# Simple capacitance problem

#### Krushnaraj Pandya

Gold Member
1. The problem statement, all variables and given/known data
There exists a potential difference V between the plates of a parallel plate capacitor of capacitance C. Now it is connected to a cell with potential V. What will be the heat generated?

2. Relevant equations
Work(on battery) - (energy stored in Capacitor) = Heat produced

3. The attempt at a solution
My logic was that since both have potential V, no charges will flow, but the answer is 2C(V)^2 ; can someone please explain? Thank you

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#### BvU

Homework Helper
Does it say HOW it is connected ?

#### Krushnaraj Pandya

Gold Member
Does it say HOW it is connected ?
ohh, there was a diagram which I converted to text here and I just noticed negative terminal of battery is connected to positive plate of capacitor and vice-versa. so will all the charge flow into the cell?

#### BvU

Homework Helper
Where else ? So: Yes! ... but that's not the whole story ...

#### Krushnaraj Pandya

Gold Member
Where else ? So: Yes! ... but that's not the whole story ...
physics problems are really exciting to uncover this way...it feels like I'm reading a murder mystery when you say "that's not the whole story...". I really like this forum.
anyway, intuitively speaking, both will have same potential at equilibrium, so does it decrease to zero for both? In that case, energy lost by capacitor is 1/2C(V)^2 and (work done is CV?), some part of my logic doesn't make sense

#### BvU

Homework Helper
so will all the charge flow into the cell
As I said: yes. At that point, you have an uncharged capacitor, connected to a cell with potential V (*) . What happens ?

(*) Nothing specific is said about the cell, so we must assume it keeps up the voltage V

#### Krushnaraj Pandya

Gold Member
As I said: yes. At that point, you have an uncharged capacitor, connected to a cell with potential V (*) . What happens ?

(*) Nothing specific is said about the cell, so we must assume it keeps up the voltage V
this seems counter-intuitive...so all the charge (Q=CV) flows into the cell, The new potential between the plates is zero and the cell's potential is unaffected...but
1) it must have had done some work to maintain that potential, since charge=CV was flown in at potential V- this work should be C(V)^2
2) The energy lost by capacitor is definitely 1/2 C(V)^2
am I wrong in saying one of these two? perhaps my conceptual clarity is lacking if so. I'd be grateful if you could remove it.

#### BvU

Homework Helper
Pity you already had the answer -- ( naively discharging ${1\over 2} CV^2$ plus charging idem sums up to $CV^2$ so you still miss a factor 2 ).

But you are asking the right questions and I agree with 2). Of 1) I'm not so sure (the $\Delta V$ is not constant) Note that the exercise asks for the heat generated, no matter where.

The new potential between the plates is zero and the cell's potential is unaffected...but
This cannot last. The cell doesn't rest until the plates also have a potential difference V. This is what I meant with
What happens ?
Everything is revealed on Hyperphysics (don't forget to click "derive expressions" and "But the ... go ?" )

physics problems are really exciting to uncover this way
We usually do not have have the right answer at hand, though ...

#### Krushnaraj Pandya

Gold Member
Pity you already had the answer -- ( naively discharging ${1\over 2} CV^2$ plus charging idem sums up to $CV^2$ so you still miss a factor 2 ).

But you are asking the right questions and I agree with 2). Of 1) I'm not so sure (the $\Delta V$ is not constant) Note that the exercise asks for the heat generated, no matter where.

This cannot last. The cell doesn't rest until the plates also have a potential difference V. This is what I meant with

Everything is revealed on Hyperphysics (don't forget to click "derive expressions" and "But the ... go ?" )

We usually do not have have the right answer at hand, though ...
sorry, I went to meditate for a while, give me a little time to understand capacitors in more detail and so respond accordingly then

#### gleem

There is a voltage change on the capacitor of 2V ( V - (-V) ) so the energy that the battery must supply (ideally) is ½C(2V)2 = 2CV22

WRT heat that should only be the result of resistance in the circuit and not from creating the electric field between the plates (ideally with no dielectric).

#### Delta2

Homework Helper
Gold Member
I suggest since the problem talks about heat generated, we may assume that there is total resistance R in the circuit.

The interesting part is that regardless what is the value of R, we can find that the total heat generated in resistance R is independent of R and it is indeed $2CV^2$. The time period that this heat will be generated depends on R (theoretically is infinite, but practically is 10RC).

To prove this we can either do the math in a RC circuit (calculate current $I_1$ as function of time for the period where the capacitor is discharging through the battery, then calculate $\int_0^{\infty}I_1^2(t)Rdt$, then calculate current $I_2$ function for the period where the capacitor is charging and again calculate $\int_0^{\infty} I_2^2(t)Rdt$ and then add ) which i believe is the harder way, or we can think in terms of energy and check what happens to energy provided/absorbed during the discharge and charging process.

