Propagation of a pure electric field

BobTPH

Joined Jun 5, 2013
8,998
So the charge-motivating factor (I guess we're calling it "electric field") propagates instantaneously?

Maybe kind of like Bell's entangled photons then? Surely there is somebody out there who will argue with you about instantaneous propagation, but it strikes me as sensible.
I said in classical physics, with is wrong, it propagates instantly.

In reality, it propagates at the speed of light. Do a calculation of how long that is for the capacitor you want to analyze and see if it makes a difference in any calculation you would do.

We use approximations all the time when doing calculations. The approximation of 0 for the time to propagate across the plates of a capacitor is a good one for any practical calculation.

Bob
 

studiot

Joined Nov 9, 2007
4,998
Well, the classical view is that the field propagates instantly across the capacitor
Sorry, Bob, I can't agree with that.

The classical view is that any disturbance (change) in the electric (or magnetic) field propagates at the local speed of light.

That was Maxwells great insight.
 
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studiot

Joined Nov 9, 2007
4,998
So the charge-motivating factor (I guess we're calling it "electric field") propagates instantaneously?
This is rubbish.

The fact that voltage lags charge by 90 degrees is electric theory 101.

Do you disagree with my definition of earth/ground then since you want to argue with it?
 
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crutschow

Joined Mar 14, 2008
34,460
The propagation time across a capacitor's plates becomes important when the signal frequency is high enough that you are concerned about the signal propagation time through the circuit. But since the propagation time through the capacitor is comparable to the time it takes to go through a similar length of wire, you don't have to normally include a separate calculation for the signal speed through the capacitor.
 

nsaspook

Joined Aug 27, 2009
13,310
I'm trying to visualize this in terms of what is acting, what is it acting through and what is affected. Simple mind I guess. Maybe no such visualization/analogy exists.
If you get stuck on the physical particles it becomes turtles all the way down. QFT is a super-set of Q.E.D. that to me seems to explain the nature of the universe in a logical manner. It does require the hump of seeing that fields and space are the fundamental aspects of physics not particles.
 

Thread Starter

cbecket13

Joined Nov 15, 2014
106
I said in classical physics, with is wrong, it propagates instantly.

In reality, it propagates at the speed of light. Do a calculation of how long that is for the capacitor you want to analyze and see if it makes a difference in any calculation you would do.

We use approximations all the time when doing calculations. The approximation of 0 for the time to propagate across the plates of a capacitor is a good one for any practical calculation.

Bob
But the same could be said of relativity. It is a totally inconsequential effect in almost all practical applications. That prevents noone from trying to explore and evaluate it.
 

BobTPH

Joined Jun 5, 2013
8,998
Sorry, Bob, I can't agree with that.

The classical view is that any disturbance (change) in the electric (or magnetic) field propagates at the local speed of light.

That was Maxwells great insight.
No, you are confusing propagation of waves with propagation of the field. They are not they same thing. Do you think actual photons go across the plates of the capacitor as it charges?

Bob
 

nsaspook

Joined Aug 27, 2009
13,310
But the same could be said of relativity. It is a totally inconsequential effect in almost all practical applications.
The GPS receiver that keeps the time and position in billions of devices world-wide depends on knowing the effects of relativity (general and special) to correct for the offset of satellite clocks IRT earth clocks as they speed around the planet.
 
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Thread Starter

cbecket13

Joined Nov 15, 2014
106
If you get stuck on the physical particles it becomes turtles all the way down. QFT is a super-set of Q.E.D. that to me seems to explain the nature of the universe in a logical manner. It does require the hump of seeing that fields and space are the fundamental aspects of physics not particles.
Let me ask this. Can I use "electric field as an effective means of communicating over considerable distance (say that of electromagnetic
The propagation time across a capacitor's plates becomes important when the signal frequency is high enough that you are concerned about the signal propagation time through the circuit. But since the propagation time through the capacitor is comparable to the time it takes to go through a similar length of wire, you don't have to normally include a separate calculation for the signal speed through the capacitor.
Again though that's no excuse for failing to understand and describe it. "It's inconsequentially small" so it doesn't matter seems unsatisfying to me.
 

