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If the 120V is the peak voltage of the sinusoid AC signal and 0.8A is the peak current, then you go:
P = [120/sqrt(2)] * [0.8/sqrt(2)]
or you can combine the sqrt(2) into one number and just do:
P = 0.5*120V*0.8A
If 120V and 0.8A are in RMS, then you just go
P = 120V*0.8A
It's pretty much all the same thing, just the root(2) is sometimes combined into an RMS number or the two root(2) that are multipled are combined to be written as 2.
Power in n AC circuit is measured in Volt Amps because the only real power is in the resistive element. This has always been a sore point with me in the USA because they describe AC generators in terms of Watts which of course is nonsense. The apparent power is what you measure when you multply current by volts and current depends on the none resistive elements in the circuit. That is why it is important to add capacitance to an AC circuit because most loads are inductive.
In AC, you have the "real power (W -> Watts)", that is the power developed in a purely ohmic-resistance circuit.
And you have the "reactive power (VAr -> Volt-Ampère reactive)", that is the power developed in a inductive/capacitive-reactance circuit.
The W and VAr forms the "Apparent Power (Volt-Ampère)."
They form a rectangled triangle, where the Real Power and Reactive Power form the legs, and the Apparent Power forms the hypotenuse.
It has to do with components in the circuit storing energy in different ways (capacitance and inductance) which causes the current AC sinusoid waveform and the voltage AC waveform to be out of phase (the sinusoids don't look the same), which affects the apparent power which is dependent on the current and voltage at that instant. Shifting the waveforms causes the voltage and current to be at different values in the same instant than if both waveforms were in phase.
It's kind of something you have to read a book for. You need to know about complex numbers.
They're not spikes. Through a purely resistive load, the current is at zero/max/min whenever the voltage is at zero/max/min. But inductive loads make it so that the current lags the voltage (the current reaches zero/max/min after the voltage does).
And you can see that this means that the power isn't the same anymore. For example, in a resistive load when current and voltage peak at the same time, that's when there is the most power since V(t=T)*I(t=T) is largest since max V and max I occur at the same time and are multiplied together, but if the current or voltage is shifted a little bit, the V*I is smaller, since now the when the maximum of the current and voltage no longer occur at the same time, so the highest V*I that you can achieve will be lower.
Imagine two identical sychronized sine waves graphed out, and multiply them together to make a 3rd graph. Now imagine that you shifted one of the first two sine waves a bit. Now multiple the shited and unshifted graph together. The graph made from two synchronized sine waves (kind of representative of power) will have higher peaks and different values than the graph made from unsynched waves.
I am lost. I will try to find a book on all this stuff. My local library has jack squat for electronics/technology books. They won't even special order them(like they are supposed to!)
It basicaly has to do with this, in AC both the current and voltage are AC waveforms. The power at any one time depends on the current and voltage at that instant in time. V*I gives you some idea of how much the power is at that instant in time. If the current or voltage is suddenly shifted in time like in inductive loads, then the power at that instant in time is different since either V or I is a different number now for V*I than mentioned earlier.
It's a little bit like destructive and non-destructive interference of sound waves. Do you know anything about that?
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