Can an open circuit voltage field charge a capacitor?

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Main Question or Discussion Point

What happens when you sandwich an uncharged capacitor between two charged capacitors?

1. Just as a piece of Styrofoam can have a static electric charge, a capacitor's dielectric material can be charged, without the requirement of capacitor plates.

2. Given 3 equal pieces of dielectric material, with two of the pieces fully charged with static electricity. If the uncharged dielectric material is sandwiched between the two charged pieces, without creating a closed circuit. Will the uncharged dielectric material gain a static electric charge?

3. Does the charged versus uncharged state of the center piece of dielectric material depend on the polarity of the charge of its neighboring charged pieces of dielectric material?

4. If the center piece of dielectric material indeed becomes charged, will shorting this capacitor discharge all three pieces of dielectric material?

5. Does the charging of the center piece of dielectric material depend on its distance from neighboring charged capacitors? In other words, does charging the center capacitor within an open circuit require exposure of the electrostatic field of its neighboring charged dielectric material.

I claim that w/o a closed circuit, the center piece of dielectric material will not become charged. Can you prove me wrong?
 

Answers and Replies

  • #2
Defennder
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1. Just as a piece of Styrofoam can have a static electric charge, a capacitor's dielectric material can be charged, without the requirement of capacitor plates.
The dielectric material is strictly insulative. It would not be charged so long as dielectric breakdown does not occur. However the presence of an electric field would polarise the dielectric medium.

2. Given 3 equal pieces of dielectric material, with two of the pieces fully charged with static electricity. If the uncharged dielectric material is sandwiched between the two charged pieces, without creating a closed circuit. Will the uncharged dielectric material gain a static electric charge?
If they are all strictly insulative materials, then the centre item would not be charged. It would, however be "stuck" due to electrostatic attraction with the other 2 materials.

3. Does the charged versus uncharged state of the center piece of dielectric material depend on the polarity of the charge of its neighboring charged pieces of dielectric material?
I don't get this one. If it's uncharged it'll stay uncharged until some of the electrons from its neighbouring insulators slowly "leak" to/from the item. If it's charged, it wil stay as charged until again leakage occurs.

4. If the center piece of dielectric material indeed becomes charged, will shorting this capacitor discharge all three pieces of dielectric material?
Charges can't flow freely because they are all insulative materials. How do you propose to "short" them?

5. Does the charging of the center piece of dielectric material depend on its distance from neighboring charged capacitors? In other words, does charging the center capacitor within an open circuit require exposure of the electrostatic field of its neighboring charged dielectric material.
Again, how do you charge a dielectric? The dielectric for a capacitor is just to ensure the charges on each plate "stay" where they are, thereby creating a potential difference via electrostatic attraction. And in a capacitor, it is the conductive plates which are charged, not the dielectric.
 
  • #3
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I claim that w/o a closed circuit, the center piece of dielectric material will not become charged. Can you prove me wrong?
Assuming the voltage applied is lower than the dielectric breakdown voltage then it won't become charged, but it will become polarized. Do you understand the difference between "charged" and "polarized" for non-conductive materials?
 
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  • #4
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Originally Posted by Defennder
The dielectric material is strictly insulative. It would not be charged so long as dielectric breakdown does not occur. However the presence of an electric field would polarise the dielectric medium.
Perhaps the following URL provides an overview of methods to charge insulative materials.
http://www.experiencefestival.com/a/Electrostatic_generator_-_Description/id/1357749 [Broken]

Perhaps I should provide some detail on the necessity of plates in the Wimshurst generator example. The following is Wimshurst’s original publication:
http://www.physics.brown.edu/physics/demopages/Demo/em/demo/Wimshurst.pdf
The following is a quote from the Wimshurst’s publication:

“Another feature is that the neutralizing current may also be broken without reducing the excitement, but then the charges alternate from positive to negative with each half revolution of the disks.”

That means that without neutralizing bars, sparks occur each half revolution of the disks, regardless of the number of plates on each disk.

The following URL mentions some causes of triboelectric effects.
http://archive.evaluationengineering.com/archive/articles/1100cope.htm [Broken]
 
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  • #5
Defennder
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I don't see how that is relevant. Your original setup consisted of a dielectric material sandwich in between a capacitor. There was no mention of friction. I don't doubt that you can charge an insulator with static electricity; examples of this phenomenon are common in everyday lives as well as introductory physics courses. I answered your questions based on the setup of a dielectric material in a capacitor or sandwiched between 2 charged insulating materials.

Secondly you did not mention in your description of the setup that a conductor would be brought in contact briefly with the dielectric material sandwiched in between the 2 charged insulators. This step is necessary in order for the sandwiched dielectric to gain a net charge by electrostatic induction. I interpret the term "charged" to mean a "net charge", not mere polarisation itself, which is why I replied as above. I did mention that the dielectric material would become polarised in the capacitor setup, but not that it would be charged.

