# Pressure Difference affecting reversibility of a process

It is said that for a finite pressure￼difference between system and the surrounding the process is irreversible. From the diagram can you please tell me how the process is irreversible ?

Does this look reversible because I have considered the weight of the piston also ? Because if I don't consider the weight then the system will change state to attain mechanical equilibrium and then to get the system back to the same position work will have to be done on the system...so by definition it won't be a reversible process

• I have to ask this. Did you copy this question from your homework? If not, can you please give more detail? Jul 15, 2016 at 14:26
• No Floris , I am studying thermodynamics again , this time more conceptually. Jul 15, 2016 at 15:05
• The policy of this site is that such "homework-like" questions should demonstrate some effort to work through the problem. Jul 16, 2016 at 2:07

A thermodynamic process is called reversible if an infinitesimal change of the external condition reverses the process.

Consider a gas enclosed by a freely moving piston in a cylinder. Let us say it is in mechanical equilibrium with the atmosphere, that is, the pressures on the piston match. If you increase the external pressure infinitesimally the piston goes downs until the system reaches equilibrium. Then decrease the external pressure infinitesimally and the piston moves upwards. the process is reversible.

Now consider that the gas and the atmosphere are not in mechanical equilibrium, let us say the atmospheric pressure is greater by a finite amount. The piston will always go down no matter if you increase or decrease the external pressure by any infinitesimal amount. So this system is, by definition, irreversible.

• Thanks alot. In the meaning of a reversible process I overlooked the part "INFINITESIMAL" small change of the external condition Jul 15, 2016 at 15:10

Take gas in a container with a piston connecting to its surrounding as an example.

When, at the moment you release the piston, the pressure difference between the surrounding and the container is very small, the piston will move slowly because there is not a lot force to accelerate it. In this situation, the system's response (temperature, pressure change) is faster than the piston motion. This process is reversal.

When the pressure difference is very large that the piston can move very fast, the molecules close to piston will move faster than those far away from the piston. These high velocity molecules will produce heat due to viscosity. The heat cannot be recovered completely so the process is irreversible.

Again, there is no perfect reversible process. You can be reversibility to describe the degree of that.

• The work done during expansion is more than the work done during compression. So when we try to compress the piston again then there is always an effect on the surrounding...is that what you mean ? Jul 15, 2016 at 15:07