# Heat change between system and surrounding

1. Aug 6, 2011

### weng cheong

how can heat change between system and surrounding be reversible?

du=q-w'

in this case, heat change, i reckon
a)since u is state function, there is no change in U, thus change in u is 0
b) 0=q-w' hence q=w' , all the heat transferred is spent on doing work
c) in order to make this reversible, w' must be maximum, so does the q

1)does it mean that if i would like to get a reversible change, the amount of heat change between system and surrounding must be huge enough?
a small amount of heat change will be irreversible?

2) i found that in order to have a infinitesimal steps in the exchange of heat, temperature difference of the system and the corresponding surrounding has to be infinitesimal.
what if the amount of q is huge enough to make a difference in the temperature of system?

Last edited: Aug 6, 2011
2. Aug 6, 2011

### Studiot

Good morning weng cheong.

Probably the simplest example of a reversible heat change is melting / freezing with the exchange of latent heat of fusion.

There is also a substantial entropy change associated with this phase transition.

go well

3. Aug 6, 2011

### weng cheong

i'm sorry, i don't get it. how does your explanation answer my question? can you explain in more detail?

actually i'm studying entropy. i came across the equation: dSsurr = -Qsys/Tsys.
Thus, i need to understand how can the heat transfer between system and surrounding be reversible that we can equate the Qsurr to Qsys.

4. Aug 6, 2011

### weng cheong

Andrew Mason,
thanks for clearing my doubt, i wish to continue the discussion here.

1) dQ = dU - dW'
i agree that to enable the reversibility, W' has to be maximum.
is there any restriction to the values of dQ and dU?
what if the dQ is too small to cause a difference in T, then dU=0, dQ=dW'? when dW' is max,
dQ is maximum too?

2) for a reversible transfer of heat, we need infinitesimal temperature difference between the system and surrounding to facilitate the infinitesimal transfer of heat.

However, how do we achieve this? does it has something to do with the fact that the surrounding is very large compared to the system?

5. Aug 6, 2011

### Studiot

What don't you get?

6. Aug 7, 2011

### weng cheong

Studiot,

for reversible heat transfer heat transfer, infinitesimal temperature difference is required.
when the system and surrounding are at the same temperature, there will be no problem. what if at the beginning of process the system and the surrounding are at different temperature(finite temperature difference)? how can infinitesimal temperature difference be achieved?

7. Aug 7, 2011

### Studiot

Sometimes being highly theoretical about a subject simplifies things and helps towards understanding, sometimes working through real world examples helps.

I suggest a few examples here.

Your question has appeared many times on PF here was a previous answer. I am sorry that the forum Tex change has messed up the formatting.

Latent heat is a good example and fusion is better than vapourisation since the volume/pressure change is so small we can neglect any work done.

Heat is exchanged between a pure substance and its surroundings on melting or solidifying.
However to strictly conform to the infinitesimal temperature difference/equilibrium requirement we should draw our system boundary between the liquid and solid phases of the pure substance. tthe solid is the system and the liquid the environment.
We know that for a pure substance the cooling curve halts at the melting/freezing point until all the solid is transformed to liquid.
So both solid and liquid are at the same temperature and heat transfers between them conform to the strict requirements for reversibility.

No work is done
All this heat equals the change in internal energy
Since the melting occurs at constant temperature the entropy change equals the heat exchanged divided by the melting temperature
The process is reversible

Yes the liquid receives its heat from the rest of the universe under slightly different conditions, but it is often convenient to ignore this.

does this help?