Redistribution of charge in a capacitor

ehabmozart

1. The problem statement, all variables and given/known data
A 0.01 F capacitor is charged by and then isolated from an 8 V power supply.
a) Calculate the charge stored... Straight forward... Q=CV
b) The capacitor is then connected across another identical capacitor which is uncharged. Describe and explain what will happen to the charge and voltage on each capacitor... This is a weak point for me.. I can't even find a clue.. All what i know is that C in parallel are given by C=C1+C2.. I really need help in this question and generally in how electrons are redistributed when a discharging capacitor is connected with another ones... Thanks in advance to whoever show his kind hand! :D


2. Relevant equations



3. The attempt at a solution

NascentOxygen

Hi ehabmozart. Well, the charge isn't going to disappear or anything like that. So all it can do is redistribute itself. If the caps are identical, how much do you think each will end up with?

ehabmozart

U mean it's gonna half.. I thought of that but what's the quantitative proof for that.. I read that C1/C2 = Q1/Q2 but i really want to know the proof.. Anyway, thanks for helping!

ehabmozart

And infact i mean, for example what if one cap. had more voltage or less capacitance.. I mean is there a rule behind that??

MalachiK

If you connect any two caps in parallel then they will have the same p.d. across them once all of the charge has finished moving. For any capacitor, if you know the p.d. and the capacitance then you can find how much charge it holds.

I think the trick is to work out the total capacitance of your two caps in parallel and then treat this combination as if it were a single cap holding however much charge you had in total on the caps at the begining. This should lead you to the new p.d. across the caps. I'm pretty sure that this sort of thinking will lead to a general sort of understanding that can be applied to problems where the capacitors aren't exactly the same at the beginning.

thisischris

I asked a similar question here: http://www.physicsforums.com/showthread.php?t=612395

NascentOxygen

Sounds right! ☺

Ratch

ehabmozart,

The first capacitor is energized to 8 volts and contains zero net charge. It does contain 8*0.01E-6 coulombs of separated charge, and (1/2)*0.01R-6*8^2 joules of energy. The second cap is initially inert. When connected, the voltages will equalize, because you cannot have one cap at a higher voltage than another. The cap with the voltage will share its energy with the cap with no energy. This will cause a current to exist from the energized cap to the other cap until equilibrium is reached. Capacitors are energy storage devices, it it makes sense to calculate their voltages from an energy standpoint.

Both caps are equal, so their capacity is "C". Since no resistor exists in the circuit, we can assume no energy loss, and the energy will be the same before and after.

(1/2)*C*8² = (1/2)*C*V²+(1/2)*C*V² ===> 64 = V²+V² , from which you can easily figure out the final voltage, and then the charge separation of each cap. Notice that it was not necessary to know the value of the cap to calculate the final voltage. If there was resistance in the circuit, then you would have to set up and solve a differential equation to get the same result. See http://www.physicsforums.com/showthread.php?t=610855

By the way, since no resistance was assumed in the circuit, the equalization current will be infinite. In a practical circuit, this is not possible, but the problem does not ask you to consider resistance, does it?

Ratch

NascentOxygen

Ratch, the purpose of these homework forums is to offer only gentle assistance to nudge students towards finding the solution to problems. You are not supposed to work through the entire solution with no interaction or input from the student. Despite the best of intentions, doing someone's homework for them is of no help to anyone.

Not only is your approach wrong, so is your answer.

ehabmozart

Well, nansent oxygen :S ... This question was not for the purpose of solving my hw.. This is not a hw.. It is a question in my book with an unclear mark scheme. I needed clarification in the full concept. Guys, thank you all for gving me the replies. I gotta an exam tomorrow! :D

Ratch

NascentOxygen,

Now you got me interested. How is the approach and answer wrong?

Ratch

MalachiK

Well I could be wrong, but doesn't your 64 = 2V² lead to V = √32 = 5.66?

On the other hand, I get exactly 4 volts which is half the inital p.d. across the first cap. Intuitively, this is what I would expect to happen if we have the same charge distributed across twice the initial capacitance.

Am I missing something here?

MalachiK

Thinking about the energy, if the initial energy of the single cap is...

E_i = 1/2 × 0.01 × V_i²

and the final energy of the system is

E_f = 1/2 × 0.02 × V_f²

Then 0.01 V_i² = 0.02 V_f²

which leads to....

V_i = 2V_f, no?

Yukoel

Well as far as I know when you connect capacitors the voltage or more technically potential difference across both of them should be in accordance with the Kirchhoff's loop rule.The connection does not contain any resistance and necessary calls for the steady state condition of the circuit .So these capacitors being identical must redistribute charge in such a way as to have same P.D. across them with opposite polarities (so that net voltage drop across the circuit is zero).Since their capacitances are the same you can easily see that they have the same charges on them and hence same P.D. .Redistribution of charge in capacitors involves loss of half the initial energy which trivially appears even in the charging case .
In short you must use Kirchoff's voltage rule while handling capacitor problems.Correct me if I am wrong.


Pardon me for violating any rule since this is my first post here.
-regards
yukoel

Ratch

MalachiK,

Yes, it does. That assumes no resistance is present in the circuit, and an infinite equilization current can occur. Also the caps will equalize instantaneously. This insures the energy before after equilibrium is the same.

Yes, you are. Before I explain it, let me say that this is why I keep harping on the fact that capacitors don't get charged, they get energized. The net charge of a cap at zero volts is the same as a cap at 100 volts, namely zero. Capacitors don't store charge, they store energy. You based your answer on the mistaken belief that one cap stored a charge and shared it equality with a second cap. But there is no conservation of charge in a cap. Caps separate charge, they don't store it. The abundance of electrons on one plate is matched by a paucity of electrons on the opposite plate for a net change of zero. It takes energy to separate charges, and that energy is what a cap stores.

So the answer I worked out of 5.66 volts is for an ideal circuit that cannot possibly exist. However, it can easily be shown that if there is any finite resistance in the circuit. The final voltage for two equal caps will be one-half of the initial energizing voltage, and the total energy of the two caps will be one-half of the energy stored in the initially energized cap. The other half of the energy will be dissipated in the resistor, and it does not matter what the finite value of the resistor is.

According to your calculation, Vi = √2Vf

Ratch

Ratch

Yukoel,

Yes, the KVL rule is in effect.

Now you ran off the track. As I said in the previous post, caps do not store, conserve, or distribute charge because they have no net charge available. In this circuit the caps store and distribute energy according what is available and their capacitance value.

It depends on the problem.

With pleasure.

Ratch

MalachiK

Yep, that's right! If I wasn't so stupid I'd have got exactly the same result as you.