Compressing Gas: Why Does It Heat Up?

[Peter G.]

When a gas is compressed in an air conditioner, It heats up.

Is this because: The pressure increases and Pressure and temperature are directly proportional, or because it is using energy to do work and
in compressing it, transfers
its energy to the gas
molecules, they absorb it as
internal energy, making the
molecules move faster, and
as temperature is the
average speed of the
molecules movement, its
temperature increases

Thanks in advance.

 

[Peter G.]

Sorry for format by The way, wrote from a cellphone

 

[mrmiller1]

Say we look at an adiabtic compresser (no heat loss) with a compression ratio of 2 that uses ideal air. Ideal adiabatic equations will show you air heats up to 90 C from standard conditions (25 C, 101.325 kPa).

P^(1-γ)*T^γ = constant
(101.325 kPa)^(1-1.4)*(298.15 K)^1.4 = 459

(202.650 kPa)^(1-1.4) * T^1.4 = 459

T= 363 K = 90 C

Like you said, the gases pressure is increased, therefore, increasing the activity of the molecules and results in heating. This concept is easiest to remember when you think about absolute zero conditions. We know at absolute zero there is no movement. This is why cryostasis must be very cold - so that we can preserve something and limit its aging and decay.

 

[Peter G.]

Cool, I get it. So when a compressor compresses a gas the increased pressure leads to greater temperature. Could this be due to increased friction between molecules?
But in the case of the gas itself doing work, as in the adiabatic expansion as a hot air parcel rises, the decrease in temperature due to expansion in an area of Low pressure would be due to the gas molecules using their internal energy to push the surrounding molecules aside?

Thanks.

 

[mrmiller1]

When the gas is compressed the molecules have a higher probability for collision and transfer momentum and, therefore, kinetic energy at the micro level (which contributes to internal energy). Pressure alone can explain the calming of the gas after expansion. If we have gas in a container then the pressure inside that container depends on how fast and often the molecules hit the sides.

I'm not sure if it pushes surrounding molecules away. When the gas is expanded the specific volume (m^3/kg) is increased, so each molecule has more room to move and collides with another less often. Because of their ceasless motion the molecules spread out naturally.

 

[Peter G.]

Ah ok. What confused me were some meteorology websites that explained air cooling as it rose due to the lower pressure higher up in the atmosphere. A hot parcel of air rising in an area of lower pressure would be forced to expand and in doing so, would use part of its internal energy. "The lower pressure at elevation is not enough to maintain the original compression in your air parcel. Instead, your air parcel expands and uses its own energy to push the surrounding air aside. Where does your parcel get the energy to push surrounding air? It takes it away from the motion of the molecules of air, resulting in slower moving molecules. As air expands, temperature drops" http://education.sdsc.edu/teachertech/downloads/climate_answ.pdf

And some other websites, such as Yahoo answers, claiming that a bicycle pump would become hot due to heat transfer since as we do work on the gas molecules, they heat up.

Thanks

 

[mrmiller1]

Very cool!

By the way you can use the same equation we talked about above for the bicycle tire. To pump air in the tire work must be done and then it is stored as internal energy and then released as heat (conversion from work to heat).

 

[Andrew Mason]

Cool, I get it. So when a compressor compresses a gas the increased pressure leads to greater temperature. Could this be due to increased friction between molecules?

Collisions between molecules are elastic. There is no friction between gas molecules. Friction is something that occurs between macroscopic objects.

Temperature is defined by the average kinetic energy of the molecules in the gas. The increase in temperature is due to the fact that compression causes the molecules in the gas to move faster. The act of compression pushes on the molecules that collide with the matter that is providing the compressing force and this increases the rebound velocity of the colliding molecules.

AM