# The real reason for a capacitor having the same amounts of + and - charges on the two plates

• I
Exactly. You seemed to be saying that is wrong in your post ("other way round"). But, no matter; we are in agreement about the Physics, which is what counts.
'the other way around' wasnt what i said

sophiecentaur
Gold Member
They were your words in the post. As there was no quoted context I made up my own mind what you were referring to. (And so could someone else.)?
but as we agree about the Physics there’s no harm done.

BvU
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2019 Award
As the bishop said to the actress: "It was me"

Trying to show f1 that it's one and the same medal

So everyone agrees on my original post?

Delta2
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So everyone agrees on my original post?
No I don't agree, it is not due to Gauss's law but due to the assumption we make in circuit theory as I explained in post #16. In real world scenario and for the time dependent case the wires are not ideal, the electric field inside the wires will not be zero, neither it will be equal in the connecting wires at the two sides , the current will not be equal at the two sides, and hence the charges on the capacitor are approximately equal (and opposite) (depends on the wavelength of current in comparison with the distance between the two sides) but not exactly equal.

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• • Klystron, etotheipi and vanhees71
No I don't agree, it is not due to Gauss's law but due to the assumption we make in circuit theory as I explained in post #16. In real world scenario and for the time dependent case the wires are not ideal, the electric field inside the wires will not be zero, neither it will be equal in the connecting wires at the two sides , the current will not be equal at the two sides, and hence the charges on the capacitor are approximately equal (and opposite) but not exactly equal.
Do you agree for ideal wires?

Delta2
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Do you agree for ideal wires?
Well yes.

Well yes.
Then why isn't the Gauss' law argument in any textbooks?

Delta2
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Then why isn't the Gauss' law argument in any textbooks?
Because many textbooks dont study a subject so deeply. It will induce uneccesary complications it will be anti pedagogical.

So everyone agrees on my original post?
Everyone agrees that Gauss's Law is correct.
The "real reason" there is no net charge on the capacitor is that charge is conserved and no gremlins put extra charge in the circuit. Of course it is supported by Gauss's Law because there is no extra charge on the circuit and Gauss was and is correct.

.

• Delta2
ZapperZ
Staff Emeritus
Most textbooks say that a capacitor whether it be a single one or one in series/parallel should have equal amounts of + and – charges on both plates and that they mostly conclude the + charges attract the same amount of – charges on the other plate without giving any reason.

Now I claim that this is supported by Gauss’ law!

When a capacitor is fully charged, there’s no electric field (no current) in the wires connecting both plates of a fully charged capacitor and there can’t be any net charge on the capacitor when enclosing the whole capacitor by a Gaussian surface.

When a capacitor isn’t fully charged, there’re 2 currents in the same direction flowing to both plates though not through the interior of the capacitor. There can’t be any net charge on the capacitor when enclosing the whole capacitor by a Gaussian surface as the whole electric flux is canceled out to 0.

Do you all agree with this argument?
I don't understand all of this. Why can't this be a simple argument based on conservation of charge?

I have an empty pail, and I fill it with water from a pond. I then lift the pail a distance h above the surface of the pond. I claim that the amount of water in the pail is equal to the amount of water missing from the pond.

What is the problem here?

Zz.

• Adesh, Klystron, russ_watters and 2 others
I don't understand all of this. Why can't this be a simple argument based on conservation of charge?

I have an empty pail, and I fill it with water from a pond. I then lift the pail a distance h above the surface of the pond. I claim that the amount of water in the pail is equal to the amount of water missing from the pond.

What is the problem here?

Zz.
When there're 2 capacitors in series, merely according to charge conservation, there could be +q, -2q, +2q, -q on the plates (left to right).

• Delta2
ZapperZ
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• BvU
Delta2
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I think it can. We have to assume that the current is everywhere the same along the branch containing the capacitor in series (which does not follow directly from charge conservation in the case of capacitors), aka well known assumption of typical circuit theory , see my post #16.

ZapperZ
Staff Emeritus
2 plates of the same conductor shouldn't conserve charge. They are independent conductors.
I think it can. We have to assume that the current is everywhere the same along the branch containing the capacitor in series (which does not follow directly from charge conservation in the case of capacitors), aka well known assumption of typical circuit theory , see my post #16.
Are we talking about the SAME thing here? The link I stated is under static condition, and the analogy that I gave is when the pail is full. There is no current flow! All the capacitors in series must have the same charge or there is a non-conservation of charge somewhere.

This is where I choose to start and see whether this is fully understood FIRST.

Zz.

Delta2
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Are we talking about the SAME thing here? The link I stated is under static condition, and the analogy that I gave is when the pail is full. There is no current flow! All the capacitors in series must have the same charge or there is a non-conservation of charge somewhere.

This is where I choose to start and see whether this is fully understood FIRST.

Zz.
I really don't understand why the configuration of #37 is prohibited by charge conservation (total charge is also 0 for this configuration), it might as well be the case, it depends how you charge the capacitors.

• feynman1 and etotheipi
ZapperZ
Staff Emeritus
I really don't understand why the configuration of #37 is prohibited by charge conservation (total charge is also 0 for this configuration), it might as well be the case, it depends how you charge the capacitors.
There are two separate issues here:

1. You are claiming that if a set of capacitor is in series and the ends are attached to a battery, that each of the capacitor can have DIFFERENT amount of charges? (see the figure in the Hyperphysics link that I gave if you are unsure of what I mean by a capacitor in series, because THAT is exactly what I'm referring to). So let me be clear that this is what you are saying.

