Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Explaining reactive power on a subatomic level

  1. Apr 22, 2012 #1
    I am currently studying electric power engineering, which in my case is most about models and approximations, which makes it hard to understand the real physics behind some of the model. I would say that I understand the definition of current and voltage. But is it possible to explain the concept of reactive power on a subatomic level? I hope my question is understandable, though im not really sure I understand it myself.
  2. jcsd
  3. Apr 22, 2012 #2
    The mechanism is through dipole formation and destruction. Basically apply an E or B field induces an electrical or magnetic dipole in the material exposed to the field which opposes the original field. The formation and destruction of the dipole is not instantaneous which is the origin of the 1st order C dV/dt and L dI/dt terms of reactance.
    Last edited: Apr 22, 2012
  4. Apr 22, 2012 #3
    Ignore the post about dipoles.

    Reactive power is not an atomic concept. It's a large-scale classical one.

    Allow me to illustrate reactive power with your front door on a windy day.

    Suppose the wind is blowing onto your door. You set up a spring on the door that gets compressed by the wind. Some days the wind blows your door open and stretches the spring. On Monday you use the energy in the spring to operate your toaster. Thats real power and it leaves your front door open and the spring unsprung (because you made toast with it).

    On Tuesday though, you leave the spring alone and it slams your door closed again and makes a whirlwind of leaves on your porch. That's reactive power.

    Reactive power comes in and goes back out every half AC cycle doing no useful net work.

    Real power always comes in on both halves of the AC cycle.

    There is no atomic description for this that would make sense.
  5. Apr 23, 2012 #4
    I think the dipole description does make sense for what you described. The dipoles store energy when their dipoles are displaced further apart from an external E-field, and release it when their internal E-field is reduced, which gives the effect of allowing more charge to be present for a given voltage, at least in the case of dielectrics. I may be mistaken, but I believe you are only trying to describe the concept of reactive power while the person above described the atomic level mechanism for this.
  6. Apr 23, 2012 #5
    Reactive power, e.g., [itex] V=L\frac{dI}{dt} \space[/itex] arises from the Maxwell equation [tex] \nabla \times \overrightarrow{E}=-\mu_o\frac{\partial \overrightarrow{H}}{\partial t} [/tex].In this equation, μo is a property of the vacuum, and does not depend on the existance of atoms. Similarly, εo is also a property of the vacuum.
  7. Apr 23, 2012 #6
    What about in medium other than a vacuum? Why does the amount of reactive power change in different medium compared to the reactive power in a vacuum?

    edit: In other words, will a circuit with a coil in a vacuum have the same reactive power as with a coil with a core? Will a circuit with a capacitor formed by two plates in a vacuum have the same reactive power as it does with 2 plates with a dielectric between them? If not, what is the mechanism to describe this reactive power change?
    Last edited: Apr 23, 2012
  8. Apr 23, 2012 #7
    Let's first understand the properties of the vacuum, i;e;, without any matter present. There are four fundamental properties of the vacuum (= free space). They are the speed of light c = 3 x 108 m/sec, the impedance of free space Zo = 377 ohms, the permeability of free space μo = 4 π x 10-7 Henrys/meter, and the permittivity of free space εo = 8.85 x 10-12 Farads per meter. They are related by the two equations [itex] c=\frac{1}{\sqrt{\epsilon_o \mu_o}} [/itex] and [itex] Z_o=\sqrt{\frac{\mu_o}{\epsilon_o}} [/itex]. Why can we store magnetic and electric energy in a vacuum? How do these relate to reactive power?
  9. Apr 23, 2012 #8
    In the case of the vacuum, the energy is not being stored in the vacuum, it is stored in the E and B field. In radiating energy, the energy storage is transferred between the electric field and the magnetic field along a poynting vector since they are orthogonal. The rate at which this energy can transfer (power) between the fields is directly related to the speed of propogation.

    In circuits with reactive fields, the energy is transferred to a field rather than being dissipated in some kind of work, and so the rate that this energy can be transferred to the field is dependent again on the speed of propagation, which is related to the permittivity/permeability constants by the equations you mentioned.

    Am I close to the answer you're hinting at?
  10. Apr 23, 2012 #9
    Not very.

    If you have a long straight wire carrying a current I in vacuum, there is a magnetic field surrounding it, and this field is proportional to the permeability of free space. If free space had no permeability, the speed of light would be infinite. This magnetic field leads directly to stored magnetic energy, which leads to inductance. There is no radiated power, hence no TEM wave, hence no Poynting vector.

    The speed of light, the impedance of free space, the permeability of free space, and the permittivity of free space are all fundamental constants, and have nothing to do with matter.
  11. Apr 23, 2012 #10
    My first statement about the radiated energy was just to show my line of thought, because I was trying to relate the fundamental properties you gave and tie them together to explain the relation of energy stored and energy transfer in an EM field in a vacuum. It was not to explain reactive power with radiated power directly, only a comparison. I then moved on to reactive fields where I was not considering radiation of the energy, but rather just energy transfer to the reactive field, which is also dependent on the permeability and permittivity (and thus by the speed of propagation of the field).

    I understand that these constants have nothing to do with matter, but they are modified when you do consider matter with relative permittivity, relative permeability, and slower than vacuum speed of light. If these fundamental constants that reactive power hinges upon are modified in other medium, then reactive power also must be modified in other medium, and so I asked what is the mechanism that causes reactive power (and as such, the relative permattivity and permeability that precedes reactive power) to depend on the medium? What is the interaction with matter that causes reactive power to be dependent on this? I have always been under the impression that it is an atomic mechanism, like the dipole example in dielectric material.
    Last edited: Apr 23, 2012
  12. Apr 23, 2012 #11
    The OP stated that he's studying power engineering.

    Reactive power in this context is nothing more than a phase mismatch between voltage and current on a power line.

    Not dipoles, no materials are necessary. The entire analysis can be done with circuit theory.

    Reactive power is when energy is transferred back and forth between source and load. This is the entirety of the explanation and it has no atomic or subatomic foundation.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook