What happens when the frequency of AC is very high?

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In summary, the conversation discusses the behavior of electrons in a wire carrying AC current at different frequencies. It is noted that at higher frequencies, the velocity of the electrons does not increase but the time period decreases, resulting in a decrease in amplitude. It is also mentioned that at extremely high frequencies, the electron does not become stationary but still moves, although its movement may be less than its diameter. The relationship between the speed of electrons and electrical current is also discussed, along with the concept of drift velocity. The conversation also touches upon the optical conductivity of free and core electrons and the effect of frequency on the electron's motion. Finally, the conversation addresses the question of whether a current can still be measured at very high frequencies and the role of the
  • #36
@phinds I agree an incomplete description but then again capacitance forms literally everywhere when we look, in fact I have made many mistakes in the past making pcb circuits without keeping this in mind.As for the capacitor's and dielectrics and current, I guess the "better" the dielectric the more polarization current will there be in it, so for high dielectric constant materials (some exotic materials up to 10 000K) the current could indeed be considerable or comparable as @Dr_Nate said earlier.
 
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  • #37
Mister T said:
The ideal mass-spring system consists of a particle of mass ##m## attached to a spring with spring constant ##k##. The frequency is ##\omega=\sqrt{\frac{k}{m}}##.

What happens to the speed of the particle as ##\omega## increases beyond all bounds?
Surely, that depends on what you are holding constant as you alter ##k##, ##m## or both to affect ##\omega##. If you are holding the speed of the particle constant then it does not change.

If you imagine a plucked violin string pulled to a fixed displacement then, as string tension increases without bound, string velocity at the zero displacement point increases without bound.

If you imagine a hammered piano string struck with a fixed energy then as string tension increases without bond, string velocity at the zero displacement point remains unchanged.

[And if you ramp up the tension on an already-excited string without damping then it depends on details of the tightening, but I believe that the typical case is that string velocity at the zero displacement point increases without bound. Without solving (or stating) the recurrence, it is also plausible that a finite bound is approached]
 
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  • #38
Dr_Nate said:
The electron doesn't have a diameter.
That is in dispute: diameter of electron
 
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  • #39
There's nothing to dispute about. It's pure "science fiction" no science :headbang:
 
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  • #40
rzyn said:
At sufficiently high frequencies the impedance of the conductor is so high the waves reflect off.
But copper reflects waves at low frequencies just as well, and there is no lower limit. For instance, the ground screen of a VLF transmitting antenna.
 
  • #41
artis said:
@phinds I agree an incomplete description but then again capacitance forms literally everywhere when we look, in fact I have made many mistakes in the past making pcb circuits without keeping this in mind.As for the capacitor's and dielectrics and current, I guess the "better" the dielectric the more polarization current will there be in it, so for high dielectric constant materials (some exotic materials up to 10 000K) the current could indeed be considerable or comparable as @Dr_Nate said earlier.
You are quite right about capacitance. Any two separated metals will have a capacitance between them, but strengths will differ greatly.

I can see why you thought I was talking about currents, but I was referring to the number of electrons. You are right to think that the magnitude of the polarization current will be dependent on the material.

I do think you are making a bit of a mistake when you say 'current' or phinds says 'actual current'. This sounds like you are referring to a DC current. I believe in post #18, tech99 was only talking about an AC current.
 
  • #42
tech99 said:
But copper reflects waves at low frequencies just as well, and there is no lower limit. For instance, the ground screen of a VLF transmitting antenna.
He didn't mention that a small portion of the wave is transmitted (and probably absorbed). If you look up RL circuits you will find you can make a low pass filter with a resistor and an inductor.

I think at this point a lot of us are cross talking because we aren't talking about a single precise circuit or material and how we are measuring it.
 
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  • #43
Dr_Nate said:
Any two separated metals will have a capacitance between them, but strengths will differ greatly.
How can that be true? The Impedance of a metal plate is very low and not dominated by permittivity. The impedance of the gap is very high and will dominate the situation.
 
  • #44
Dr_Nate said:
Any two separated metals will have a capacitance between them, but strengths will differ greatly.

sophiecentaur said:
How can that be true?

EG, any two wires, PCB tracks have mutual capacitance between them. and increasing the freq. makes the effects worse
But you should already know that :wink:
 
  • #45
davenn said:
EG, any two wires, PCB tracks have mutual capacitance between them. and increasing the freq. makes the effects worse
But you should already know that :wink:
Your statement implies that the metals make a difference. It's the geometry and the dielectric that count.
 
  • #46
Going all the way back to post #1; I see …
Saptarshi Sarkar said:
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.
That is incorrect. One second in each direction is a frequency of 0.5 Hz.
For 1 Hz, the current would repeat with a period of one second, so it would flow for half a second in each direction.
The frequency f in Hz is related to the period T in seconds by f = 1 / T.

For a current of one ampere, one coulomb charge of electrons will pass any fixed point on the conductor per second. The distance the electrons travel in one direction before reversing is proportional to the period of the waveform. For higher frequencies, the period and the distance traveled become less, but whatever the frequency, for the same current, the same number of electrons pass any point on the conductor per unit time.

The equation and graph of; F = 1 / T; is asymptotic to both axes. Neither the frequency nor the period can be zero, the graph simply doesn't go there.
The turn on transient of a DC current makes it the start of an AC current that will change sometime in the future. It may have a very long period, but it can never have zero frequency.
 
  • #47
sophiecentaur said:
Your statement implies that the metals make a difference.

not at all, and if that is what you read, then you misunderstood, I made no mention of metal types or the dielectric type
I just stated an eg., (example) of mutual capacitance and its effect as freq increases.
 
  • #48
davenn said:
if that is what you read, then you misunderstood,
Ahh, I see the problem. I should have been replying to @Dr_Nate . Sorry for the misunderstanding.
 
  • #49
sophiecentaur said:
Ahh, I see the problem. I should have been replying to @Dr_Nate . Sorry for the misunderstanding.
I don't follow how my statement implies that the metals make the difference. When I said the strengths differ greatly, I was referring to the capacitance, which as you mentioned depends on the dielectric and the geometry.
 
  • #50
Dr_Nate said:
I don't follow how my statement implies that the metals make the difference.
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.
 
  • #51
sophiecentaur said:
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 🤷‍♂️
 
  • #52
tech99 said:
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.
 
  • #53
Saptarshi Sarkar said:
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.
 
  • #54
Paul Colby said:
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.
 
  • #55
tech99 said:
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.
 
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