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Reactive power?

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vlad777

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If we have a cap connected to AC source it's power is imaginary.
We have energy in time so it must be power, but this energy did not
convert to another form so this power is imaginary. What this energy
did do is change it's place.

So I am trying to draw this analogy with a ball in space.
Let's say this ball is moving and has kinetic energy but this energy is not
converting to anything so it's power is zero.
But since we have energy in time than can it be said, that this ball has imaginary power?

Many thanks.
 
Hi,

Reactive power is considered to be not real power but we know this because when we put energy into a system we do not see it dissipate. But looking at the storage of the energy only does not tell us about the whole system. If we had a perfect storage system we could store energy forever, but once we tap into that energy with any real system we loose energy and the loss of energy is the real part. But this whole reactive non reactive thing is mainly used in AC analysis not time domain analysis.

An electrical analog to the ball would be applying a DC current to a capacitor, and the velocity is the voltage. We apply current, the voltage rises. We apply a force, the ball moves. In the absence of an energy eater, both systems maintain their final state forever. But we dont usually refer to this as reactive because that is when we apply an AC current. When we apply an AC current we see a different kind of response that follows some rules that involve sine waves. Without the sine wave it is not really an AC circuit so the terms 'reactive' and 'imaginary' dont apply in this sense.
 
vlad777,

If we have a cap connected to AC source it's power is imaginary.

Are you referring to the energy stored/released in the electric field of the capacitor, and the rate it is stored/released? There is nothing "imaginary" about that electric field energy. It is every bit as "real" as any other energy you will run into.

...but this energy did not convert to another form so this power is imaginary.

What does the above sentence mean? What other energy conversion are you talking about?

What this energy did do is change it's place.

Change place with what or where?

So I am trying to draw this analogy with a ball in space.

Drop the analogies. They are only good for explaining a point, not for showing how things work. The capacitor will store and release electric field energy twice every cycle. If there is no resistance, no energy will be lost as heat. The energy for storage will come from the circuit at the beginning of the cycle, and the circuit will receive back the energy when the cycle starts to reverse. Now, ask your questions.

Ratch
 
Hi,

Reactive power is considered to be not real power but we know this because when we put energy into a system we do not see it dissipate. But looking at the storage of the energy only does not tell us about the whole system. If we had a perfect storage system we could store energy forever, but once we tap into that energy with any real system we loose energy and the loss of energy is the real part. But this whole reactive non reactive thing is mainly used in AC analysis not time domain analysis.

An electrical analog to the ball would be applying a DC current to a capacitor,

So the main question is:
Do we call this power imaginary because:
1) energy did not convert to heat or mechanical motion,
2) because energy went back to the source
3) or the energy stayed in the system?

Another question:
If we just charge up a cap there will be no energy conversion but if we measure voltage and current
while cap is charging there will be power, so is this power imaginary?

I always seem to make a misleading title :) .I am interested in imaginary not so much in reactive.

Are you referring to the energy stored/released in the electric field of the capacitor, and the rate it is stored/released? There is nothing "imaginary" about that electric field energy. It is every bit as "real" as any other energy you will run into.

Energy is real but power is imaginary.

What does the above sentence mean? What other energy conversion are you talking about?

It did not convert to heat or mechanical motion.

Change place with what or where?

Energy stays electrical but instead in the power source, now it is in capacitor.
 
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So the main question is:
Do we call this power imaginary because:
1) energy did not convert to heat or mechanical motion,
2) because energy went back to the source
3) or the energy stayed in the system?

Another question:
If we just charge up a cap there will be no energy conversion but if we measure voltage and current
while cap is charging there will be power, so is this power imaginary?

I always seem to make a misleading title :) .I am interested in imaginary not so much in reactive.



Energy is real but power is imaginary.



It did not convert to heat or mechanical motion.



Energy stays electrical but instead in the power source, now it is in capacitor.


Hi,


I think i understand your question better now. We would not call the power imaginary, we would simply call it zero.
If you have a battery charged up but not driving anything, the power is not imaginary, it is not there at all so it is zero.

For an AC circuit (again i stress the AC nature of this) it's called imaginary when the current is 90 degrees out of phase with the voltage. So there is apparent power but it doesnt actually dissipate any energy, so we call it apparent power. We dont really call it imaginary anyway, we might call it complex power though.

Oh yes, notice that in the AC circuit we have BOTH current and voltage, but no power, but with the battery we have no current at all.
 
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If we have a ball bouncing up and down without losses can we say that complex power = 0+j*x .
Apparent power equals imaginary part of complex power?

Edit:
OK now I realize I am talking about a ideal mechanical harmonic oscillator so I have to look into that.
Wikipedia suggests some analogy but I don't think power is defined.
https://en.wikipedia.org/wiki/Harmonic_oscillator#Equivalent_systems

Velocity of a weight and force of a spring are 90 degrees out of phase
which means this system can be modeled with complex numbers.