2. The example I gave in my first post has nothing to do with capacitor in series or parallel. It is simply a standard scenario of a capacitor being charge by a battery, i.e. a simple closed circuit. or an RC circuit if you will. I need to know if this is fully understood as a case of charge conservation FIRST. Because if it isn't, then I had to dig more elementary case of electrostatic charging! I have no idea where this is going, but at some point, I have to established a common knowledge that everyone can agree to! Otherwise, and this appears to be the case here now, we are talking about different things doing different events!

A simple capacitor being charged by a battery. At equilibrium, charge on one is equal to charge on the other, but opposite in sign. Again, as my example with water in the bucket, what is the issue here?

Zz.

• davenn
Delta2
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There are two separate issues here:

1. You are claiming that if a set of capacitor is in series and the ends are attached to a battery, that each of the capacitor can have DIFFERENT amount of charges? (see the figure in the Hyperphysics link that I gave if you are unsure of what I mean by a capacitor in series, because THAT is exactly what I'm referring to). So let me be clear that this is what you are saying.
Yes this is the issue except that I had not in mind a DC battery but rather the AC case with very high frequency such that the wavelength of current in the circuit is in comparison with the dimension of the branch containing the capacitors in series.
2. The example I gave in my first post has nothing to do with capacitor in series or parallel. It is simply a standard scenario of a capacitor being charge by a battery, i.e. a simple closed circuit. or an RC circuit if you will. I need to know if this is fully understood as a case of charge conservation FIRST. Because if it isn't, then I had to dig more elementary case of electrostatic charging! I have no idea where this is going, but at some point, I have to established a common knowledge that everyone can agree to! Otherwise, and this appears to be the case here now, we are talking about different things doing different events!

A simple capacitor being charged by a battery. At equilibrium, charge on one is equal to charge on the other, but opposite in sign. Again, as my example with water in the bucket, what is the issue here?

Zz.
This example I believe is fine.

ZapperZ
Staff Emeritus
Yes this is the issue except that I had not in mind a DC battery but rather the AC case with very high frequency such that the wavelength of current in the circuit is in comparison with the dimension of the branch containing the capacitors in series.
So, can you explain to me at what point from the OP's original post did it somehow morphed into this situation? Have we all agreed upon the simplest case first before going into this rather unusual state?

Zz.

Delta2
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So, can you explain to me at what point from the OP's original post did it somehow morphed into this situation? Have we all agreed upon the simplest case first before going into this rather unusual state?

Zz.
Well you decided to bring in the charge conservation principle, which works for your example but not for the example of capacitors in series in the AC case or when ,in generally ,they are being charged in an irregular way (not via a battery).
My thoughts on this regarding conservation of charge and KCL (or that the current along the same branch is everywhere the same):
Conservation of charge is equivalent to the continuity equation:
$$\nabla\cdot \vec{J}=-\frac{\partial \rho}{\partial t}$$
But this is not enough to infer KCL. We have to assume that $$\nabla\cdot \vec{J}=0$$ everywhere along the circuit and from this (by integrating both sides over a closed surface S that encloses the junction point and using divergence theorem) we can infer KCL.

ZapperZ
Staff Emeritus
Well you decided to bring in the charge conservation principle, which works for your example but not for the example of capacitors in series in the AC case or when ,in generally ,they are being charged in an irregular way (not via a battery).
My thoughts on this regarding conservation of charge and KCL (or that the current along the same branch is everywhere the same):
Conservation of charge is equivalent to the continuity equation:
$$\nabla\cdot \vec{J}=-\frac{\partial \rho}{\partial t}$$
But this is not enough to infer KCL. We have to assume that $$\nabla\cdot \vec{J}=0$$ everywhere along the circuit and from this (by integrating both sides over a closed surface S that encloses the junction point and using divergence theorem) we can infer KCL.
That's fine, but that wasn't my question. I wanted to know, starting from the OP's question in the first post, how that morphed into this. How is this relevant within the scope of what I perceived the OP was asking?

Zz.

Delta2
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That's fine, but that wasn't my question. I wanted to know, starting from the OP's question in the first post, how that morphed into this. How is this relevant within the scope of what I perceived the OP was asking?

Zz.
I don't know its just a forum's thread, conversation can divert (a little or a lot) from the original topic.

ZapperZ
Staff Emeritus
I don't know its just a forum's thread, conversation can divert (a little or a lot) from the original topic.
Once again, that's fine. It occurs a lot in this forum. However, when you jumped all over my post using a situation that is beyond the scope of what I think the OP is asking, then you are being unfair.

I can easily cite many discussions on here in which, if we apply a more general or unusual situations, the standard and common explanation simply will not work or incomplete. Every time there's a discussion on the photoelectric effect, for example, and the claim that increasing intensity of the light source that has photon energy below the work function will not cause any electron emission, I will let that pass by even though I have personally done many experiments where this is clearly not true. Why? Because within the scope of the question at that level, this is an added complication that is totally unnecessary and irrelevant.

At this point, I have no idea what the OP knows or have understood, because the situation just got way too complicated and confusing. I don't see a clearly-established baseline.

Zz.

Delta2
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Once again, that's fine. It occurs a lot in this forum. However, when you jumped all over my post using a situation that is beyond the scope of what I think the OP is asking, then you are being unfair.
Sorry for being unfair I blame @feynman1  he has the tendency of diverging from the thread's original topic in the threads he makes.

At this point, I have no idea what the OP knows or have understood, because the situation just got way too complicated and confusing. I don't see a clearly-established baseline.

Zz.
My summary from this thread: in the DC case it holds that the charges in capacitors plate are opposite and equal (and we can explain this with various ways, like with gauss's law and ideal wires (and ideal capacitor) , or with conservation of charge). But it doesn't necessarily hold in the AC case, not when the frequency becomes too high.