# Isothermal Processes

mech-eng
I have become almost sure but have only some small doubts. Are all isothermal process actually ideal gas equation PV=mRT? If all such processes are occur in closed systems, this is so. Because it is isothermal the temperature is constant, R is constant and so is mass for a closed system. So the left side of the equation should be PV, which is a polytropic process with n=1. But my doubts comes from that in books PV=mRT is always called ideal gas equation, not a polytropic process with n=1 and when it comes to a isothermal process they use ideal gas equation.

Thank you.

Mentor
I have become almost sure but have only some small doubts. Are all isothermal process actually ideal gas equation PV=mRT? If all such processes are occur in closed systems, this is so. Because it is isothermal the temperature is constant, R is constant and so is mass for a closed system. So the left side of the equation should be PV, which is a polytropic process with n=1. But my doubts comes from that in books PV=mRT is always called ideal gas equation, not a polytropic process with n=1 and when it comes to a isothermal process they use ideal gas equation.

Thank you.
Real gases do not exactly obey the ideal gas law. And solids and liquids definitely do not obey the ideal gas law.

Also, in irreversible processes, even though the outer boundary of the gas may be held at constant temperature, the temperature is not constant throughout the body of the gas, and varies with spatial position and time. However, some would regard this as an isothermal process.

mech-eng
mech-eng
Real gases do not exactly obey the ideal gas law. And solids and liquids definitely do not obey the ideal gas law.

Also, in irreversible processes, even though the outer boundary of the gas may be held at constant temperature, the temperature is not constant throughout the body of the gas, and varies with spatial position and time. However, some would regard this as an isothermal process.

What I meant to ask is neither for reals gases, nor if is there any real isothermal process. In this link second question is both related to ideal gas and its being a polytropic process. The second question in the link directly uses ideal gas equation as a polytropic process. So are all polytropic processes actually ideal gas equation?

http://home.iitk.ac.in/~suller/lectures/lec5.htm

Thank you very much.

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Mentor
What I meant to ask is neither for reals gases, nor if is there any real isothermal process. In this link second question is both related to ideal gas and its being a polytropic process. The second question in the link directly uses ideal gas equation as a polytropic process. So are all polytropic processes actually ideal gas equation?

http://home.iitk.ac.in/~suller/lectures/lec5.htm

Thank you very much.
Sorry. I don't understand your question. In the 2nd problem, they are analyzing the isothermal reversible expansion of an ideal gas. If the question is, "for a constant temperature reversible expansion, is it possible to satisfy an polytropic equation for a real gas, or does it have to be an ideal gas?", I think the answer is that it has to be an ideal gas.

mech-eng
Sorry. I don't understand your question. In the 2nd problem, they are analyzing the isothermal reversible expansion of an ideal gas. If the question is, "for a constant temperature reversible expansion, is it possible to satisfy an polytropic equation for a real gas, or does it have to be an ideal gas?", I think the answer is that it has to be an ideal gas.

1. Can there be any polytropic process with the indice equals to 1 which is described by an equation right hand side is not mRT?

2. Can there be any isothermal polytropic process having an indice other than 1?

I hope I am clear enough now.

Thank you.

Mentor
1. Can there be any polytropic process with the indice equals to 1 which is described by an equation right hand side is not mRT?

2. Can there be any isothermal polytropic process having an indice other than 1?

I hope I am clear enough now.

Thank you.
In my judgment as an experienced engineer, I see very little value in your spending your valuable time speculating about esoteric things like this. Your time would much better be served getting practice solving thermodynamics problems.