Aw please, c'mon now Cruts... Don't turn this into a one upmanship debate here please. How can you have different amount or charges on 2 sides???

You misunderstood my post.
It has nothing to do with one upmanship.
I was trying to clarify a point that seemed to be muddied.

Where did I say or imply there were different amounts of charge on the 2 sides of a capacitor?
The two sides always have equal and opposite charge (give or take an electron or two ).

I think you are confusing voltage with the electric field.
It's true that the electric field is zero at the midpoint between the two plates of a capacitor, but that doesn't translate to a voltage level of zero.
As has been stated, voltage requires two points, you can't define a voltage at a single point.
You need to understand that distinction or you will continue to be confused.

 

ozsavran,

There is no contradiction, for the reason crutschow explained. At the risk of superfluously expanding on what he said: in electronics just as in physics definitions are crucial. Look up the physics or electronics definition of voltage - they are the same. It is the electric potential energy difference between two points. There is no such thing as an absolute voltage at a point - only relative to some reference point. So calling the midpoint between the two plates of a capacitor as zero volts has no meaning - to be meaningful it has to be explicitly stated or implicitly understood to be relative to some point.

My Dear Elder,

Well, sure I see your particular point there. The capacitor midpoint itself can be at +100V relative to actual Earth ground of your building. Which is generally a meter long galvanized steel rod you literally hammer down into ground somewhere in your basement here. For all practical intents and purposes we can assume earth as the absolute ground, final reference point of 0V for all everything else.

In my second circuit below, I made a 10V pp AC source by adding two 5V AC sources in series and putting earth ground between them. So there is no ground relative to anything, but just physical real earth ground. Simulation including both scopes and voltmeters show 2 sides of the capacitor at 180d voltage phase difference. It has to be.

Perhaps one or both of you guys take the time to simulate this same circuit yourself or setup a little actual experiment to show us where I am wrong in my little simulation?

 

This is an important point in understanding the physics and terminology. Yes, we often assume ground is zero volts, but earth ground is not at 0V; it has no meaning to say it is at zero volts for the reason I explained above. An ideal ground is a body in which you can pump as much current/charge as you want and its voltage will not change. So to realize that physically the body would be very large and would have zero internal resistance. For most purposes earth acts as an adequate ground, but it has nothing to do with its voltage. Depending on the circuit or system you are talking about we often use ground as the reference, therefore the voltage between the reference and itself is zero volts or between any two points at ground potential will be by definition zero. Perhaps that is why you thought ground was 0V.

It is interesting to note that the earth's surface is negatively charged. So if there were some universal absolute 0V reference somewhere then earth ground would probably not be at 0V with respect to that reference.

 

Aw please, c'mon now Cruts... Don't turn this into a one upmanship debate here please. How can you have different amount or charges on 2 sides???

DG Elder said it himself above:

> What you can say is that with respect to the point half way between the two plates of the capacitor the voltages on the capacitor plates are equal
> and opposite. And in an AC circuit their voltages with respect to that midpoint are 180 degrees apart. This can be inferred from the physics of
> capacitors and the o-scope waveforms, but you can not directly measure it in your circuit.

If my word as a newbie here is not good enough please take it upto DrAI and take his challange yourself.

> In this case with the capacitor and voltage, you are absolutely right. The two voltages are 180 degrees out of phase. But can you figure out how to
> come to this conclusion? The answer may not be very obvious, but it's there. I thought i would give you a little time to think about it again now that
> you have the confirmation that it really is out of phase and it must be out of phase.

I will have to declare this thread as [SOLVED] if nobody is going to sue me for it.

Hi,

I am a little sad to see so many disagreeing with this simple concept.
As far as i can see you got the right result. We can prove this in a couple different ways.

The rest of this post also addresses some of the other previous replies.

First, we have to realize that the viewpoint we are taking is partly an internal one, but we can turn it into an external one pretty easy i think, but probably more discussion would be interesting anyway. Also there certainly are times when our new viewpoint is not wise, but then again there are times when it is.
For example, we all know that the capacitance is proportional to the inverse of the thickness of the dielectric. We also know that two capacitors in series with capacitance C results in an external measurement of half the capacitance of either one.
Taken these two together, we can construct a simple model. We take the original capacitor, stick a plate right in the middle of the dielectric, and now we have two capacitors in series each are twice the original capacitance so the overall capacitance is the same as the original, yet now we have a central plate for reference.
Measuring the voltage from each outer terminal to the central one, we see a difference of 180 degrees, or just an opposite polarity. It's that simple, and we dont even need a circuit.
If we want a circuit, all we have to do is connect two equal value resistors in series also in series with the capacitor and that gives us a forced central ground reference point too. It's not imaginary it's a real central voltage point. If you dont like the two equal resistors, then let their resistance approach infinity...we still get a zero reference point in theory.
Still dont like that? Charge one cap, then discharge partly into two smaller caps in series. How do the two caps get equal but opposite voltages? Did we just make a power supply splitter?