Thread Starter

cbecket13

Joined Nov 15, 2014
106
The GPS receiver that keeps the time and position in billions of devices world-wide depends on knowing the effects of relativity (general and special) to correct for the slowing of satellite clocks IRT earth clocks as they speed around the planet.
There is some argument as to whether or not GPS actually relies on relativistic principles as I understand it. Ready as always to be corrected/convinced.
 

Thread Starter

cbecket13

Joined Nov 15, 2014
106
No, you are confusing propagation of waves with propagation of the field. They are not they same thing. Do you think actual photons go across the plates of the capacitor as it charges?

Bob
At the risk of tedium, I'd like to propose another thought experiment and I genuinely do not know the answer to it and I'm not trying to convince/convert anyone;

Our two plate capacitor causing so much controversy, I decide to simplify it. I am in a vacuum (which the qm crowd will say doesn't exist). I have a generator of. ac - say a photo electric machine or similar.

I have a single wire that connects my generator to a single plate oriented perpendicular to the wire, like our characteristic concept. But there is only one plate to our capacitor.

Now I fire up my oscillator and send my ac wave down the wire (maybe you don't think it will go, I don't. know. My wave hits the plate and charges it up, then removes the charge and so on.

Now in our two plate capacitor, this is the point where the opposite charge is drawn to the other plate. But we have no second plate. But whatever was producing our attracting whatever is still being caused at my source plate.

Now if I follow the Maxwell fans out there, it seems that we would find electromagnetic radiation, transverse and mutually stimulating waves of electric and magnetic energy, eminating from my lone plate. The others would say god only knows. But does something leave the plate? If so, what is it, how does it leave, where and how fast does it goes there and what is it's effect on our universe.
 

studiot

Joined Nov 9, 2007
4,998
You are still proposing an impossible situation. period.

Your experiment falls down at the first hurdle.

What have you connected the second terminal of your AC generator to?
 

Thread Starter

cbecket13

Joined Nov 15, 2014
106
In most practical applications near-field communications using the decoupled magnetic or electric fields are confined to the area directly under the influence of the source charges.

http://en.wikipedia.org/wiki/Near_field_communication
Why? How does the decoupled field know what's near and what's far afield? Is 6 inches near field? In the world of microcircuits, it seems not. You just arbitrarily define some nebulous "near" field?
 

nsaspook

Joined Aug 27, 2009
13,310
Why? How does the decoupled field know what's near and what's far afield? Is 6 inches near field? In the world of microcircuits, it seems not. You just arbitrarily define some nebulous "near" field?
It's not nebulous. There are basic conditions that must be met in each region but how they blend near the radiator/antenna depends on frequency and the structure of the radiator. Some methods using near field evanescent coupling have the capability to extend the range of near field transfers.
http://www.antenna-theory.com/basics/fieldRegions.php
http://research.microsoft.com/pubs/70504/tr-2007-143.pdf
 
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BR-549

Joined Sep 22, 2013
4,928
There are two types of field propagation that we know about so far. Both types require both fields(E and B).

At slow alternations in static circuits, you can max one and min the other, allowing one to be ignored.

1. Static. Static does not mean still or not moving. Static means connected. The field lines are physically connected to the charge. These lines can change amplitude, polarity and physical direction(angle). The propagation speed is constant relative to media. This is the field propagation thru a capacitor or media(dielectric or conductor) if you will. It is a surface source not a point source.

2. Radiation. Two types...monopole and dipole.
A single charged conductor in space can radiate only in one way. It has to be physically moved very quickly. It doesn't have to be moved far(hardly any), but it has to be jerked very hard and fast.
With two charged conductors in space, you can radiate by changing the polarity of the conductors at the right time in accordance with the length of the conductor.
The field lines are cut and then repulsed(ejected, emitted, there are not self-propagating) from source. Requires equal amplitudes of E and B fields to be cut at the right time. It comes from a point source.
Once cut and emitted....the amplitude, polarity and direction(other than divergence) does not change.
The radiation can only be absorbed at a certain rate. This rate remains constant even with the divergence. If the media can not accommodate that rate, it will not be absorbed.
 
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