Ideal Gases under Adiabatic Compression

In summary: It is true that the temperature of the air in the pump will remain constant as it passes through the valve, but the overall temperature of the gas will still increase due to the work done on it during compression. This is because the Joule Thompson effect only applies to a single gas phase, while the overall gas in the pump is undergoing a phase change from gas to liquid. So, in summary, the internal energy of an ideal gas will increase under adiabatic compression and the temperature will also increase due to the work done on the gas. The Joule Thompson effect does not apply in this situation.
  • #1
aa1607
2
0
I'm having trouble understanding what happens to the internal energy of an ideal gas being compressed adiabatically.

If DU = DQ + DW,

then as we do work PdV compressing the gas, since in adiabatic processes DQ=0, W the change in internal energy is non-zero, so U must increase.

But if we're talking about an ideal gas, as I keep hearing, (such as in this lecture where we're told an ideal gas shouldn't increase in temperature when we compress it in a bicycle pump:)

http://www.youtube.com/watch?v=g14939TMTCE#t=36m30s

U is a function only of T, and so T ought not to vary as we increase the pressure, because it ought to be compensated for by a (countering) decrease in V, in accordance with the Ideal Gas Law: PV = nRT.

So should U stay constant or should it increase under adiabatic compression if the gas is ideal?

Can someone help me with this?

Thanks a lot!
 
Physics news on Phys.org
  • #2
aa1607 said:
I'm having trouble understanding what happens to the internal energy of an ideal gas being compressed adiabatically.

If DU = DQ + DW,

then as we do work PdV compressing the gas, since in adiabatic processes DQ=0, W the change in internal energy is non-zero, so U must increase.

But if we're talking about an ideal gas, as I keep hearing, (such as in this lecture where we're told an ideal gas shouldn't increase in temperature when we compress it in a bicycle pump:)

http://www.youtube.com/watch?v=g14939TMTCE#t=36m30s

U is a function only of T, and so T ought not to vary as we increase the pressure, because it ought to be compensated for by a (countering) decrease in V, in accordance with the Ideal Gas Law: PV = nRT.

So should U stay constant or should it increase under adiabatic compression if the gas is ideal?

Can someone help me with this?

Thanks a lot!


U increases under adiabatic compression even if the gas is ideal. The change in U is equal to the work done on the gas. Air compressed in a bicycle pump is very close to an ideal gas. I found the discussion in the video very confusing. When the heated air passes from the pump through the valve into the tire, its higher temperature should remain nearly constant from one side of the value to the other, according to the Joule Thompson effect. You can look up the Joule Thompson parameter as a function of temperature and pressure for air (or calculate it from the non-ideal gas parameters for air), and you will find that the jt effect at bike tire pressures will be much smaller than the temperature change for adiabatic compression of air.
 
  • #3
Thanks for the reply, I understand that now. The video is actually quite misleading.
 

1. What is an ideal gas?

An ideal gas is a theoretical concept used in thermodynamics to describe the behavior of gases at low pressures and high temperatures. It assumes that the gas particles have no volume and do not interact with each other, making the gas highly compressible and following the ideal gas law.

2. What is adiabatic compression?

Adiabatic compression is a process in which a gas is compressed without any heat exchange with its surroundings, meaning there is no transfer of energy in the form of heat. This results in an increase in pressure and temperature of the gas.

3. How does an ideal gas behave under adiabatic compression?

Under adiabatic compression, an ideal gas follows the relationship PV^γ = constant, where P is the pressure, V is the volume, and γ is the specific heat ratio. This means that as the volume decreases, the pressure and temperature of the gas will increase.

4. What is the specific heat ratio (γ) for an ideal gas?

The specific heat ratio, also known as the adiabatic index, is a constant value that depends on the type of gas. For an ideal gas, the specific heat ratio is usually considered to be 1.4.

5. What are the applications of ideal gases under adiabatic compression?

Ideal gases under adiabatic compression have practical applications in various fields such as engineering, meteorology, and thermodynamics. They are used in gas turbines, internal combustion engines, and weather forecasting models. Adiabatic compression is also utilized in the compression stroke of a four-stroke engine to increase the efficiency of the engine.

Similar threads

A Adiabatic Compressible Flow in a Converging Duct
    • Classical Physics
Replies
6
Views
1K
I Why is estimation of ##\frac{Pv}{RT}=1+BP+CP^2+...## interesting?
    • Classical Physics
Replies
2
Views
569
A Why are the Navier-Stokes equations inconsistent in this case?
    • Classical Physics
Replies
18
Views
1K
I Graphical difference between adiabatic and isothermal processes.
    • Thermodynamics
Replies
1
Views
641
I How Do You Calculate Initial Pressure and Final Volume in Adiabatic Expansion?
    • Classical Physics
Replies
7
Views
1K
I Question regarding dh, du, cp, cv for ideal gases
    • Classical Physics
Replies
3
Views
1K
B Adiabatic or Isobaric process?
    • Classical Physics
2
Replies
61
Views
5K
I Does the Euler method give a better insight into adiabatic processess?
    • Classical Physics
Replies
6
Views
812
I Non-adiabatic expansion in Diesel and Otto cycles
    • Classical Physics
Replies
1
Views
496
I Irreversible adiabatic processes & entropy change (clarification needed)
    • Thermodynamics
Replies
22
Views
2K

Hot Threads

Recent Insights

Back
Top