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Ch 32: Alternating CurrentWorksheetSee all chapters
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Alternating Voltages and Currents
Inductors in AC Circuits
Capacitors in AC Circuits
Power in AC Circuits
Resistors in AC Circuits
Series LRC Circuits
RMS Current and Voltage
Phasors for Inductors
Phasors for Capacitors
Phasors for Resistors
Resonance in Series LRC Circuits
Impedance in AC Circuits

Concept #1: Power in AC Circuits


Hey guys, in this video we're going to talk about power in AC circuits. What elements are emitting power, what elements are not emitting power, what the average power is and things like that. Alright, let's get to it. In AC circuits the only element to have an average power not equal to zero is, what do you guys think it is? It's the resistor. This is because whatever energy enters a capacitor or an inductor equals the energy that leaves it. Capacitors and inductors are elements that store energy. A capacitor stores charge to store electric potential energy and an inductor stores current to store a magnetic potential energy but either way they only store energy, the resistor is what's actually bleeding from the circuit. So if you were to plot the other element's power as a function of time you would see that on average all the peaks would cancel all the valleys and you would have no power on average.

The maximum power of a resistor is going to be that maximum voltage across the resistor times the maximum current. Now as a function of time you can say that the power equals the current as a function of time squared times the resistance. This gives us the following graph of power and current versus time. You can see that current, like we expect, is going to have no average value because anything that's positive any peak above the horizontal cancels with the negative peaks below the horizontal but power stays above the horizontal it just bounces above the horizontal so it's always positive and therefore it has nothing to cancel it out so on average it is absolutely not zero. The average power emitted by an AC circuit is going to be one half of the maximum power, this is because the power's peaks are completely symmetric so the average is going to be one half of the maxima so it's going to be one half the V max times I max and if you substitute the maximum values for their RMS values you find that this actually equals VRMS times IRMS. So the average power depends upon the RMS voltage and the RMS current which is an interesting result. We're not talking about the RMS power here we're talking about the true average of the power and it doesn't depend upon the average of the voltage or the average of the current. It can't because those are zero but it does interestingly enough depend upon the RMS values of the voltage and the RMS value of the current.

Let's do a quick example. An AC source operating at a maximum voltage of 120 volts is connected to a 10 ohm resistor. What is the average power emitted by the circuit? Is it equivalent to the RMS power which would be IRMS squared times R? Don't forget the average power we're just going to say is one half times the maximum voltage times the maximum current. So first what's the maximum current? Well that's just the maximum voltage which we know divided by R which is 120 volts that's the maximum voltage divided by 10 ohms which is 12 amps. So we can say that the average power is one half times 120 volts right which is the maximum power, sorry, the maximum voltage times 12 amps which is the maximum current and that whole thing is going to equal 720 watts.

Now if I take this maximum current I can then say that the RMS current is the maximum current over the square root of 2 which is 12 over the square 2 which is going to be 8.49 amps. That I can take and I can find IRMS squared times R is 8.49 squared times 10 which is indeed 720 watts so yes that does match up and this is something we touched upon earlier that the average power depends upon RMS values so this is a form of power emitted by a resistor we should absolutely be able to just plug in RMS values for it and get the average power out. Alright guys, that wraps up our discussion on power and AC circuits. Thanks for watching.