A question about capacitor

what if one plate of a capacitor is connected to the power, the other end is NC, is this capacitor neutral or charged?

Thanks in advance.

desperatejobseeker,

what if one plate of a capacitor is connected to the power, the other end is NC, is this capacitor neutral or charged?

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Hmm, something tells me you don't quite know what a capacitor is or does. A capacitor is an electrical energy storage device. It stores its energy in an electrostatic field. This field is caused by a voltage difference between its two plates. This voltage difference is caused by a excessive amount of electrons on one plate and an equally deficient number of electrons on the opposite plate. No matter how high the voltage is between the plates, the net charge is the same as no voltage between the plates, specifically zero. Because the net charge of a capacitor never changes from zero, it is the wrong to say a capacitor is "charged". A capacitor is never charged. It is energized.

To answer your question. Nothing will happen when you connect only one lead of a capacitor to a voltage source. That will not make a voltage difference happen.

Ratch

Good to know, many thanks.

Rather I shall say, capacitor is charge storage device. "what if one plate of a capacitor is connected to the power, the other end is NC, is this capacitor neutral or charged?" Q = CV. if more voltage applied across capacitor, and there is path from supply to ground, capacitor will store more charge and in other word, more electrons in the plate.

Here you say if another plate of capacitor is not connected , you mean floating, you can say that there is no path for electrons to charge the capacitor, and nothing comes to capacitor plate.

varunkant2k,

Q = CV. if more voltage applied across capacitor, and there is path from supply to ground, capacitor will store more charge and in other word, more electrons in the plate.

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The point I am trying to make is that a capacitor stores energy, not charge. Any charge added to one plate is offset by an equal removal of charge on the other plate for a net charge of zero. The plates are imbalanced with respect to charge, but that is not the same as storing charge. That is why I aver that a capacitor should be described as being energized, not charged.

Ratch

Energy is nothing but E= QV = CV2. See if one terminal is connected to ground and another terminal is connected to 5 Volts, positive plate will carry +5V negative plate will carry 0V (GND). If you increase voltage difference between plates they have to counter it and Positive plate will absorb more electrons , and to counter the positive holes created , ground plate will take more electrons to cancel the effect.
As you said their are no change in the charge ( it looks like that but not in real) but you can see the charge density across plates are now very dense. You can say it, the charge has increased. If Q increased energy increased.

varunkant2k,

Energy is nothing but E= QV = CV2.

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Energy is the ability to do work. Describing energy quantitatively by a formula, right or wrong is not the correct way to define it. The above formula is not correct for energy stored a capacitor. See http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html.

If you increase voltage difference between plates they have to counter it and Positive plate will absorb more electrons , and to counter the positive holes created , ground plate will take more electrons to cancel the effect.
As you said their are no change in the charge ( it looks like that but not in real) but you can see the charge density across plates are now very dense. You can say it, the charge has increased. If Q increased energy increased. .

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The capacitor plates do not absorb electrons. Electrons accumulate on the negative plate, but they are not absorbed. I said there is no change in the net charge. The charges in a capacitor become separated on the plates, but the total net charge has not changed. There is no charge density across the plates. There is an accumulation of charge on one plate and a deficiency of electrons on the other plate, but no charge density across the dielectric. For instance, in a vacuum capacitor, there is no material including electrons between the plates. However, there is an electric field across the dielectric which contains energy. Holes do not exist outside of P-type semiconductor. The sea of electrons found in wires and other conductive materials would completely neutralize them in an instant. Holes can only exists in semiconductors where electrons are fairly scarce. Immobile positive ions exist on the positive plate of a capacitor after the plate gives up its valance electrons.

Ratch

Hi Ratch, You explained it clearly, but I do not agree with your proposition about net charge constant.
See, there are two scenario, one where net charge is constant, another where is gets distributed.
Let me give one example,
Let charge 1F capacitor with 5V, and disconnect supply.( you have voltage of 5V across Cap). Now you connect two parallel capacitors of 1F each across 5V charged capacitor (total three caps). What do you think, how much votage will be across each capacitor? Its straight, 5/3V. I am telling charge across C1 was distributed equally. Again you connect all C1 C2 and C3 in series. how much voltage / charge will be at each node/cap?
So here you can say net charge will be constant.
Now If you say capacitor stores energy not Charge, I do not have problem. You see these two are interdependent.

varunkant2K,

You explained it clearly, but I do not agree with your proposition about net charge constant.
See, there are two scenario, one where net charge is constant, another where is gets distributed.
Let me give one example,

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I think you will agree after I solve the problem you presented.

Since a capacitor is a energy storage device, and you give voltages, I will work the problem using only energy and voltage. The energy in the first capacitor is (1/2)*C*E^2 or 12.5 joules. That is the amount of energy we have to work with, and unless we add energy, that is our base. Now, we distribute the energy across three caps of the same 1f value, the voltage will be 12.5 = (1/2)*3*E^2, from which E = 2.89v . Now we put them in series for a total voltage of 8.66v . The three caps in series have a total capacitance of (1/3)f . This gives a total energy stored of (1/2)*(1/3)*8.66^2 = 12.5j . The energy is conserved and the charge imbalances across the plates of each cap are irrelevant. Nevertheless, the net charges of each cap before and after each reconnection and energizing are the same, i.e. zero.

Ratch

the net charges of each cap before and after each reconnection and energizing are the same, i.e. zero.

