# Homework Help: First law of thermodyamics: undestanding required

1. Jun 10, 2015

### Sahil Dev

1. The problem statement, all variables and given/known data
Understanding the first law of thermodynamics

2. Relevant equations

3. The attempt at a solution

I have been doing a self-study course on heat transfer. I have been reading from the book by Incropera and all.

I understand that the first law of thermodynamics forms the very basic as fundamentals like the conduction equation are all derived from the fist law.

I have a few questions concerning the first law of thermodynamics and shall be grateful if helped. I understand that some might advise to do a complete course on thermodynamics first before heat transfer course. Surely, I will but need to look into basics of heat transfer a bit urgently.

Concerning the first law of thermodynamics;

We first have the first law of thermodynamics for a closed system being stated as;

Firstly, the first law of thermodynamics addresses the ways in which energy can cross the boundaries of a system.

• For a closed system (a region of fixed mass), there are 2 ways: heat transfer through the boundaries and work done on or by the system.
For a closed system (a region of fixed mass);

i.e. the change in the total energy stored in the system is equal to the net energy transferred to the system minus the net work done by the system

I have got the following questions here;

a) What is a closed system? Is it a system- from the boundaries of which mass cannot leave (hence the term closed) ?

b) Secondly, can anyone give a physical simple example of a closed system?

c) What you mean by work done by the system? Any example will be really helpful

We then say that the first law of thermodynamics can be applied to a control volume (or open system), a region of space bounded by a control surface through which mass may pass. Mass entering and leaving the control volume carries energy with it; this adds a third way energy can cross the boundaries of a control volume.

d) I have a little confusion again. Above it is said, for an open system we have a THIRD way energy can pass through the control volume.

I am not very clear with the three ways energy can pass through the system. What exactly are the 2 ways in the closed system and third way in the open system that define how energy can pass through the system?

Please can anyone help? I shall be really grateful

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2. Jun 10, 2015

### Topolfractal

A closed system is a system that doesn't dissipate energy through friction, I think. It is a system that doesn't lose or gain energy to outside sources, hence closed. Now a good picture of work is pushing a rock up a hill. A good example of work is compressing a gas. As the pressure is increased, the temperature of the gas goes up ,and the internal energy of the gas goes up. The two methods of energy transfer for a closed system are work and heat conduction. The third method is for application to open systems, which is implied from the definition. The definition of an open system is a closed system that exchanges energy with its surroundings. A good example of a closed system is gas in a sealed plastic box. The system is the box and the gas. Compressing the gas (and the box) does work on it and raises the energy. The hot gas transfers heat over to the box till equal in temperature. Now an open system would be the gas being released into the atmosphere. A certain amount passes through an imaginary control surface as it expands every second.

3. Jun 10, 2015

### Staff: Mentor

This is mostly not correct. Please don't give answers to questions if you are not sure. It will only confuse the members more than they are already.

Chet

4. Jun 10, 2015

### Topolfractal

Sorry I didn't know thank you for correcting me

5. Jun 10, 2015

### Topolfractal

Please can you tell me what I did wrong, so Sahil Dev and I don't get more confused. It would greatly benefit both of us.

6. Jun 10, 2015

### Topolfractal

I do agree though that I didn't answer as clearly as I should have or as organized as I could have, but I mainly reiterated what Sahil Dev posted in a explanatory ( disorganized ) fashion.

7. Jun 10, 2015

### Staff: Mentor

It is a system for which mass cannot enter or leave through the boundaries of the system, but for which work and heat can be exchanged with the surroundings at the boundary of the system.

A gas contained in a cylinder with a piston.

If the gas in the cylinder expands against an external force, the gas is doing work on its surroundings.
I very strongly urge you not to start trying to get an understanding of the first law applied to an open system until you have had some experience applying it to closed systems. With that being said, I will answer your questions.

Two of the ways are the same as for a closed system: heat and work. The third way is that mass can enter and leave through the boundaries of the system (i.e., the control volume), and this mass carries energy with it in the form of internal energy and kinetic energy. This is energy passing across the boundary of the system (i.e., the control volume).

Again, I strongly urge you to get practice solving many many problems applying the first law to a closed system before you start looking at open systems.

Chet

8. Jun 10, 2015

### Topolfractal

Thank you, I see my error, and will work to improve.

9. Jun 10, 2015

### Staff: Mentor

10. Jun 11, 2015

### Sahil Dev

Thank you very much for the reply.

You said;

I'm sorry for this but I'm from the Civil Engineering background, so just wanted to be clear on the above statement.

Do you mean as in the picture below;

http://www.learnthermo.com/images/ch07/Lesson-B/7B-11-piston-cylinder+P+F.png

That is if I pull the piston , gas in the cylinder expands (due to the external force that I'm applying by pulling the piston) and the gas compresses if I push the piston due to the external force I'm applying by pushing the piston?

11. Jun 11, 2015

### Staff: Mentor

The figure is not quite correct. The piston is never pulling on the gas, and the force is in the wrong direction. If the piston has a negligible mass, to get the gas to expand, you ease up on the downward force on the piston, while to compress the gas, you increase the downward force.

12. Jun 11, 2015

### Sahil Dev

Ok.

