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Explaining reactive power on a subatomic level 
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#1
Apr2212, 05:01 PM

P: 4

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
Apr2212, 05:34 PM

P: 281

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.



#3
Apr2212, 07:15 PM

P: 1,781

Ignore the post about dipoles.
Reactive power is not an atomic concept. It's a largescale 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. 


#4
Apr2312, 01:04 AM

P: 834

Explaining reactive power on a subatomic level



#5
Apr2312, 11:16 AM

P: 4,663

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, μ_{o} is a property of the vacuum, and does not depend on the existance of atoms. Similarly, ε_{o} is also a property of the vacuum.



#6
Apr2312, 11:44 AM

P: 834

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? 


#7
Apr2312, 12:35 PM

P: 4,663




#8
Apr2312, 01:09 PM

P: 834

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? 


#9
Apr2312, 01:36 PM

P: 4,663

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. 


#10
Apr2312, 01:56 PM

P: 834

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. 


#11
Apr2312, 09:02 PM

P: 1,781

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. 


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