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hi Ratch, this sentence I can complete for Energy also. I don't disagree with your energy conservation theory, its well proved. But I can not say net energy or net charge is zero?
Question was If you connect supply across capacitor, it charges or not? It energies or not?
If I follow your idea, supply shouldn't dissipate any charge and hence zero current, once it is connected across Capacitor?
You do not want to tell this, right? There is instantly a charge sharing across supply and Capacitor, and proportionally it gets charged. And supply do losses some charge/current.
I am also telling that, Electric energy increases with proportion to charge trapped in the capacitor. Actually capacitor uses electrical field to trap the charge.

varunkant2k,

the net charges of each cap before and after each reconnection and energizing are the same, i.e. zero.
hi Ratch, this sentence I can complete for Energy also.

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And still be correct? No, the net charge of each capacitor is the same before and after, that is zero. In the problem, the cap was energized, not charged, to 12.5 joules.

But I can not say net energy or net charge is zero?

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Net charge is always zero. Net energy is whatever you start out with. In the problem you started out with a cap energized to 5 volts.

If I follow your idea, supply shouldn't dissipate any charge and hence zero current, once it is connected across Capacitor?

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The voltage supply did supply voltage and charge carriers to the capacitor, and received them back in return. For every electron given to the capacitor, one electron was returned back to the voltage supply. That is the way it works for any battery or voltage source. There was a net quantity of zero charges lost or gained by the cap and voltage source.

You do not want to tell this, right? There is instantly a charge sharing across supply and Capacitor, and proportionally it gets charged. And supply do losses some charge/current.

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A voltage source acts like a water pump. Does a water pump lose or gain water?

I am also telling that, Electric energy increases with proportion to charge trapped in the capacitor. Actually capacitor uses electrical field to trap the charge.

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The net charge is zero, so no charge is "trapped". Energy is not trapped, it is stored.

Ratch

The voltage supply did supply voltage and charge carriers to the capacitor, and received them back in return. For every electron given to the capacitor, one electron was returned back to the voltage supply.

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Hi, did you read or did you measured sometime?
I think you need to read once again and if possible measure the supply, if it is having no charge dissipation.

varunkant2k,

Hi, did you read or did you measured sometime?

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Measure what and how?

I think you need to read once again and if possible measure the supply, if it is having no charge dissipation.

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Do you mean the voltage supply losing charge? What does that mean? For every electron that leaves a voltage source, another electron returns. It has to be that way or an imbalance of charges would occur. That never happens in a voltage supply, does it?

Ratch

I think, you did not get my point. I have asked , did you measured supply current to capacitor, sometime?
Ok, you told "What does that mean? For every electron that leaves a voltage source, another electron returns."
Really?
If supply is connected across capacitor (initial voltage zero) to ground, what the current direction do you expect?
I expect, a current spike direction from supply to ground. I know electrons are coming to supply, thats how current flow. But I don't expect electrons to go back to ground, do you? Please clarify this.

varunkant2k,

I am having a hard time understanding your question. Ground is an abitrary designation in a circuit that has no effect on current. Perhaps if you posted a circuit that outlines your question, I could answer it.

Ratch

Please see the first comment ( main question #1) of the thread. Instead of NC , I connected ground.

Ground is an abitrary designation in a circuit that has no effect on current.

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Really its very tough, every time you come up with some new terminology. I don't think any current without ground (ground referenced source) .

varunkant2k,

I don't think any current without ground (ground referenced source) .

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You said current, but I think you mean voltage. Voltage can me measured between any two points, not necessarily ground.

Ratch

Nice discussion.

Although Ratch is right, that a capacitor literally doesn't store charges because the current sum through it's terminals is zero, this viewpoint apparently contradicts common explanations of capacitor operation. "Charging a capacitor" (or a battery) seems to be a clear technical description at first sight. The process can be described by an equation:

Q = ∫I dt = C V

How do both sides go together? The answer is, that when "charging" a capacitor, the charges on both capacitor "plates" are separated, but it's sum stays constant.

Having this in mind, we can keep on talking about capacitors in usual terms, including "charging" and "discharging", I think.

I jump into this hot discussion.
There is no problem if you connect one pole of capacitor to live mains wire but don't touch the other end weather capacitor is charged or not.:lol:
Some information already available on net,

**broken link removed**
http://www.kpsec.freeuk.com/components/capac.htm
http://freecircuits.org/2012/01/capacitors-basics-working/
http://en.wikipedia.org/wiki/Capacitor
http://electronics.howstuffworks.com/capacitor.htm
**broken link removed**

Ratch does have a point. If you calculate the net charge for plate A and B in respect to some middle point, you could say the net charge remains constant. I haven't dug into the meaning of charge. But I'd say there's also a electric charge between the plates. That would mean that there really is a charge in "charged" capacitor.

Also "For every electron that leaves a voltage source, another electron returns." as Ratch said is true in basic circuits, where we don't for example break atoms to create charges etc.

I think the initial question is actually great! Ofcourse if you don't make a closed circuit, it doesn't do anything according to classic circuit theory, where some effects are negligible. But consider a case where you had a huge potential difference in respect to ground/earth soil/other huge conductor/electron storage. If you isolate yourself completely from the ground by whatever means(resistive and good enough capacitive isolation) and then charge yourself to 100k voltage, you will have a charge against ground because of huge difference in charge carriers (often electrons). And they try to spread on the conductor as evenly as possible, so they will cause a current/spark to another conductor. All this need huge conductive material volumes, and the material needs to be able to accept the charge carriers, but even human body is enough for alot. But you cannot obviously prove this easily with two capacitors.