As DaleSpam said, you have to distinguish between "polarised" and "charged" for insulative materials.
 
  • #6
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There is a difference between charged and polarized. Charged material contains voltage's energy. Polarized material contains polar molecules.

Some texts claim that dielectric material becomes polarized when exposed to an electromagnetic field.

Consider that when a vacuum capacitor becomes charged, the vacuum contains a voltage gradient, but the vacuum is not polarized. We know that the vacuum within vacuum tube diodes and triodes contained a voltage gradient, because the voltage gradient accelerated thermionic electrons. We know that the tube's vacuum was not polarized because the vacuum itself contained no atoms or molecules that could be polarized.

Anything (including conductors, resistors and capacitors) that contains a voltage gradient contains voltage’s energy. If polarization occurs w/o a voltage gradient, then that polarized material is w/o a voltage’s energy. The dielectric material within a charged capacitor indeed contains a voltage gradient.
 
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  • #7
Defennder
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Where did I say a vacuum can be polarised? I was referring to dielectric materials,not vacuum. Furthermore, where did I deny that the insulating dielectric cannot contain a potential gradient? The textbooks you cite are referring to dielectric insulative materials, and clearly not vacuum in particular. It is apparent that in the case of a vacuum, there is nothing to be polarised, so what point are you trying to make here?

I still don't see how this connects with your original questions.
 
  • #8
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Where did I say a vacuum can be polarised? I was referring to dielectric materials,not vacuum. Furthermore, where did I deny that the insulating dielectric cannot contain a potential gradient? The textbooks you cite are referring to dielectric insulative materials, and clearly not vacuum in particular. It is apparent that in the case of a vacuum, there is nothing to be polarised, so what point are you trying to make here?

I still don't see how this connects with your original questions.
Perhaps Defennder should re-read his 1st response:
The dielectric material is strictly insulative. It would not be charged so long as dielectric breakdown does not occur. However the presence of an electric field would polarise the dielectric medium.
In my responses to Defennder’s 1st response, I (tried to) put forth the idea that:
1. Dielectric material can be charged w/o the requirement of dielectric breakdown, albeit one of the charging methods is the triboelectric effect.

2. The charge within dielectric material is not because of polarization of dielectric material, because a vacuum is dielectric material. A vacuum is dielectric material, because a vacuum is a poor conductor of electricity and a vacuum can store voltage’s energy.

Perhaps dielectric material simply contains voltage's energy, instead being a converter of voltage's energy to polarization energy.
 
  • #9
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There is a difference between charged and polarized. Charged material contains voltage's energy. Polarized material contains polar molecules.
Almost, a charged material has a different number of positive and negative charges, there is a net charge. A polarized material has an uneven distribution of charges, but no net charge, e.g. a surface layer of polar molecules with all of the + ends pointed out and the - ends pointed in.
 
  • #10
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Almost, a charged material has a different number of positive and negative charges, there is a net charge. A polarized material has an uneven distribution of charges, but no net charge, e.g. a surface layer of polar molecules with all of the + ends pointed out and the - ends pointed in.
Thank you DaleSpam for identifying which characteristics differ in a charged versus polar molecule. I really did not know that charged material has a different number of positive versus negative charges. I am having some trouble verifying your net charge hypothesis. Perhaps you could either provide some more detail, or provide references that support your hypothesis.

Your incite into the nature of polarized material is fantastic. Many people think that most polarized molecules are bipolar, thus result from displacement of a single charge. Do I understand you correctly; the surface layer of a polar molecule has multiple + ends pointed out and multiple – ends pointed in? I did even not know that the + ends of a polar molecule point out, or that the – ends point in.
 
  • #11
Defennder
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Perhaps Defennder should re-read his 1st response:
Perhaps pzlded should re-read his original post:

pzlded said:
What happens when you sandwich an uncharged capacitor between two charged capacitors?
1. Just as a piece of Styrofoam can have a static electric charge, a capacitor's dielectric material can be charged, without the requirement of capacitor plates.
My response which you convenient quoted completely out of context was written with the bolded part of your original post in mind. You provided the setup, neglected to mention friction or any way in which it could be used to charge the insulator, nor provided means by which charging by electrostatic induction can be used and then you baselessly accused me of claiming that insulators cannot be charged at all.

pzlded said:
In my responses to Defennder’s 1st response, I (tried to) put forth the idea that:
1. Dielectric material can be charged w/o the requirement of dielectric breakdown, albeit one of the charging methods is the triboelectric effect.
This is trivial. Nowhere have I denied that insulators may be charged. Again I reiterate that your setup says nothing about friction or partially discharging the polarised material, which can be done easily if it is conductor, but not if they are dielectrics. Quoting out of context is disingenous. I have already said this was very common and self-evident.

pzlded said:
2. The charge within dielectric material is not because of polarization of dielectric material, because a vacuum is dielectric material. A vacuum is dielectric material, because a vacuum is a poor conductor of electricity and a vacuum can store voltage’s energy.

