2nd law Thermodynamics -

[davcams]

I have an interesting question that I thought I'd ask here as I may not have adequate knowledge to even ask the question, but something occurred to me while learning thermo dynamics, specifically the second law, and how it relates to the gas laws, pressure, temperature, etc.

 

When a cylinder of gas is compressed, work is being done on the gas in that cylinder. Easy. But could it not also be said that work is being done on the "universe" as (in a sense) it reduces pressure on the surrounding environment, so where is the work really being done? I get it that the universe is infinite, and it is (again, in a sense) nonsense to say the volume of the universe would change, but wouldn't it "decrease" by the same amount that the volume of the container is decreased? Whether this has ANY effect on things, I have no idea at this stage, but for some reason this is bugging me.

 

Does this then have any effect on heat in the same way volume?

[EdEarl]

Hmm, interesting question. I should have thought of it. LOL

 

It isn't air in the Universe that is affected, only air around the earth. There is such a large volume that any change would be insignificant, but my guess is that there must be a tiny, tiny reduction in pressure, volume and number of molecules. It would be close to free expansion and near constant energy.

[studiot]

Good morning, davcams and welcome to SF.

 

Your question relates to the first law, not the second. Do you know the first law, it is normally studied before the second.

 

Thermodynamic analysis divides the universe into two parts, separated by a boundary and specifies a process.

 

So you have

 

1) The system

2) The rest of the universe, called the surroundings.

3) The system boundary between 1 and 2

4The system process

 

The First Law relates to processes which transfer energy across the system boundary. It states a sort of system piggy bank for energy where all forms of system energy are stored. This is called internal energy. All transfers from the surroundings into the system go into this piggy bank - they increase the internal energy. Similarly all transfers from the system to the surroundings decrease the system internal energy. The internal energy of the surroundings is considered to be infinite.

 

Remember that work is a form of energy transferred so is counted as a form of energy transferred in the First Law.

 

Now for your question, the compression of a container of gas.

The volume of the gas decreases and the volume of the surroundings increases.

 

There is only one lot of energy transferred.

 

We say work is done on the system by the surroundings.

 

It does not matter whether we consider the work done by the surroundings or the work done on the system. It is the same lot of work.

 

When we come to calculate the amount of this work we can either consider the system or the surroundings, the result is the same.

 

Sometimes one calculation is much easier than the other and so that is the one we go for. Work is pressure times volume change.

 

Does this help?

[EdEarl]

We say work is done on the system by the surroundings.

Work done by the surroundings includes the energy used to run the compressor, does it not?

[studiot]

Work done by the surroundings includes the energy used to run the compressor, does it not?

 

 

 

 

Not really.

 

The compressor will not be 100% efficient so some of the energy used to run it will remain in the surroundings and not be transferred across the system boundary.

 

Only energy transferred across the system boundary counts in the First Law.

 

A further consideration is that when you compress the gas, it will warm up so some heat will leak back to the surroundings. If this happens this heat will also need to be taken into account in a First Law calculation.

 

Success in thermodynamics often boils down to correctly drawing the system boundary. That is properly identifying the system and its surroundings. This is where people often get into difficulty.

Edited June 15, 2013 by studiot

[Enthalpy]

If the air has pressure P_A and the gas in the cylindre P_C (at some time since P will vary), then the piston displacement making dV volume reduction:

- gets P_A*dV work from the atmosphere

- gives P_C*dV work to the gas in the cylinder

- becomes (P_C-P_A) *dV work from the force on the piston.

 

So a force is necessary when P_C differs from P_A - usually the case, for instance because P_C varies upon compression.

[studiot]

Enthalpy, haven't you got your pressure difference back to front ?