[alkufi]

Hello Brilliant
1-I wonder If the reversible steps in thermodynamic is actual?


if the answer be yes >>> what the advantage from the reversible process law ??!! 

[renge ishyo]

If we could ever do a reversible process the advantage would be that we would have an efficiency of 1 or that all the energy we extract from a process could be entirely converted to work with no heat losses to the environment. This would be superior to all existing heat engines that have efficiencies that are way less than 1, as these engines "waste" a lot of energy in heat losses to the environment when performing their task. This is all well and good except for one *small* problem...carrying out a process reversibly requires that infinitely small changes to the system be enacted at each step such that the system can always return to its previous step at any point during the process. Doing this requires that an infinite number of steps take place in order to ensure that no irreversible steps leak in. Even if every step took only one second, having an infinite number of steps means that the reversible process would take an infinite amount of time!

This idea is backed by our modern concept of time which is related to the entropy increasing...in a reversible process the change in entropy can be zero which means that the passage of time would not be observed in such a process. Therefore working in a reversible fashion, while thermodynamically efficient with regards to energy loss, is completely inefficient when it comes to getting a job done in a suitable time frame.

[alkufi]

but why the scientist put the reversible steps law such as

w= nRT ln (v1/v2)

that the advantage from this law??

[renge ishyo]

The advantage of using reversible processes is simply that we can use calculus and partial differential equations to derive the results, and these results are independent of the materials used to construct the particular system. It also provides a maximum amount of work that we can obtain from a given process which is nice to know.

Sure, it would be nice to be able to calculate the results of non-reversible processes, but to my knowledge at least it simply can't be done (calculus cannot be directly used in this case because the measureable steps are not in uniform infinitisemal jumps)....the amount of work you "actually" get out of a system is always less then the reversible engine and just how less depends on how the system was designed, the materials involved, etc. So it would be hard to find general relations to describe what can happen in various cases.

What is normally done to compare engines is the maximum work for a process is calculated for the reversible engine. Then the actual work you get out of the engine is measured experimentally. Taking the actual work and dividing it by the maximum work gives the efficiency which can then be used to compare to other efficiencies obtained from other engines running under the same conditions. In this way you can see which engine is more efficient to use.

[alkufi]

Thanks

but I need more this not all story

[srihari]

Sorry for starting up an old topic again .
But I felt it better to revive this than start a new one.

I'd like to know how exactly one can extract more work in reversible process
than in irreversible  ? I'd be grateful if detailed explanation can be given ..
this has troubled me quite  lot ...

regards
Srihari

[renge ishyo]

In a reversible process, you can convert all of the heat energy entirely to work with no losses incurred in the process. The losses are prevented (in theory) by very carefully ensuring that with each step you take that you can go back and forth with the previous step as you gradually move forward (each step is reversible because you can "reverse" the process to the conditions of the previous step at each step as you go along). To attempt to get a qualitative understanding of how this works, imagine that you are standing atop a stairway, and I hand you a glass filled to the brim with water such that if you make the slightest *Ignore me, I am impatient* the water will spill out of the glass. I then instruct you to walk down the stairs without spilling a drop. Further suppose that evaporation or condensation of water from the atmosphere cannot take place and can't skew the results of the experiment. The goal is to reach the bottom of the staircase with exactly the same amount of water as you started with. Can you accomplish it?

Well, you might imagine that if you moved very slowly and carefully down the steps that you might be able to prevent even the slightest amount of water from spilling out of the glass. Furthermore, as you move down the stairway without spilling a drop you can always go back to the previous step up, and the glass will contain the same amount of water as it had before you had moved down to the lower step. In other words, so long as you don't spill ANY of the water at any point on the stairs, then the process will be reversible all the way down the steps. At the bottom of the stairway the amount of water would be the same as it would be if we measured the amount on top. Hence, we can say that this amount of water represents the maximum amount of water you can have at the bottom of the staircase regardless of what steps or tricks you used to get the glass down the staircase without spilling.

On the other hand, suppose you tried this in actual practice and somewhere in the process you spilled a drop out of the glass at any point. The moment this happened, you would find that you cannot go back up to the previous step with the same amount of water as you had before. Furthermore, even if you are perfect the rest of the way down the stairs, you are from that point forward doomed to have less water at the end than what you started with. In other words, the loss of a single drop of water was an *irreversible* change. Once you lose any water you can't get it back, and the overall process will leave you with less water at the end than what you started with. As you can imagine you can have many drops spill out as you go down the stairs depending on how the person carries out the task, so the amount of water any one person can have at the bottom of the stairway varies depending on what happens as they carry down the glass. Furthermore, you might suppose that it is much more likely that someone will spill a drop at some point while trying this making the irreversible process much more likely to take place than the reversible one. In fact, you might find that in practice the reversible process never actually takes place as some loss seems to take place regardless of how hard the person tries to keep it from spilling.

In reality, we are talking about energy instead of water in thermodynamics. We can convert energy from heat to macroscopic work without loss if we use a theoretical reversible engine that can carry out the process without losing anything to the environment. In reality, we lose heat to the environment as the process takes place and in doing so we always end up with less work coming out than the energy we put in to begin with from the heat. Just how much work we get out depends on how much we lose, which depends on how the process is carried out.

Of course, it must be emphasized that a mathematical treatment is strongly proffered to holistic approaches such as this if you want a more detailed and accurate understanding of entropy. I recommend starting with H.C. Van Ness, "Understanding Thermodynamics" for a good balance between the two approaches.

[srihari]

okie
thanks
I'll definitely remember this analogy .

regards
Srihari