What happens when the frequency of AC is very high?

[Dr_Nate]

It's because the only possible variables you mentioned were the "two metals". If you had used the word "capacitance" or mentioned geometry or dielectric then there would have been no problem. Readers are not mind readers.

But, I did say capacitance twice and quoted somebody talking about parasitic capacitance in PCBs

 

[Paul Colby]

On the other hand, a vacuum capacitor does not radiate.

I'm baffled by this. One can make a patch antenna without dielectric[1] and it will radiate because an aperture field is still present. The same radiation integral would be used with or without a dielectric.

[1] one would support the patch above a ground plane with the feed wire.

 

[ThatGuyIS]

Summary:: How is the motion of electrons in very high frequency AC?

If I consider a wire carrying AC current, I know that at an AC frequency of 0Hz, the current will always in the same direction. If I change the frequency to 1Hz, the current will flow left to right for 1 second and then right to left for 1 second.

I guessed that at these higher frequencies, as the voltage is the same, the velocity of the electron will not increase but the time period will decrease, so an single electron will move in an SHM whose amplitude will decrease as frequency increases. If this is correct, what will happen as the frequency becomes extremely high? Does the electron become stationary?

Shoot off a quick burst of EMP and see what you get...

Speed is the distance traveled by an object where as, velocity is distance traveled by an object per unit time in a particular direction. I think it’s a logic problem with definition.

 

[tech99]

I'm baffled by this. One can make a patch antenna without dielectric[1] and it will radiate because an aperture field is still present. The same radiation integral would be used with or without a dielectric.

[1] one would support the patch above a ground plane with the feed wire.

If a capacitor has only vacuum between its plates, where is there an accelerated charge to cause radiation? In the case of the patch, it often is used as a slot antenna, four slots being formed around the edges of the patch. With slot antennas, it is the acceleration of the charges on the metal which do the radiating. It is also possible to have radiation from the feed wire.

 

[Paul Colby]

If a capacitor has only vacuum between its plates, where is there an accelerated charge to cause radiation? In the case of the patch, it often is used as a slot antenna, four slots being formed around the edges of the patch. With slot antennas, it is the acceleration of the charges on the metal which do the radiating. It is also possible to have radiation from the feed wire.

You've answered your own question, in the capacitor plates and feed wire. A free standing parallel plate capacitor fed with a harmonic current will radiated, there will be an aperture field present around the edge of the plates.

This entire thread has had a fixation on charges and their motions. From a radiation/current flow perspective it's quite reasonable to approach all these problems from a classical boundary value problem approach where $J = \hat{n}\times H$ is taken as the surface current for example. That's not to say there isn't significant physics in the conduction of charges, however, for many (dare I say all) one need not do so as these effects are accounted for as material constitutive relations.