Velocity * force = power
 
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vlad,

So the main question is:
Do we call this power imaginary because:
1) energy did not convert to heat or mechanical motion,
2) because energy went back to the source
3) or the energy stayed in the system?

If energy did not convert to heat or mechanical, doesn't that mean it stayed in the system?
If energy goes back to the source, doesn't that mean that it stayed in the system? After all, the source is part of the system.

Reactive power is usually plotted on the j-axis or "imaginary" axis of the power triangle. Google for "power triangle".

I always seem to make a misleading title .I am interested in imaginary not so much in reactive.

There is nothing "imaginary" about the energy stored in capacitance and inductance. It will knock you on your butt like any other energy source.

Energy is real but power is imaginary.

How can the rate of energy storage or usage (power) be imaginary?

If we have a ball bouncing up and down without losses can we say that complex power = 0+j*x .
Apparent power equals imaginary part of complex power?

You really have to go back to square one and read up on the power triangle.

Ratch
 
Ratchit,

If energy did not convert to heat or mechanical, doesn't that mean it stayed in the system?
If energy goes back to the source, doesn't that mean that it stayed in the system? After all, the source is part of the system.

You have a point, I have to think about this.

How can the rate of energy storage or usage (power) be imaginary?

In a power triangle reactive power is imaginary.


Edit:

Power is not defined for the whole system but "across" a component, in our case a cap.
So imaginary power means all of energy is returned to source.
Of course all this movement of energy has some rate (imaginary power).
 
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I hope you mean in the sense that it is out of phase with resistive power.

Voltage being out of phase with current means that power is sometimes negative ie returning to source.

------

One thing about science is that it is context-weak. Namely, same laws and equations should work in different contexts.
This is why I'm looking for reactive power in mechanical oscillator.

And (complex) numbers are context-free (abstract) and you assign physical meaning to them.
 
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Thanks, man.

So there are two of them (analogies), and all this time I was wondering why did MrAl say "velocity is voltage"
when I would say that force is voltage :)

EDIT:

Wow, lubricity is inverse friction, wouldn't think of that.
 
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If we have a ball bouncing up and down without losses can we say that complex power = 0+j*x .
Apparent power equals imaginary part of complex power?

Edit:
OK now I realize I am talking about a ideal mechanical harmonic oscillator so I have to look into that.
Wikipedia suggests some analogy but I don't think power is defined.
https://en.wikipedia.org/wiki/Harmonic_oscillator#Equivalent_systems

Velocity of a weight and force of a spring are 90 degrees out of phase
which means this system can be modeled with complex numbers.

Velocity * force = power

Hello again,

If all you want to do is model a system then you can call the imaginary axis anything you please. In an AC circuit it has special meaning however because we distinguish between the real power which is calculated with a simple multiplication of E times I and apparent power which is calculated the same way but isnt a real loss of energy.

For example, i can model the side panel of a cardboard box using a complex number by assigning the edge along the bottom to the real axis and the edge along the height as the imaginary axis. This doesnt mean the height is really imaginary though.
 
MrAl,

Now I have this experiment in mind.
If we have heavy cylinder that has rotational energy.
Also we have a pin-slot mechanism that isolates motion in one direction (so it just goes up and down as cylinder rotates).
If now this up down con-rod acts on a spring, we have analogical system to ac source and a capacitor.
Force and velocity are 90 degrees out of phase and force*velocity=power.
At first semi-cycle cylinder would be acting against the spring giving it energy and in second semi-cycle
cylinder would receive energy from the spring, and there would be apparent power with only reactive component.

I think this is more then cardboard example.
Circle,sin,cos and complex numbers go in hand, and we are using all this properties.


Edit:
Actually the spring has to be at rest at the middle point so there would be four sub-cycles.
Compress,decompress,lengthen,return.
 
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Hi vlad,


Well it appears to me that you just want to call something imaginary? Im not sure what you want to show here. That we can use imaginary numbers for mechanical motion as well as AC circuits? I gave the cardboard box example because that is the most basic. The mechanical motion example is a good one, but now i am not sure what it is that you want to show or prove.

(yes stretch is one of the partial cycles)
 
Well it appears to me that you just want to call something imaginary?

You are correct. We use negative numbers to describe dept or direction etc.
So what physical things are imaginary numbers good for describing?
I am guessing anything that has amplitudes out of phase.
 
Hi,

Anything that has rectangular components that are 90 degrees out of phase with each other.
Another way of looking at it is that the imaginary axis is in another dimension.

In AC though it seems to have special meaning because for example the capacitor driven by an AC voltage source has current:
j*w*C*E

and when multiplied by E we get still get an imaginary number.
 
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