# Propagation of a pure electric field

Discussion in 'General Electronics Chat' started by cbecket13, Nov 19, 2014.

1. ### cbecket13 Thread Starter Member

Nov 15, 2014
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How does an electric field propagate from a changing charge source, to a point that is subjected to that field?

For example, apply a charge to one plate of a capacitor such that all parts of the plate receive the charge essentially simultaneously. An opposite charge accumulates at the opposing plate.

What is the mechanism that causes the opposite charge to be stimulated at the other plate? What is the speed of propagation of that mechanism?

Nov 15, 2014
106
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Test rig :

3. ### alfacliff Well-Known Member

Dec 13, 2013
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that looks like a good circuit to check for noise on the pot.

4. ### cbecket13 Thread Starter Member

Nov 15, 2014
106
3
So you think the pot noise would be large, possibly larger than the p-n junction noise? So remove the pot and check noise levels with an equal value fixed (good) resistor at the same supply voltage? Or remove the p-n in some way and then measure the remaining noise.

5. ### studiot AAC Fanatic!

Nov 9, 2007
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You are mixing up the signal which is a (changing) voltage and charge.

Which are you interested in?

Nov 15, 2014
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Voltage

7. ### MikeML AAC Fanatic!

Oct 2, 2009
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Are you asking how fast does a capacitor charge? Depends on the current available to charge it, and the capacitance.

If you transpose the equation there, you will see that the rate-of-change of voltage across the capacitor plates is proportional to I/C

Last edited: Nov 19, 2014
8. ### cbecket13 Thread Starter Member

Nov 15, 2014
106
3
The current available to charge which? Assume current available is not limited on either side and the whole surface of of each plate charges simultaneously.

9. ### MikeML AAC Fanatic!

Oct 2, 2009
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The current in/out the top of the capacitor is equal to the current out/in at the bottom of the capacitor. (Kirchoff's law). The voltage across the plates of the capacitor depends on how much current, and how long it was applied, and inversely on the value of capacitance (plate area, gap, and dielectric constant).

10. ### alfacliff Well-Known Member

Dec 13, 2013
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when you apply a charge to one plate, it repels the like charge from the other, and attracts the oposite charge on the other. as far as how fast the electric field propagates through the cap, someone else will have to answer that.

11. ### crutschow Expert

Mar 14, 2008
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A changing electric field travels at the speed of light.

12. ### MikeML AAC Fanatic!

Oct 2, 2009
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Charge re-distribution along an conductor occurs at speeds approaching the speed of light. Look at a wave traveling along an antenna wire, or inside a two-wire transmission line. The time it takes charge to distribute itself inside a capacitor would be so small as to be unnoticable.

13. ### cbecket13 Thread Starter Member

Nov 15, 2014
106
3
I think my point is being missed. My q question is more basic than that.

One plate charges. This induces an opposite charge to accumulate in the other plate. How did it do that? By what mechanism?

Was there some kind of action-at-a-distance? Were particles or waves exchanged? Was something else propagated across the gap that induced the opposite charge to accumulate.

What is that something and how and how fast does it move? What medium does/doesn't that something move in.

14. ### cbecket13 Thread Starter Member

Nov 15, 2014
106
3
If that change is transverse. What if it is not?

15. ### BobTPH Senior Member

Jun 5, 2013
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361
The electric force is propagated by virtual photon exchange and happens at the speed of light. If you put a charge on one plate, the other plate sees it d / c seconds later where d is the distance between the plates in meters, and c is the speed of light in the material between the plates.

Bob

16. ### cbecket13 Thread Starter Member

Nov 15, 2014
106
3
If that change is transverse. What if it is not?

17. ### cbecket13 Thread Starter Member

Nov 15, 2014
106
3
Virtual photon? I have not heard that term. What is the difference between a virtual photon and a real one? I'm not an EE or a physicist or a mathematician, by the way. For a discussion on this topic, I am a somewhat informed layman. Say I barely got through Physics for Business Majors 101.

18. ### MikeML AAC Fanatic!

Oct 2, 2009
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A capacitor is a simple, two-terminal device. It has a simple relationship between current through it, and voltage across it which I linked you to. What else do you want?

What capacitor behavior that can be measured, observed from outside of the capacitor (voltage across it, current flow into the wires going to it) are you disputing?

19. ### BR-549 AAC Fanatic!

Sep 22, 2013
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The formal mechanism that your looking for is called an charge attraction and repulsion.
This force is transmitted thru electric field lines.
Modern physics says that electric field lines are one dimensional lines that have polarity and direction.
Electrons or negative charge have negative polarity lines and radiate in, in every direction.
Protons or positive charge have positive polarity lines and radiate out, in every direction.
Without any medium, i.e. space.....these lines propagate at the speed of light.
With any medium we know of now....this propagation speed will be reduced and some field lines will be lost.
The strength of the lines is constant, but because it's a point source, the flux decreases with distance.
In other words, the difference between a 1 volt field and a 100 volt field is not the strength of the field lines......but the number of field lines.
Also field lines repel like field lines and attract unlike field lines.

This is very fundamental and should have been studied early in this subject.

I suggest that you study basic electricity(charge and current) and capacitor action.

After a good understanding of electric fields...........you might want to study magnetic fields.
They are harder to comprehend.

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20. ### cbecket13 Thread Starter Member

Nov 15, 2014
106
3
Who said I'm disputing anything? I must disagree that it is a"simple" device. It appears borderline mystical to me. This thing, crossing a gap taking with it the energy to induce electrons there to do this or that. Amazing if you ask me.

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