Coffee Room
Discuss anything here - everything that you wish to discuss with fellow engineers.
12915 Members
Join this group to post and comment.
devesh9749 • Jun 30, 2008

# compressing an ideal gas

hello everyone,
since i am a crazy engineer so i had this crazy doubt that,

If we have an insulted container having a gas like say helium(which is close to ideal gas) and has a piston on top.
Now if we put a weight on the piston and allow the gas to be compressed to some volume will the gas get cooled?

can anyone please can tell whether my reasoning is correct or wrong?
devesh9749 • Jul 2, 2008
Jeanius • Jul 6, 2008
If you're looking at an ideal gas, then you're working with (phonetically) pivnert (PV=nRT).

Both n and R would remain constant throughout the push, so we really need to focus on the PV and T portions. What you'd really have to work out is the ratio between P and V in any specific case, as pressure will increase and volume will decrease. Think of a liquid oxygen tank expelling gas. The nozzle freezes as pressure decreases, right?
Ernesha • Apr 28, 2009
One of the postulates of Kinetic Molecular Theory is that ideal gases have no forces of attraction or repulsion between molecules. So when you compress an IDEAL pressure increases but with the same ratio Volume decreases so in the relation PV=nRT, T is not affected. But in the case of NON-IDEAL gases the force of attractions and repulsions ARE present and so when the gas is compressed it experiences heavy repulsions from the other molecules due to which its rms velocity increases and then K.E and ultimately its Temperature. But in case of IDEAL gases the rms velocity is not altered by the compression.
Corvus • Apr 28, 2009
The gas will heat up as you pressurize it
g_rakesh2 • Apr 28, 2009
devesh9749
hello everyone,
since i am a crazy engineer so i had this crazy doubt that,

If we have an insulted container having a gas like say helium(which is close to ideal gas) and has a piston on top.
Now if we put a weight on the piston and allow the gas to be compressed to some volume will the gas get cooled?

can anyone please can tell whether my reasoning is correct or wrong?
hello devesh
PV=nRT is ideal gas law.

for ideal gas
By compressing it, you're changing the volume (decreasing) and the pressure (increasing).

It's possible that the combination of PV could remain constant, e.g. so that nRT remains a constant, so that T remains a constant.
have you ever seen air baloon seller.
baloon seller is having a tank in which a light gas is stored by copmressing it like helium if you touch that tank it will feel hot.
agreed ernesha!!!
Rohan_sK • Apr 28, 2009
The thermodynamic process in which the tempreature of the gas remains constant is called an ISOTHERMAL PROCESS. As all thermodynamic processes it is goverened by thr gas equation PV = mRT, where T is kept constant by providing a constant cooling reservoir around the system.
As for Devesh, where he wants to reduce the tempreature of the gas as well despite of compression, then the process will not be a standard process. It will need a calculated cooling procedure from outside, and the equation governing would be p1v1^n = p2v2^n, where value of 'n' would be the deciding factor.
jhbalaji • Jul 26, 2009
Whether it's Constantly applied Pressure or variable mate...
skipper • Jul 26, 2009
As others have pointed out an ideal gas is isothermal (is an isotropic medium). An ideal gas can be compressed without heating, temperature stays constant and pressure increases linearly. Isothermal expansion and adiabatic compression are the keys to a successful heat engine, But all real engines lose heat.

A real gas does heat up because of nonlinearity. If an ideal gas existed it would be monatomic and smooth, with no dipoles of charge or mass, the spin would be irrelevant for each atom; in a real gas this isn't true except at some very general level that ignores quantum effects.
😎
rajulucky2520 • Feb 17, 2011
hello davesh,
ur doubt can be clarified by viewing it in thermodynamics point of view.As u insulted the container no heat tranfer takes place then dQ=0,as gas is compressed work done is -Pdv. then apply fist law of thermodynamics for closed system....dQ-PdV=U2-U1..then it becomes -PdV=U2-U1 it implies U2
ISHAN TOPRE • Feb 27, 2011
As PV=nrT is the law,if we vary V then P will change.Thereby maintaining T constant.So temperature will remain constant.
But as it is a piston cylinder arrangement with no room for extra volume,the gas will get heated up.
Hey basic Chemistry says, when molecules are tightly packed, they get heated up! So, obviously when you apply pressure, the gas heats up.
At the same time, when hydrogen or oxygen (non ideal gases) are compressed, on the other hand, they get super cooled! Basic Physics!
devesh9749
If we have an insulted container having a gas like say helium allow the gas to be compressed to some volume will the gas get cooled?
Work has been done on the gas. Since the process as described is adiabatic, the gas will get heated.

Bioramani