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Reactive Power

Reactive Power in AC circuit

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What is meant by reactive power in AC circuits. Power is joules per second. Real power is the the energy dissipated in the resistor per second. Then what is meant by the reactive power? Does this mean the energy being stored in capacitor and inductor per second? Or does it mean the stored energy supplied per second by the inductor or capacitor.

If we have a voltage source connect with RLC network and after some time the inductor and capacitor are suddenly detached from the network and then they are supplied to another resistor with diffent value without voltage source then of course they will supply the energy at a new rate to the new resistance depending upon value the resistance. Will this not make the value of stored energy per second different from the value of energy supplied per second? And doesn’t that give the two different values of reactive power? Doesnt this mean that the reactive power is now changed for the same capacitor and inductor?acreactive-powerShareCiteEditFollowFlagasked 1 hour agoAlex59011 gold badge66 silver badges1919 bronze badges

  • Hmmm are you sure this one isn’t a duplicate? – K H 1 hour ago
  • Ah, I never knew what is the mysterious thing called “reactive power”. Now I am locking down and got nothing to do. Perhaps I should Google something and see if I can answer. True, Reactive, and Apparent Power – All About Circuits: allaboutcircuits.com/textbook/alternating-current/chpt-11/… – tlfong01 1 hour ago   
  • I found the AAC’s explanation very confusing. So I googled again and found this one: Reactive Power – Electronics Tutorials electronics-tutorials.ws/accircuits/reactive-power.html. – tlfong01 1 hour ago   
  • I found ET’s tutorial on reactive power better than AAC. I would give ET 4 out of 10 marks, but AAC only 2 out of 10 marks. Either of them not at all answers my question but makes me more confused. I thought for two minutes and concluded that I don’t understand the tutorial because I do not have enough knowledge (so called prerequisites) to understand the question, not to mention the answer. – tlfong01 41 mins ago   
  • Anyway, so I googled again, or actually wikied again. This time I found Wiki’s column on AC power is good, and I gave it 7 out of 10 marks. One good thing about Wiki is that it tells the whole story, from the very beginning. And it is this “the very beginning of the story” that clarifies my mind that it is the “root cause” of not being able to understand the prerequisite knowledge. In other words, the prerequisite of the prerequisite, or blind spot of the big picture, or “missing link” of the path to Eureka. AC power Wiki: en.wikipedia.org/wiki/AC_power – tlfong01 17 mins ago    
  • I read the OP’s question again and found his thought experiment of mobile/switching LC interesting. I guess he brings out this thought experiment because he has the same blind spot or miss link like mine. Ah, locking down jogging/supper time. See you later. – tlfong01 1 min ago   Edit   

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2 Answers

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Reactive power is a concept that comes out in AC circuits where the voltage or current sources have a sine wave shape with a certain fixed frequency f.

Picture this circuit: a sine wave voltage source V that charges and discharges a capacitor C. They are in parallel.

v(t) = V * sin (2 * pi * f * t)

V and f are fixed.


Look at this picture from Wikipedia:

enter image description here

Initially the capacitor is discharged.

The current flows from the source to the capacitor. We like that.

When Vc reaches its maximum, that is V, the capacitor GIVES BACK the current to the source.

The bad thing for the voltage source is that it is receiving current from the capacitor while V is still positive. When V is positive, the voltage source is supposed to push power to the load.


Reactive power is the power that the capacitor C returns back to the voltage source.

Power is the blu line in the picture. It changes in time. If you take its integral in the 90-180 and 270-360 intervals you have the reactive power.ShareCiteEditFollowFlaganswered 48 mins agoEnrico Migliore77033 silver badges77 bronze badgesAdd a comment1

Capacitors and Inductors are Reactors and account for the reactive power in an AC circuit. When you measure the current and voltage in an AC circuit, you measure what is called Apparent Power, which will appear to be greater than the actual True Power being used in the circuit if the circuit has reactors in it. They store and release energy, causing current to flow in the process even though the Reactive Power isn’t being used and contributing to the “Power” you “see”. To recognise the difference between the energy being stored and being used, we refer to True Power in W(watts), Reactive Power in VAR (volt-amps reactive) and Apparent Power in VA (volt-amps).

The Apparent Power is not simply the sum of the True Power and the Reactive Power. Instead it’s a Pythagorean relationship where the Apparent power is the long side of a square triangle, the True Power is the horizontal line and the Reactive Power is the vertical line. You can apply trigonometric rules and the Pythagorean Theorem to calculate values on different sides of the triangle from your known values.ShareCiteEditFollowFlaganswered 1 hour agoK H3,47388 silver badges2424 bronze badgesAdd a comment

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