Perhaps dielectric material simply contains voltage's energy, instead being a converter of voltage's energy to polarization energy.
Dear pzlded, can you distinguish the difference between "polarised" and "charged" ? Please look at DaleSpam's last post more carefully for the difference. There is no "charge" within a dielectric material in a capacitor setup. If you want to charge it, it is by means of as you said tribo-electric effect and electrostatic induction. As for your point of vacuum being dielectric material, you are splitting semantical hairs. You want to fault textbooks for leaving out the trival possibility that vacuum, which is literally empty space cannot be polarised, when it is readily apparent.

And it turns out, if I'm not mistaken, that there is actually something known "vacuum polarization" in quantum physics, thought I don't think that is relevant here.
 
  • #12
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Perhaps you could either provide some more detail, or provide references that support your hypothesis.
It isn't really a hypothesis, it is just the definition of polarized and charged. For reference, what I am describing is straight out of my 2nd semester freshman physics text book: Serway, "Physics for Scientists and Engineers" 3rd edition. Look at Chapter 26 with a special emphasis on section 26.7 and the figures on page 727.
 
  • #13
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Hi DaleSpam,

The force between the charges is said to be the source of voltage’s energy. Near the + plate, there is a net excess of + charges; near the – plate, there is an excess of – charges. Kirchoff’s current law specifies that “current in equals current out”, therefore + charges in equal + charges out. In other words, if the + plate gets more charge than the – plate, the charges got there in violation of Kirchoff’s law.

In the diagram of a polarized molecule, the little +’s and – ‘s represent the field from the + charge at one end of the molecule and the – charge at the other end.
 
  • #14
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In other words, if the + plate gets more charge than the – plate, the charges got there in violation of Kirchoff’s law.
That is OK, Kirchoff's law is not a fundamental law and gets broken all the time. For example, Kirchoff's law doesn't work in antennas either.

You use Kirchoff's law for analyzing small circuits and you usually only apply it at nodes of your circuit diagram.
 
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  • #15
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Hi Defennder,

Perhaps I did misinterpret your response. Let us start over. I would like you (or others) to provide whatever assistance you can in clarifying a problem. I would like to know more about the nature of the field beyond a capacitor’s plates.

A vacuum separates two charged insulators. The charged insulators attract each other.
Does the portion of electrostatic field within the vacuum contain a voltage gradient?

If you claimed that electrostatic field between the insulators has a voltage gradient:
Why am I not been able to detect the voltage gradient with a high impedance voltmeter?

If you claimed that, the electrostatic field within the vacuum has no voltage gradient:
How do point charges produce both a field within an insulator with a voltage gradient, and the attractive field within a vacuum that does not have a voltage gradient?
 
  • #16
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If you claimed that electrostatic field between the insulators has a voltage gradient:
Why am I not been able to detect the voltage gradient with a high impedance voltmeter?
Are you sure about that? I would think you would be able to detect it. Perhaps not with a standard voltmeter, but certainly with a device designed for the purpose.
 
  • #17
Defennder
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If you claimed that electrostatic field between the insulators has a voltage gradient:
Why am I not been able to detect the voltage gradient with a high impedance voltmeter?
You probably have to use a sensitive electrometer to detect it. I'm not familiar with experimental devices, so I won't be able to help you here, sorry.
 
  • #18
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Placing a conductor between two charges does not change the electrostatic force between the charges. Adding capacitor plates to each of the two isolated charged insulators can reduce the voltage gradient between the two insulators to zero, w/o changing the force between the two charged insulators.

Point charges do not produce a force w/o a voltage gradient. Is there a theoretical explanation of how a conductor can reduce a voltage gradient w/o reducing force between two charges?
 
  • #19
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Placing a conductor between two charges does not change the electrostatic force between the charges. Adding capacitor plates to each of the two isolated charged insulators can reduce the voltage gradient between the two insulators to zero, w/o changing the force between the two charged insulators.
Your first sentence is correct, but not the second sentence. The definition of the voltage field is that it is the gradient of the voltage is the e-field, and the definition of the e-field is that it is the force on a test charge. So if the voltage gradient is reduced at the insulator then the force is necessarily reduced also, by definition.

Note that a conductor in the middle will have a zero e-field inside the conductor, but will not change the e-field outside the conductor (for an infinite sheet charge and conductor at least, I am not sure about more complicated geometries).
 
  • #20
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Hi DaleSpam,

Thank you for your thoughtful opinion.

The conductors in an electronic vacuum tube are tube elements that control (not short) the vacuum voltage gradients between elements. There is no current in the conductor inserted between to charged insulators within a vacuum, because there are no point charges in the vacuum. W/O current, the conductor does not convert vacuum volts to thermal energy or magnetism.