Thinking in terms of energy:
For the discharging process your post #7 is correct. The energy provided by the battery is $\int VIdt=V\int Idt=VQ=CV^2$. The energy provided by the capacitor is $1/2CV^2$. All this energy is lost as heat in the resistance R. So total heat for this phase is $3/2CV^2$

For the charging process the energy lost in the resistor as heat will be $1/2CV^2$. How can you prove that? (think again how much energy the battery provides and how this energy is split into heat in the resistor and electric field energy in the capacitor)

Last edited:

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#### Krushnaraj Pandya

Gold Member
Everything is revealed on Hyperphysics (don't forget to click "derive expressions" and "But the ... go ?" )
I read this and it cleared up a lot of things, thank you

#### Krushnaraj Pandya

Gold Member
This cannot last. The cell doesn't rest until the plates also have a potential difference V. This is what I meant with
so, the capacitor discharges but since the cell still has a potential V, the plate that had positive charge initially will start getting negative charge and vice versa till the capacitor is again at a potential V? Am I right?

#### Krushnaraj Pandya

Gold Member
There is a voltage change on the capacitor of 2V ( V - (-V) ) so the energy that the battery must supply (ideally) is ½C(2V)2 = 2CV22

WRT heat that should only be the result of resistance in the circuit and not from creating the electric field between the plates (ideally with no dielectric).
Usually the resistance takes up half the energy supplied in the form of heat, I understood your point that the battery must ideally supply 2C(V)^2...but wouldn't that mean that all the energy supplied transformed to heat, since the answer is also 2C(V)^2. where am I going wrong?

#### Krushnaraj Pandya

Gold Member
I suggest since the problem talks about heat generated, we may assume that there is total resistance R in the circuit.

The interesting part is that regardless what is the value of R, we can find that the total heat generated in resistance R is independent of R and it is indeed $2CV^2$. The time period that this heat will be generated depends on R (theoretically is infinite, but practically is 10RC).

To prove this we can either do the math in a RC circuit (calculate current $I_1$ as function of time for the period where the capacitor is discharging through the battery, then calculate $\int_0^{\infty}I_1^2(t)Rdt$, then calculate current $I_2$ function for the period where the capacitor is charging and again calculate $\int_0^{\infty} I_2^2(t)Rdt$ and then add ) which i believe is the harder way, or we can think in terms of energy and check what happens to energy provided/absorbed during the discharge and charging process.

Thinking in terms of energy:
For the discharging process your post #7 is correct. The energy provided by the battery is $\int VIdt=V\int Idt=VQ=CV^2$. The energy provided by the capacitor is $1/2CV^2$. All this energy is lost as heat in the resistance R. So total heat for this phase is $3/2CV^2$

For the charging process the energy lost in the resistor as heat will be $1/2CV^2$. How can you prove that? (think again how much energy the battery provides and how this energy is split into heat in the resistor and electric field energy in the capacitor)
Everything above "thinking...energy" was above me; probably because current electricity is still the next chapter in my textbook, I know that heat generated is independent of R though. Regardless, I should be able to derive the answer from what I have learned.
Anyway, you mention that I'm correct for the discharging process and C(V^2) is supplied by battery and half of that by capacitor; how did you say all this energy is lost as heat though? doesn't a part of 3/2 C(V^2) get used to charge the capacitor again and the other part as heat
For the charging process the energy lost in the resistor as heat will be 1/2CV^2. How can you prove that? (think again how much energy the battery provides and how this energy is split into heat in the resistor and electric field energy in the capacitor)
I got this part. The battery provides C(V^2), the capacitor takes up half of it and half is lost as heat

#### gleem

Usually the resistance takes up half the energy supplied in the form of heat,
Could you supply the source of this statement?

The energy supplied in charging a battery goes into the work needed to "squeeze" additional charges into the conductors that form the capacitor. Any heat generated would be do to the transporting the charges through a conductor with intrinsic electrical resistance. If you do away with resistance then no heat is generated. Bring charges closer together by itself does not generate heat.

#### Krushnaraj Pandya

Gold Member
Could you supply the source of this statement?

The energy supplied in charging a battery goes into the work needed to "squeeze" additional charges into the conductors that form the capacitor. Any heat generated would be do to the transporting the charges through a conductor with intrinsic electrical resistance. If you do away with resistance then no heat is generated. Bring charges closer together by itself does not generate heat.
I read that in charging a capacitor a cell does work CV^2 and energy stored by capacitor is half of that. therefore half is lost as heat. That was the source of my statement. I understood that heat generated is due to the resistance though and not by bringing charges together. But I'm not sure how it'll affect things here any differently mathematically

#### Delta2

Homework Helper
Gold Member
Anyway, you mention that I'm correct for the discharging process and C(V^2) is supplied by battery and half of that by capacitor; how did you say all this energy is lost as heat though? doesn't a part of 3/2 C(V^2) get used to charge the capacitor again and the other part as heat
where could all this $3/2CV^2$ energy go? The capacitor discharges during the discharging period so it gives away energy, a battery can only provide energy (the battery cant absorb and store energy ) so we got no choice, the energy provided is lost as heat on the resistance R. To see this clearly you have to do the math on the RC circuit as I said in the previous post.

