Is Slow Compression Speed Sufficient for Quasistatic Gas Compression?

[aaaa202]

Have already received excellent help in understanding this but might need a bit more.

Suppose we have a gas inside a cylinder with a piston in it. Now my teacher said that to compress the gas quasistatically we would need to press in the piston with a speed, that is slow compared to the speed of sound, which makes sense as this is the speed at which pressure differences travel.
Now my question is: Don't we also need to press in the piston with a force only infinitely larger than the force from the internal pressure? I mean if we have a gas in a cylinder with a piston of area 0.01m^3 and pressure of 100Pa such that the internal force on the piston initially is 1N. Then we couldn't really achieve a quasistatic compression by exerting 100N on the piston could we? But is this maybe accounted for in saying that the speed should be sufficiently slow?
Overall I think this way of thinking of a proces is quite weird? Do they occur often in nature? Because it would seem that to compress something quasistatically you would the whole time have to keep an eye on not exerting a too large force..

 

[shyguy79]

I'm not really sure what you are asking... the reason for quasi static compression is to minimise the increase in heat which goes along with compressing a gas - and an increase in heat means an increase in entropy and will be an irreversible process. So by changing the pressure infinitely slowly then the process may remain reversible - it you to carry it out over and over again.

I'm not sure if quasi static compression occurs in nature - I'd be inclined not to agree because of the entropy principle of the universe but like I said it's more for the reversible nature of the process.

Hope this helps

 

[aaaa202]

But suppose we have a cylinder with a pressure of perhaps 1Pa and an area of 0.01m^2. The force is then 0.01N. Now say we slam that piston 100N. That compression wouldn't be quasistatic right? I mean, if we think of quastistatic doesn't it mean, that we start at equilibrium, then press down just infinitisimally more to reach a new equilibrium after dx and so on..

 

[shyguy79]

But suppose we have a cylinder with a pressure of perhaps 1Pa and an area of 0.01m^2. The force is then 0.01N. Now say we slam that piston 100N. That compression wouldn't be quasistatic right?

Right!

Are you saying that isn't it difficult to compress quasi statically then yes... it is virtually impossible because to compress quasistatically as you have to compress infinitely slowly. How it is generally done is compress slowly, allow the gas to cool and return to equilibrium and then compress a tiny bit more... and repeat

 

[asteriatic]

I can provide some clarification on the concept of quasistatic compression. Quasistatic compression refers to a process in which a gas is compressed slowly and smoothly, allowing it to remain in thermodynamic equilibrium at all times. This means that the system is always in a state of balance, with no sudden changes in pressure, temperature, or volume. In order to achieve this, the compression must be done at a slow speed, as you correctly noted, to allow for pressure differences to travel through the gas.

In regards to your question about the force needed for quasistatic compression, it is important to understand that the force exerted on the piston is not the only factor. The speed of compression is also a crucial factor in achieving a quasistatic process. This is because the speed of compression determines how quickly the gas molecules can adjust to the changes in pressure and volume. If the compression is done too quickly, the gas molecules will not have enough time to adjust and the process will not be quasistatic.

In your example, the force exerted on the piston should be sufficient to overcome the internal pressure of the gas, but it does not necessarily have to be infinitely larger. As long as the speed of compression is slow enough, the process can still be considered quasistatic.

In nature, quasistatic processes are not uncommon. For example, the expansion and contraction of gases in a car engine or the compression of air in a scuba tank are both quasistatic processes. They may seem strange at first, but they are actually very important in understanding the behavior of gases and other thermodynamic systems.

In summary, quasistatic compression is a slow and smooth process that allows a gas to remain in thermodynamic equilibrium at all times. The speed of compression is just as important as the force exerted, and both must be carefully controlled in order to achieve a quasistatic process.