Am I right, then, to say;

1) If the gas in the cylinder expands against an external force (i.e. if you ease up the downward force on the piston), then, the gas is doing the work on the piston (i.e. on its surroundings as you had quoted in the post before the above).

2) Whereas, if the downward force is increased, the gas compresses and it is the piston that does the work on the gas.

Are 1 and 2, right?

13. Jun 11, 2015

### Staff: Mentor

Yes.

14. Jun 15, 2015

### Sahil Dev

15. Jun 15, 2015

### Sahil Dev

Thank you very much for the reply. My internet had a problem from the past few days and so couldn't logon.
I'm sorry to ask a bit more clarification on the above.

Regarding a closed system,
Now,

1) Am I right to say that the above example of the gas doing the work on the piston or piston doing the work on the gas is an example of work exchanged with the sorroundings?

2) Can you give an example of heat exchanged with the sorroundings considering a closed system?

16. Jun 15, 2015

### Staff: Mentor

Yes.
The cylinder containing the gas at temperature T1 is put into contact with a constant temperature bath at temperature T2. Heat flows from the constant temperature bath into the gas (through the cylinder wall, the gas boundary) until the gas has attained the temperature T2.

Chet

17. Jun 15, 2015

### Sahil Dev

Thanks,

1) when the gas does work on the sorroundings (then it expands as a result of easing up the downward force on the system), what is the form of energy involved here (it is not 'heat' energy- for sure).
2) Similarly, when piston does wowk on the gas (if the force on the piston is increased), what is the form of energy involved (Again it is not heat energy for sure)
3) Now, considering a system is closed. We have a gas (say 'ammonia') inside the cylinder at temperature T1. We have gas (say 'benzene') outside the cylinder at temperature T2 and T2 is greater than T1. Also, mass density of benzene is greater than ammonia. Can I say that, since the system is CLOSED, benzene will not eneter the cylinder BUT there will be exchange of heat and there will be a heat flow from outside the cylinder into the cylinder ?
4) Had the system been an open system, there will be mass tarnsfer which will take heat along with it. Am I Right?

18. Jun 15, 2015

### Staff: Mentor

No. It's mechanical work, not heat. Heat transfer takes place (by conduction, convection, and radiation) when the average temperature of the system during the process is different from the temperature at the boundary of the system
No. It's mechanical work, not heat. Heat transfer takes place (by conduction, convection, and radiation) when the average temperature of the system during the process is different from the temperature at the boundary of the system
Yes. If the system is taken as the benzene inside the cylinder, then there is exchange of heat from outside the system into the system.
If mass actually does enter or leave a system (i.e., control volume), it carries internal energy with it.

Chet

19. Jun 16, 2015

### Sahil Dev

You said, YES, if the system is taken as benzene inside the cylinder. But, as I said above, insider the cylinder is ammonia and outside benzene. It is a closed system.

In a closed system, there is no exchange of mass (i.e. no mass can enter) but as we said there can be exchange of heat and work with the sorroundings. Then, in such a case can there be heat transfer (due to high temperature of BENZENE gas)from outside of the cylinder to inside the cylinder?

20. Jun 16, 2015

### Staff: Mentor

Oops. I meant ammonia inside the cylinder. So that is your closed system, and the benzene is part of the surroundings. So there can be heat transfer from the benzene to the ammonia through the boundary , and that counts as Q.

21. Jun 16, 2015

### Sahil Dev

Thank you, again.

Considering the same problem.

Let us say the system is not an open system.

Now, there is mass transfer - mass of benzene from the outside of the cylinder enetring the inside of the cylinder - carrying along with it heat.

In such a case, will the quantity of heat that is tarnsferred vary depending upon if the system is closed or open ?

I would say, the quantity of heat tarsnfered will be the same regardless if th system is closed or open. Am I right? If wrong, why?

22. Jun 16, 2015

### Sahil Dev

With respects
Sahil

23. Jun 16, 2015

### Staff: Mentor

I really don't know what you are asking here. Please define precisely what your system includes and does not include.

Chet

24. Jun 16, 2015

### Sahil Dev

The system is the same : cylinder filled with ammonia and outside the cylinder is benzene.Considering the above system as open system.

Hence, there is mass transfer which carries with it heat.

In such a case, will the quantity of heat transfer vary had the system of (cylinder) been a closed system?

25. Jun 16, 2015

### Staff: Mentor

This doesn't seem clear to me. I don't know whether your system consists of the ammonia and the benzene, or just the ammonia.

If the system is just the ammonia, then the cylinder is its boundary with the surroundings, and the ammonia is part of the surroundings.

If the system includes both the ammonia and the benzene, then the system description is incomplete; you haven't described where the boundary of this system is located, which separates the system from its surroundings. And you haven't indicated whether this boundary is rigid (no work) or insulated (no heat), or something else.

The reason you are struggling so much with all of this is that you haven't even solved your first problem using the first law for a closed system. Now, after you clear up the confusion concerning the definition of your system above, I want you to go to your thermodynamics book and try to solve at least one problem for a closed system. If you have trouble solving the problem, I will help you. That way you can get some practical experience working with the concepts of work W, heat Q, and internal energy U. Until you have worked this first problem, I am not going to try to spend any more of my time trying to address your questions.

Chet