Perhaps you could more precisely explain the interaction between the conductor and the volts between the two-isolated (open circuit) charged insulators.
 
  • #21
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The conductors in an electronic vacuum tube are tube elements that control (not short) the vacuum voltage gradients between elements. There is no current in the conductor inserted between to charged insulators within a vacuum, because there are no point charges in the vacuum. W/O current, the conductor does not convert vacuum volts to thermal energy or magnetism.
OK, that is true except for the small transient currents when you first insert the conductor. So what?

Perhaps you could more precisely explain the interaction between the conductor and the volts between the two-isolated (open circuit) charged insulators.
Are you familiar with the Gauss' Law derivation of the E-field between two infinite sheets of opposite charge? If so, then you can follow the same procedure to determine the E-field when you have a conductor between them. You will find that the E-field between the charge sheet and the conductor is unchanged. You can even use Gauss' Law to determine the amount of charge on each face of the conductor.
 
  • #22
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The conductors in an electronic vacuum tube are tube elements that control (not short) the vacuum voltage gradients between elements. There is no current in the conductor inserted between to charged insulators within a vacuum, because there are no point charges in the vacuum. W/O current, the conductor does not convert vacuum volts to thermal energy or magnetism.
OK, that is true except for the small transient currents when you first insert the conductor. So what?

Perhaps you could more precisely explain the interaction between the conductor and the volts between the two-isolated (open circuit) charged insulators.
Are you familiar with the Gauss' Law derivation of the E-field between two infinite sheets of charge? If so, then you can follow the same procedure to determine the E-field when you have a conductor between them. You will find that the E-field between the charge sheet and the conductor is unchanged. You can even use Gauss' Law to determine the amount of charge on each face of the conductor.
 
  • #23
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Isn’t there a problem with voltage gradients bound by conductors?

The amount of energy above a horizontal charge sheet equals the energy below the charge sheet. Placing a conductor on one side of a charge sheet converts (shorts) half the charge sheet’s energy. The short converts voltage's energy to magnetic or thermal energy.

Because of the short, devices that contain a linear voltage gradient bounded by conductors (including capacitors, vacuum tubes and resistors), are missing half their voltage’s energy. In an LC oscillator, when moving electrons from magnetism completely convert to a sheet of charge: the plates short out 50% of the resulting voltage’s energy.


The linear voltage gradient within resistors is another problem. The only configuration of point charges that produce a linear voltage gradient is an infinite plane. A resistor does not normally contain an infinite sheet of charge.
 
  • #24
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Isn’t there a problem with voltage gradients bound by conductors?
The E-field inside a conductor is 0. I don't see why that is a problem.

Placing a conductor on one side of a charge sheet converts (shorts) half the charge sheet’s energy. ... Because of the short, devices that contain a linear voltage gradient bounded by conductors (including capacitors, vacuum tubes and resistors), are missing half their voltage’s energy.
This is incorrect. The E-field is only 0 inside the conductor itself. The E-field outside the conductor it is unaffected. You can derive that using Gauss' law.

The linear voltage gradient within resistors is another problem. The only configuration of point charges that produce a linear voltage gradient is an infinite plane. A resistor does not normally contain an infinite sheet of charge.
How is that a problem? Consider an analogy: an infinite sheet of mass generates a uniform gravity field (the force on a mass is constant), a kinetic friction force is also constant. Is there a problem? Why not?

I don't know what your real question is, but I get the feeling that you are not asking it. Every time I answer a quesiton it results in two more almost unrelated questions. Can you please take some time and think about what your underlying point of confusion is? All of these things that you list as problems are obviously not problems since these principles work and basic electronics depend on them. Therefore the problem is simply one of understanding, so please think carefully about the real question that you have and try to distil it into one or two fundamental points.
 
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  • #25
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Hi DaleSpam

I am trying to reconcile the difference between shorting a capacitor and shorting the voltage between capacitors. I suppose I should first ask about ‘sheets of charge’.

A voltage gradient within a charged peanut shaped piece of Styrofoam does not require conductive plates. Therefore, a voltage gradient can exist within a vacuum w/o conductive plates. Both Styrofoam and a vacuum are insulators that can contain a voltage gradient. How can the conductor-less vacuum contain ANY point charges or sheet of point charges?

Shorting any portion of a voltage gradient reduces only that portion of the voltage gradient to zero. Therefore, shorting a portion of a voltage gradient within a vacuum reduces only that portion of the vacuum voltage gradient to zero. When a portion of a voltage gradient within a vacuum is shorted, there is no release or flow of point charges from the vacuum to the conductor.

In the original problem, the two isolated insulators could have been composed of vacuum dielectric material; the area between the insulators could also have been composed of vacuum dielectric material. :cool:
 

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