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Alternating CurrentBasic ConceptsEE Questions / AnswersElectric Circuit Analysis

Why Power in Pure Inductive and Pure Capacitive Circuit is Zero?

Why Power is Zero (0) in Pure Inductive, Pure Capacitive or a Circuit in which Current and Voltage are 90° Out of Phase?

These types of questions are asked in electrical or electronics engineering interviews. So let’s back to the basic to clear the basic and important concepts of electrical engineering.

1.  Why Power in a circuit is Zero (0) in which Current and Voltage are 90° out of phase?

If Current and Voltage are 90 degree out of phase, then the power (P) will be zero. The reason is as follow:

We know that power in single phase  AC Circuits:

P= V I Cos θ

Where;

  • P = Power in Watts
  • V = Voltage in Volts
  • I = Current in Amperes
  • Cos θ = Power factor of the circuit i.e. phase difference between current and voltage waves.

If angle between current and Voltage are 90° (θ = 90), then

Power = P = V I Cos (90°) = 0 ….. → [Cos (90°) = 0]

So if we put Cos 90° = 0, then the overall power of the circuit will be zero (i.e. In pure Inductive circuit or capacitive circuit where voltage is leading by 90° from current and current is leading 90° from voltage respectively).

2.  Why Power in Pure Inductive Circuit is Zero (0).

We know that in pure inductive circuit, current is lagging by 90 degree from voltage (in other words, Voltage is leading 90° from current) i.e the phase difference between current and voltage is 90 degrees.

As explained above, if current and voltage are 90° out of phase from each other like in pure inductive circuit, the total power of the circuit would be 0 as follow.

P= V I Cos θ

if angle between current and Voltage are 90° (θ = 90), then

Power P = V I Cos (90°) = 0

[ Note that Cos (90°) = 0]

This shows that in case of pure inductive circuit, the total power of the circuit would have zero as Cos 90° = 0.Why-Power-in-pure-Inductive-Circuit-is-Zero-0

3.  Why Power in Pure Capacitive Circuit is Zero (0)?

We know that in pure capacitive circuit, current is leading by 90° from voltage (in other words, Voltage is lagging 90° from current) i.e the phase difference between current and voltage is 90 degree.

As mentioned above, if current and voltage are 90° out of phase from each other like in pure capacitive circuit, the total power of the circuit would be zero as follow.

Power in AC Circuit

P= V I Cos θ

if angle between current and Voltage are 90 ( θ = 90) Degree. then

Power P = V I Cos (90°) = 0

[ Note that Cos (90°) = 0]

This shows the same as like pure inductive circuit i.e. in case of pure capacitive circuit, the total power of the circuit would be zero as Cos 90° = 0.

Why-Power-in-pure-Capacitive-Circuit-is-Zero-0

In short, in a circuit where voltage or current leading or lagging 90° (phase difference = 90°) behind each other, the positive cycle cancel the negative which leads to the zero average power of the circuit i.e. the total power factor of the ac circuit is zero which leads to the zero average power. 

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15 Comments

  1. Stephen R. Stanton says:

    What is difference between Capacitance (C) and capacitor reactance (Xc)????

    1. Capacitance = the ability of a body to store an electrical charge.
      C = Q/V
      and
      Capacitive reactance = an opposition to the change of voltage across an capacitor.
      its a type of resistance in AC circuits.
      Capacitive reactance Xc= 1/2πfC

    2. Kashif Hussain says:

      Please ans me we say capacitor is use for improve power factor but the prove is reciprocal?

    3. Capcitance is a fixed value in farads, capacitor reactance is a function of frequency, so x farads at x herts = x reactance

  2. Riza Acuna says:

    Thank you. Very useful

  3. Anonymous says:

    What is the concept of lagging by 90 degree leading by 90 degree in capacitor and an inductor?

  4. Atiq Khan says:

    Plz tell me we know that current decrease when value of resistor in a circuit increases so then why electric heater draw max current instead of having max resistance..

    1. Current Decreases??… Obviously, because the resistance opposes the current to flow. but it not means that there is no flow of electrons. Did you hear about Joule’s law… P = I^2Rt as well as Heating effect of current in the conductor ?? that is the answer.

    2. Resistor will oppose the max current.(which means draws more current)which converts in to heat energy.

  5. Onarinde John Babatunde says:

    How can I have the pdf or Microsoft word format of the Basic Electrical Engineering Questions and Answers?

  6. It is based on I and V
    IN capacitor I leads V by 90
    In inductor I lags V by 90

  7. YARED mulugeta says:

    How can i construct electric transmitter by magnetic field ?

  8. Ramnarayan Chaudhary says:

    How can I have the pdf or Microsoft word format of the Basic Electrical Engineering Questions and Answers?

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