Using a hot gas to drive a piston: entropy reduction?

In summary, the conversation discusses the relationship between the movement of gas molecules and the movement of a piston in an insulated cylindrical container. It questions whether the conversion of energy from disordered movement to ordered movement violates the second law of thermodynamics. The conclusion is that in a real application, the presence of friction would ultimately result in an increase in entropy, negating any entropy decrease from the conversion of energy.
  • #1
omoplata
327
2
Suppose we have an insulated cylindrical container with a piston inserted from one end. Suppose the volume confined by the piston is full of a hot gas. Now let the gas drive the piston so that the volume is increased.

Did the entropy of the system decrease because some of the energy of the disordered random-directional movement of the gas molecules (disordered movement) was converted to the linear (one-directional) kinetic energy of the piston (more ordered movement)?

How does this not violate the second law of thermodynamics?
 
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  • #2
If you consider the walls of the cylinder adiabatic, best performance you can achieve is an isentropic change of state and the entropy stays the same (all the internal energy would have been transformed into work). In a real application you would have friction increasing the entropy of the system by transforming internal energy also into heat at the surfaces between piston and cylinder. So no entropy decrease after all.
 

1. What is the purpose of using a hot gas to drive a piston?

The purpose of using a hot gas to drive a piston is to convert the thermal energy of the gas into mechanical energy, which can then be used to perform work.

2. How does using a hot gas to drive a piston reduce entropy?

When a hot gas is used to drive a piston, it expands and does work on the piston, which decreases its internal energy and thus its entropy. This reduction in entropy helps to maintain the efficiency of the system.

3. What factors affect the efficiency of using a hot gas to drive a piston?

The efficiency of using a hot gas to drive a piston depends on factors such as the temperature and pressure of the gas, the design of the piston and cylinder, and the type of gas being used.

4. Can any type of gas be used to drive a piston?

No, not all gases are suitable for driving a piston. The gas must have the ability to expand and do work, and it should also have a high specific heat capacity to maintain a consistent temperature throughout the process.

5. What are the applications of using a hot gas to drive a piston?

Using a hot gas to drive a piston has various applications, including in internal combustion engines, steam engines, and gas turbines. It is also used in refrigeration and air conditioning systems, as well as in some industrial processes.

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