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 5 hours agoAlex60011 gold badge66 silver badges1919 bronze badges

  • 1Hmmm are you sure this one isn’t a duplicate? – K H 5 hours 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 5 hours 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 5 hours 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 4 hours 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 4 hours 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 4 hours ago   
  • @tlfong01, the comments section isn’t really the place for your personal blog. It’s for seeking clarification on questions and answers although we do allow for a moderate amount of chat or humour. – Transistor 1 hour ago

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4 Answers


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 5 hours agoK H3,49388 silver badges2424 bronze badgesAdd a comment1

Reactive power

In ac circuits, energy flows into and out of energy storage elements (inductances and capacitances). For example, when the voltage magnitude across a capacitance is increasing, energy flows into it, and when the voltage magnitude decreases, energy flows out. Similarly, energy flows into an inductance when the current flowing through it increases in magnitude. Although instantaneous power can be very large, the net energy transfered per cycle is zero for either an ideal capacitance or inductance.

When a capacitance and an inductance are in parallel energy flows into one, while it flows out of the other. Thus, the power of a capacitance tends to cancel that of an inductance at each instant in time.

The peak instantaneous power associated with the energy storage elements contained in a general load is called reactive power and is given byQ=VRMSIRMS⋅sin(θv−θi)Q=VRMSIRMS⋅sin⁡(θv−θi)

The physical units of reactive power are watts. However, to emphasize the fact that QQ does not represent the flow of net energy, its units are usually given as Volt Amperes Reactive (VARs).

Importance of reactive power

Even though no average power is consumed by a pure energy-storage element, reactive power is still of concern to power-system engineers because transmission lines, transformers, fuses, and other elements must be capacble of withstanding the current associated with reactive power. It is possible to have loads composed of energy-storage elements that draw large currents requiring heavy-duty wiring, even though little average power is consumed. Therefore, electric-power companies charge their industrial customers for reactive power as well as for total energy delivered.


Electrical Engineering Principles and Applications by Allan R. HambleyShareCiteEditFollowFlaganswered 4 hours agoCarl1,16311 gold badge55 silver badges1919 bronze badges

  • Can you please clarify how much of your answer was from this textbook? Did you copy the text or paraphrase? – Elliot Alderson 1 hour ago

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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.

enter image description here
Vc(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 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 integrals in the 90-180 and 270-360 intervals you have the reactive power over the period T = 1/fShareCiteEditFollowFlagedited 3 hours agoanswered 5 hours agoEnrico Migliore77833 silver badges77 bronze badgesAdd a comment1

What is meant by reactive power in AC circuits.

  • Real power is V⋅I⋅cos(θ)V⋅I⋅cos⁡(θ)
  • Reactive power is V⋅I⋅sin(θ)V⋅I⋅sin⁡(θ)
  • Apparent power is V⋅IV⋅I (derived from the above using Pythagoras)
enter image description here

Picture from hereShareCiteEditFollowFlaganswered 4 hours agoAndy aka335k1818 gold badges274274 silver badges583583 bronze badgesAdd a comment

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