For the charge view, if the charge is separated equally then there must be a central point we can call zero volts.

A question came up as to a voltage needing two points to measure. Yes, sure, in the general case, but then again zero volts is not any voltage so there is nothing to measure.

So what about another application?
The model for a floating capacitor will show equal voltages but opposite polarity where each voltage is 1/2 of the full capacitor voltage when it has a DC voltage across it. Why would that be so? The circuit has no ground so the solution has to give equal rights to both sides. (One caution here is that not every simulator will show this, but you can still show it with a little clever setup).

Any other application?
How about a capacitor antenna? If we create an electric field outside of the floating capacitor that permeates the dielectric, what will happen to the two free ends of the capacitor?

What about when we charge a capacitor, then move it into a circuit with a single resistor in parallel. How do we get opposite voltages at each end of the resistor.

The battery is an even better example. There is no preference in nature for a battery floating in free space to put any emphasis on either terminal as both terminals are equally important.

Also, any uniform field gradient will have to have a central point that is common and there is no reason why we cant call that point (actually would be a surface) zero volts. Yes, we dont HAVE to do this, but we COULD if we wanted to.

So the way i see it, we dont have to call the central part of the capacitor or battery zero volts but there will be times when it just makes sense to do so.

Try this:
Set up a simulator of your choice with ONE battery and no other connections. Do a dynamic DC analysis. See what voltages it displays on each terminal. Then, color yourself surprised

Isnt absolute ground at infinity in all solid angle directions?
Measuring the battery terminals with that reference means we see two voltages, one positive and one negative. I think that's the short answer.

 

I'm a little sad to see such Didactic sins being committed for such dubious arguments.

 

I'm a little sad to see such Didactic sins being committed for such dubious arguments.

Easy for you to say.

 

It's true that the electric field is zero at the midpoint between the two plates of a capacitor, but that doesn't translate to a voltage level of zero.

The electric field is not zero at the midpoint if there is a voltage across the plates.

 

It's true that the electric field is zero at the midpoint between the two plates of a capacitor, but that doesn't translate to a voltage level of zero.

The electric field is not zero at the midpoint if there is a voltage across the plates.

True.
I wasn't thinking clearly.

 

True.
I wasn't thinking clearly.

Has happened to me more than a few times here.

 

I'm a little sad to see such Didactic sins being committed for such dubious arguments.

Hi,

First i think you have to prove that any argument is dubious. Merely stating that doesnt "make it so", unless you are on Star Track

Second, it is unfortunate that sometimes we have to assume the role of teacher, but only in a manner of speaking because anyone listening here can always challenge the "lecture" if you will, unlike many lecture environments. The challenge should come in the form of some logical argument however, as just blurting out "Hey that's not right" doesnt help because if the original argument was true then that statement is false and worthless, and if the argument really was not right then nobody ever knows why.

We all are more or less allowed to have our own opinions if we live in free country, and those opinions may contradict fact either real or fiction, but if it really is fiction we usually want to know why. I am sure you would have good grounds to state a given fact or anti fact, but if we dont know what it is we cant talk about it.

 

As someone said, we are all allowed to have our own opinions, but not our own facts.

 

As someone said, we are all allowed to have our own opinions, but not our own facts.

Hi,

That's cool

I might disagree just a tiny bit though, in that what we believe are facts may not be facts, but we still might hold them as such. So in a way we can have our own facts, at least until someone comes along and contradicts one of them and we see the light

 

IRT the zero voltage midpoint between two capacitor plates. What are the conditions when that is true. It's when the stored energy between plates is zero. If the two terminals A & B are both at zero Electric potential V that's obviously true but what if A & B are both at the same exact non-zero electric potential across the capacitor? In that condition the voltage across the capacitor is also zero, the electric field at midpoint is zero, so it's also in a zero net Electric potential energy state (not charged). For a typical electronics AC signal coupling requirement this is what we desire in the capacitor. For any other condition of unbalanced voltages there is a energy storage condition with a voltage across the capacitor and no midpoint zero. If the element were a resistor the conditions would be the same but it would dissipate energy instead of storing it.

 

Here is the definition according to the International Electrotechnical Commission.
This is a fact.

voltage
(electric) tension

scalar quantity equal to the line integral of the electric field strength E along a specific path linking two points a and b:

See this link for the rest of it.

http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=121-11-27


If you do the math using SI units you end up with (Newtons/Coulomb)*meters = Joules/Coulomb.= V.

