capacitor confusion

Thread Starter

e-learner

Joined Apr 25, 2015
30
This is what I understand about capacitor .Plz correct me if I understand something wrong about this.

A capacitor stores electrical energy. The voltage connected to the capacitor produces + and - charge by taking electrons from one plate and collecting on another plate. And the capacitor tries to maintain a constant voltage across its terminal and it opposes any change of voltage by flowing current in opposite direction.

But in conductors, a closed path is necessary so that there is continuous flow of electrons. In a capacitor since the two plates are separated by an insulating medium so the electrons cannot form a continuous path of flow and the electrons are gathered on one the plate of the capacitor and a electric field exists between two plates. A capacitor resists a volatge change by producing current. Does this means the current flows from one plate to another via the battery and stops when it reaches the other plate? then how can current flow when the path is discontinuous? What hapens inside the plates?
 

Hypatia's Protege

Joined Mar 1, 2015
3,203
how can current flow when the path is discontinuous?
The 'dirty little secret' is that (leakage aside) current does not flow through capacitors! -- Although it may be said to flow into or out of same...

Moreover -- Please note:
Capacitors react to changes in EMF (aka 'voltage')
Whereas
Inductors react to changes in Current

As an aid to comprehension please consider that, from a mathematical perspective, reactors may be seen as supplying the 'imaginary component' of impedance -- specifically the 'i' (or 'j') term in rectangular expression or the phase angle in polar (vector) expression...

Best regards
HP:)
 
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ErnieM

Joined Apr 24, 2011
8,011
I would beg to differ about that dirty little secret, as would James Maxwell himself. A displacement current does indeed flow between the plates, with units of current density (amps over area). It does indeed have an associated magnetic field.

If such a current did not exist your radio would not work.

The only "secret" here is that the current is not formed by moving charge carriers (electrons).

Practically you can ignore this little fact and just pretend that every electron that runs to one plate comes out the other plate. In reality, they are different electrons, but one could never tell the difference.
 
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Thread Starter

e-learner

Joined Apr 25, 2015
30
I watched the video and it says that there is no current inside the plates of the capacitor which was confusing me as it violates KCL. After reading another book I found that displacement current do exist between the plates of the capcitor which is equal in magnitude to conduction current which is flowing outside.
I would be to differ about that dirty little secret, as would James Maxwell himself. A displacement current does indeed flow between the plates, with units of current density (amps over area). It does indeed have an associated magnetic field.

If such a current did not exist your radio would not work.

The only "secret" here is that the current is not formed by moving charge carriers (electrons).

Practically you can ignore this little fact and just pretend that every electron that runs to one plate comes out the other plate. In reality, they are different electrons, but one could never tell the difference.
I did not understand what you mean when you say curent is not formed by moving charge carriers? The definition of curent is flow of amount of charge per unit croos sectional area. Is this definition incomplete?
 

nsaspook

Joined Aug 27, 2009
6,726
I would be to differ about that dirty little secret, as would James Maxwell himself. A displacement current does indeed flow between the plates, with units of current density (amps over area). It does indeed have an associated magnetic field.

If such a current did not exist your radio would not work.
James Maxwell's 'displacement current' actually proves that radio-waves do not need a current or media (dielectric ether) to propagate. The vacuum displacement current does not qualify as a current (bound charges in a dielectric). It is a co-artifact (with the magnetic field) that occurs together with the magnetic field when charges move. Do we have a magnetic force between two regions of space bearing a vacuum displacement current? The answer is no, so it can't be at the most fundamental level a real current.

Using 'displacement current' as the equivalent of current in the circuit as a 'system' completes the open circuit flow of external charge so it's useful.
 
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ErnieM

Joined Apr 24, 2011
8,011
As Fireside Theater put it "Half a watt come in here, must go out there."

I watched the video and it says that there is no current inside the plates of the capacitor which was confusing me as it violates KCL. After reading another book I found that displacement current do exist between the plates of the capcitor which is equal in magnitude to conduction current which is flowing outside.
I didn't watch that video but it seems you know more than the person who made it.

I did not understand what you mean when you say curent is not formed by moving charge carriers? The definition of curent is flow of amount of charge per unit croos sectional area. Is this definition incomplete?
Well, consider KCL applied to a cap from one lead to the center between the plates. Since we don't have a complete loop the current in must equal the current out. One current is electrons moving along to (or away) from the plate. The other current is not made of electrons, but must indeed be a current to satisfy KCL.

KCL comes out of Maxwell's equations. It is universally true.

"Current is made of moving charges" is an approximation for some situations.

Try this instead of some un-reviewed personal opinion video.
 

nsaspook

Joined Aug 27, 2009
6,726
Yes, Kirchoff's Laws universally apply in lumped circuit theory. If you eliminate EM fields there are no non-conservative fields either. Is this the real world of real circuits? KVL and KCL result from solutions of Maxwell's equations with specific conditions, stay within those conditions and all is peachy.

http://www.learner.org/vod/vod_window.html?pid=604
Skip to about 20 min in for displacement current.
 
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nsaspook

Joined Aug 27, 2009
6,726
As you see in the video Maxwell cuts the Gordian knot with a mathematically correct but (as we know today) physically impossible solution using Luminiferous aether as the vacuum displacement current carrier. Today it's explained by relativity with Maxwell–Lorentz equations. Transformation of the electromagnetic field that provide a solution without a fictitious (analogous to a Fictitious force) displacement current equal to the "real" current in the wires.
 

Thread Starter

e-learner

Joined Apr 25, 2015
30
Yes, Kirchoff's Laws universally apply in lumped circuit theory. If you eliminate EM fields there are no non-conservative fields either. Is this the real world of real circuits? KVL and KCL result from solutions of Maxwell's equations with specific conditions, stay within those conditions and all is peachy.

http://www.learner.org/vod/vod_window.html?pid=604
Skip to about 20 min in for displacement current.
thanks for ur help but I could not play that video .
 
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