#### gleem

read that in charging a capacitor a cell does work CV^2 and energy stored by capacitor is half of that. therefore half is lost as heat. That was the source of my statement. I understood that heat generated is due to the resistance though and not by bringing charges together. But I'm not sure how it'll affect things here any differently mathematically
In reality heat is generated but the amount is also dependent on the specific characteristics of the cell used. the energy to charge a capacitor is ½CV2 not because of heat but because as the capacitor charges it requires increasingly more work to add charges. The first charge requires basically no work while the last charge must be brought in against the electric field of the all the previous charges.

#### Krushnaraj Pandya

Gold Member
where could all this $3/2CV^2$ energy go? The capacitor discharges during the discharging period so it gives away energy, a battery can only provide energy (the battery cant absorb and store energy ) so we got no choice, the energy provided is lost as heat on the resistance R. To see this clearly you have to do the math on the RC circuit as I said in the previous post.
Here is my step-wise understanding of what is happening-
1) capacitor discharges- 1/2 CV^2 lost as heat
2)Battery does work CV^2 to charge capacitor fully again
3)of this work, half is stored up by capacitor and half is lost as heat
At the end then we have a charged capacitor and CV^2 of lost heat. What step did I miss or which one is wrong?
As for the RC circuits...I'm afraid that's in the next chapter so I don't have even basic knowledge regarding them yet. But I'm sure there must be a way to understand this by what I already know since it is given in the book before RC circuits are introduced.

#### Krushnaraj Pandya

Gold Member
In reality heat is generated but the amount is also dependent on the specific characteristics of the cell used. the energy to charge a capacitor is ½CV2 not because of heat but because as the capacitor charges it requires increasingly more work to add charges. The first charge requires basically no work while the last charge must be brought in against the electric field of the all the previous charges.
Ah! right, I understand the integral used in the derivation now. Thank you for clearing up this concept in my mind.

#### Delta2

Homework Helper
Gold Member
Here is my step-wise understanding of what is happening-
1) capacitor discharges- 1/2 CV^2 lost as heat
2)Battery does work CV^2 to charge capacitor fully again
3)of this work, half is stored up by capacitor and half is lost as heat
At the end then we have a charged capacitor and CV^2 of lost heat. What step did I miss or which one is wrong?
As for the RC circuits...I'm afraid that's in the next chapter so I don't have even basic knowledge regarding them yet. But I'm sure there must be a way to understand this by what I already know since it is given in the book before RC circuits are introduced.
step 1) is not completely described. a correct 1) would be :
1) Capacitor discharges, so 1/2 CV^2 lost as heat, but also charge Q=CV has flown through the battery V, so battery has done work W=QV=CV^2 on this charge and this work is eventually lost again as heat in the resistance, so total CV^2+1/2CV^2 lost as heat.

I did a mistake earlier when I said that a battery cant absorb energy, a battery is considered to absorb energy when the current through battery is from the positive to the negative terminal. But if you do a scheme and connect the battery and the capacitor with reversed polarities you ll see that the current through battery during the discharging process, is from the negative to the positive terminal, so the battery gives away energy.

#### Krushnaraj Pandya

Gold Member
step 1) is not completely described. a correct 1) would be :
1) Capacitor discharges, so 1/2 CV^2 lost as heat, but also charge Q=CV has flown through the battery V, so battery has done work W=QV=CV^2 on this charge and this work is eventually lost again as heat in the resistance, so total CV^2+1/2CV^2 lost as heat.

I did a mistake earlier when I said that a battery cant absorb energy, a battery is considered to absorb energy when the current through battery is from the positive to the negative terminal. But if you do a scheme and connect the battery and the capacitor with reversed polarities you ll see that the current through battery during the discharging process, is from the negative to the positive terminal, so the battery gives away energy.
Ah! I understand clearly now. Thank you very much for your patience in explaining this to me.
Also, I meant to ask- are all threads permanently archived on the site? If someday I wish to revisit this question, it'd be nice just to be able to open it again a year later and understand things easily

#### Delta2

Homework Helper
Gold Member
Ah! I understand clearly now. Thank you very much for your patience in explaining this to me.
Also, I meant to ask- are all threads permanently archived on the site? If someday I wish to revisit this question, it'd be nice just to be able to open it again a year later and understand things easily
Well as far as I know yes. Well to be accurate almost all threads, there are very few threads that could be deleted for various reasons (like if they become too religious, or attempt to discuss non mainstream physics , or because they are pure nonsense).

"Simple capacitance problem"

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