The point being you need to know the E-field along the path between two defined points to calculate V. So it is relative.

In principle, point a could be set at infinity beyond all field influences, and defined as zero potential; then you could calculate an absolute voltage for point b. But it is an unknowable and impractical definition, probably not physical and I admit outside of my wheelhouse. And it won't be helpful in determining the absolute voltage between two plates of a capacitor.

 

................
I might disagree just a tiny bit though, in that what we believe are facts may not be facts, but we still might hold them as such. So in a way we can have our own facts, at least until someone comes along and contradicts one of them and we see the light

But just believing they are facts doesn't make them facts.
Look at things like global warming or evolution.

 

But just believing they are facts doesn't make them facts.
Look at things like global warming or evolution.

Hi again,

I am not sure if you realize it, but didnt you just prove my point?

This is the way i see it:
1. There are proponents on both sides of both arguments.
2. By stating that global warming or evolution is a proof that facts exist somehow naturally in the universe you are actually using your own beliefs to try to prove that other beliefs are wrong.

I agree however that once everyone agrees with a given so called 'fact' then we have a more solid ground. But take a look in the past and see what 'facts' have changed, even in science, and especially in medicine. The problem is, getting everyone to agree on any one thing because there are so many different beliefs. Then once everyone agrees, a new discovery could change the facts yet again.

Take a quick look at religions of the world. Almost none of them agree except on the basic issues. Does that mean that there are no facts? Each group has there own set of facts that they hold dearly.

 

IRT the zero voltage midpoint between two capacitor plates. What are the conditions when that is true. It's when the stored energy between plates is zero. If the two terminals A & B are both at zero Electric potential V that's obviously true but what if A & B are both at the same exact non-zero electric potential across the capacitor? In that condition the voltage across the capacitor is also zero, the electric field at midpoint is zero, so it's also in a zero net Electric potential energy state (not charged). For a typical electronics AC signal coupling requirement this is what we desire in the capacitor. For any other condition of unbalanced voltages there is a energy storage condition with a voltage across the capacitor and no midpoint zero. If the element were a resistor the conditions would be the same but it would dissipate energy instead of storing it.

Hi,

It sounds like you are saying that you do not believe that there could not be a midpoint (actually an entire surface) between the two plates that is 1/2 the total potential unless the potential difference is zero. That cant be right, right?
That's because voltage potential is not something that just exists at a point, it is a distribution. In other words, if we have +5 volts at some point in the circuit referenced to zero volts (whatever point we are calling that) then somewhere else in the circuit there is a potential of +4v, +3v, etc., even if it is inside a component.

In circuit analysis there is no golden rule for what point we must call the reference node, which we assign 0v to. It is completely arbitrary although we often pick the most convenient point, convenient to the context of the analysis. That is, convenient to what we are trying to find out.
For example, if we wanted to make a tiny 1/2 distance measuring device based on the field in a capacitor, one way would be to measure the total voltage across the cap and then try to measure a point where we see exactly 1/2 the voltage. That requires two 'volt meters'. But another way would be to measure from one plate to the center with one meter and from the other plate to the center with another meter. If we keep both meters oriented in the same direction, we get the same voltage across both meters, and since they are in series that means that one terminal of the cap is negative and one is positive with respect to the center.

Also, the only capacitor that would not exhibit a voltage drop in an AC circuit would be an infinitely large capacitor or with an infinitesimally low frequency. Any other cap will experience a voltage drop, no matter how small. So for any practical purpose any cap will have at least some voltage drop for AC.
For example, a 1uf cap at 1Mhz will have a voltage drop. Going down in frequency, this drop will decrease. It's not until we reach 0Hz that we see no drop provided the AC signal started and ended at the right phase. Coupling caps are made large enough to minimize this voltage drop, but there will always be some.

 

Hi,

It sounds like you are saying that you do not believe that there could not be a midpoint (actually an entire surface) between the two plates that is 1/2 the total potential unless the potential difference is zero. That cant be right, right?

I was only talking about this mystical mid-point zero voltage for a 180 phase shift from a capacitor. 1/2 the total non-zero potential across the capacitor is not zero.

 

I was only talking about this mystical mid-point zero voltage for a 180 phase shift from a capacitor. 1/2 the total potential across the capacitor is not zero.

I'm just an ignorant bystander here. But I thought I'd let you know that it's been a while since I've learned so much in so little time.

 

.................
Take a quick look at religions of the world. Almost none of them agree except on the basic issues. Does that mean that there are no facts? Each group has there own set of facts that they hold dearly.

Not the way I see it.
Each religion has their own set of beliefs, not facts, even the stuff they agree on.