# I Heat absorbed by a gas

#### Nikhil Rajagopalan

Summary
Thermodynamics , Path independence of heat absorbed.
Dear Experts,
While going through the explanation why heat absorbed by a gas is path dependent, there were two examples of comparison.
1. A gaseous volume of 2 liters expanding slowly to 4 liters , supplying heat from a heater keeping the temperature of the gas constant at 300K.
2. A gas allowed to free expand from 2 liters to 4 liters by bursting a membrane.

In the second case, there is no heat absorbed by the gas and no work done by the gas. Volume becomes twice and pressure becomes half. It seems easy to comprehend.
In the first case, how does the state variables change. Does the temperature of the gas decrease while expanding. Is the state equation PV = nRT applicable here?

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#### mfb

Mentor
Does the temperature of the gas decrease while expanding.
The problem statement says the temperature stays constant.
Is the state equation PV = nRT applicable here?
That law is always true (for ideal gases).

#### Chestermiller

Mentor
Is the state equation PV = nRT applicable here?
For an irreversible expansion such as with the membrane, the pressure calculated from the ideal gas law does not describe the force per unit area on any real or conceptual surface. There is a "viscous" contribution to the force per unit area (for a typical Newtonian fluid gas like air).

"Heat absorbed by a gas"

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