply the volts times the amps. When they are 90° out of phase, then no net power is delivered over any full cycle. In between values result in some power, but always less than Volts x Amps.

The power factor is one way to quantify this. It is the fraction of Volts x Amps that results in real power, and is also the cosine of the phase shift between the two.

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  • \$\begingroup\$ Yes. What I meant Is that when I randomly choose an phase angle/shift of the voltage, one of the current adjusts to it. In some examples that I've done, no matter the angle of voltage, difference between it and cureent's angle is the same. Is this universal? \$\endgroup\$ – Desperado Aug 31 '15 at 10:46

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