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Homework Help: Real and reactive power?

  1. Feb 18, 2009 #1
    1. The problem statement, all variables and given/known data

    What is the exact difference between real an reactive power?

    2. Relevant equations

    As far as i know

    real power =V*I*cos(teta)
    reactive power = V*I*sin(teta)

    3. The attempt at a solution
    I have heard, in physics reactive power is not considered as its work is zero?
    But how significant it in electrical field?
  2. jcsd
  3. Feb 18, 2009 #2
    Reactive power is the part that just goes back in forth in the circuit. Sometimes it is positive and sometimes negative, but over the whole AC cycle it averages to zero. Real power averages to something positive over the whole AC cycle.
  4. Feb 19, 2009 #3
    Just a little tidbit of information.... Although reactive power does indeed do no work, it is very important. The blackout of 2003 was caused in part by the lack of reactive power.
  5. Feb 22, 2009 #4
    A nice basic intro to work from, taken from my marine engineering notes:
    In DC circuits, power is a result of the current and voltage multiplied together. This is acceptable because the current and voltage are in phase with each other. It is acceptable in AC circuits where the load is totally resistive. In reality the load onboard is more inductive (due to motor windings) and resistive (due to heating/lighting etc). In inductive circuits with AC, the constant changing on current and voltage causes a constantly changing magnetic field. This induces and emf into the windings which opposes the source creating it (Lenz'z Law) causing an inductive reactance. Reactance (whether due to inductance or capacitance) and resistance together combine to give impedance.

    Remember that in AC circuits, a solenoid actually resists the current BUT does not use any power (and therefore do any work) in doing so. SO balancing this relationship of REACTANCE and RESISTANCE gives IMPEDANCE and that is used to form the power factor triangle (you gave the equations in your question). When this goes out of shape, you have blackouts and lots of wasted energy. Back in the good old days (I suppose World War 2 time'ish) industrial works and factories in England used to get a reduction in their electric costs if they could prove they had a good power factor, of say 0.8+.

    Hope it helps.
  6. Feb 26, 2009 #5
    Reactive power involves an actual current flow, and hence heating, even though it produces no useful work. Because of this current, and the associated losses, it is is highly undesirable.

    Power factor correction, alluded to by XaseR, is still very much practiced today. This is one of the reasons for the popularity of synchronous motors because with the proper excitation they can look like a capacitor on the line, thus offsetting inductive loads. Utility companies do not like customers with very bad power factors, and they will charge extra for such service.
  7. Feb 26, 2009 #6


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    Some more description, which was almost said above: reactive power is the energy flow used in the creation and collapse of magnetic (inductors) and electric fields (capacitors) in the system. The energy could be desribed as sloshing back and forth as the fields build and fall, though this does not work. The sloshing still creates current flow, and thus greater requirements for on the electrical transmission system (conductor sizes, etc).

    The 'theta' in the OP is defined as the phase angle between the oscillating current i(t) and voltage v(t)
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