Loading [MathJax]/jax/output/HTML-CSS/jax.js
Skip to main content
Library homepage
 

Text Color

Text Size

 

Margin Size

 

Font Type

Enable Dyslexic Font
Physics LibreTexts

3.7: Adiabatic Processes for an Ideal Gas

( \newcommand{\kernel}{\mathrm{null}\,}\)

Learning Objectives

By the end of this section, you will be able to:

  • Define adiabatic expansion of an ideal gas
  • Demonstrate the qualitative difference between adiabatic and isothermal expansions

When an ideal gas is compressed adiabatically (Q=0), work is done on it and its temperature increases; in an adiabatic expansion, the gas does work and its temperature drops. Adiabatic compressions actually occur in the cylinders of a car, where the compressions of the gas-air mixture take place so quickly that there is no time for the mixture to exchange heat with its environment. Nevertheless, because work is done on the mixture during the compression, its temperature does rise significantly. In fact, the temperature increases can be so large that the mixture can explode without the addition of a spark. Such explosions, since they are not timed, make a car run poorly—it usually “knocks.” Because ignition temperature rises with the octane of gasoline, one way to overcome this problem is to use a higher-octane gasoline.

The figure on the left is an illustration of the initial equilibrium state of a container with a partition in the middle dividing it into two chambers.  The outer walls are insulated. The chamber on the left is full of gas, indicated by blue shading and many small dots representing the gas molecules. The right chamber is empty. The figure on the right is an illustration of the final equilibrium state of the container. The partition has a hole in it. The entire container, on both sides of the partition, is full of gas, indicated by blue shading and many small dots representing the gas molecules. The dots in the second, final equilibrium state, illustration are less dense than in the first, initial state illustration.
Figure 3.7.1: The gas in the left chamber expands freely into the right chamber when the membrane is punctured.

Another interesting adiabatic process is the free expansion of a gas. Figure 3.7.1 shows a gas confined by a membrane to one side of a two-compartment, thermally insulated container. When the membrane is punctured, gas rushes into the empty side of the container, thereby expanding freely. Because the gas expands “against a vacuum” (p=0), it does no work, and because the vessel is thermally insulated, the expansion is adiabatic. With Q=0 and W=0 in the first law, ΔEint=0 so Einti=Eintf for free expansion.

If the gas is ideal, the internal energy depends only on the temperature. Therefore, when an ideal gas expands freely, its temperature does not change; this is also called a Joule expansion.

The figure is an illustration of a container. The walls and bottom are filled with a thick layer of insulation. The chamber of the container is closed from above by a piston. Inside the chamber is a gas. There is a pile of sand on top of the piston, and a hand with tweezers is removing grains from the pile.
Figure 3.7.2: When sand is removed from the piston one grain at a time, the gas expands adiabatically and quasi-statically in the insulated vessel.

A quasi-static, adiabatic expansion of an ideal gas is represented in Figure 3.7.2, which shows an insulated cylinder that contains 1 mol of an ideal gas. The gas is made to expand quasi-statically by removing one grain of sand at a time from the top of the piston. When the gas expands by dV, the change in its temperature is dT. The work done by the gas in the expansion is dW=pdV; dQ=0 because the cylinder is insulated; and the change in the internal energy of the gas is

dEint=CVndT.

Therefore, from the first law,

CVndT=0pdV=pdV

so

dT=pdVCVn.

Also, for 1 mol of an ideal gas,

[d(pV) = d(RnT), \nonumber \]

so

pdV+Vdp=RndT

and

dT=pdV+VdpRn.

We now have two equations for dT. Upon equating them, we find that

CVnVdp+(CVn+Rn)pdV=0.

Now, we divide this equation by npV and use Cp=CV+R. We are then left with

CVdpp+CpdVV=0,

which becomes

dpp+γdVV=0,

where we define γ as the ratio of the molar heat capacities:

γ=CpCV.

Thus

dpp+γdVV=0

and

lnp+γlnV=constant.

Finally, using ln(Ax)=xlnA and lnAB=lnA+lnB, we can write this in the form

pVγ=constant.

This equation is the condition that must be obeyed by an ideal gas in a quasi-static adiabatic process. For example, if an ideal gas makes a quasi-static adiabatic transition from a state with pressure and volume p1 and V1 to a state with p2 and V2, then it must be true that p1Vγ1=p2Vγ2.

The adiabatic condition of Equation ??? can be written in terms of other pairs of thermodynamic variables by combining it with the ideal gas law. In doing this, we find that

p1γTγ=constant

and

TVγ1=constant.

A reversible adiabatic expansion of an ideal gas is represented on the pV diagram of Figure 3.7.1. The slope of the curve at any point is

dpdV=ddV(constantVγ)=γpV.

The figure is a plot of pressure, p on the vertical axis as a function of volume, V on the horizontal axis. Two curves are plotted. Both are monotonically decreasing and concave up.  One is slightly higher and has a greater curvature. This curve is labeled  “isothermal.” The second curve is below the isothermal curve and has  a slightly smaller curvature. This curve is labeled “adiabatic.”
Figure 3.7.3: Quasi-static adiabatic and isothermal expansions of an ideal gas.

The dashed curve shown on this pV diagram represents an isothermal expansion where T (and therefore pV) is constant. The slope of this curve is useful when we consider the second law of thermodynamics in the next chapter. This slope is

dpdV=ddVnRTV=pV.

Because γ>1, the isothermal curve is not as steep as that for the adiabatic expansion.

Example 3.7.1: Compression of an Ideal Gas in an Automobile Engine

Gasoline vapor is injected into the cylinder of an automobile engine when the piston is in its expanded position. The temperature, pressure, and volume of the resulting gas-air mixture are 20oC, 1.00×105N/m2, and 240cm3, respectively. The mixture is then compressed adiabatically to a volume of 40cm3. Note that in the actual operation of an automobile engine, the compression is not quasi-static, although we are making that assumption here.

  1. What are the pressure and temperature of the mixture after the compression?
  2. How much work is done by the mixture during the compression?
Strategy

Because we are modeling the process as a quasi-static adiabatic compression of an ideal gas, we have pVγ=constant and pV=nRT. The work needed can then be evaluated with W=V2V1pdV.

Solution
  1. For an adiabatic compression we have p2=p1(V1V2)γ, so after the compression, the pressure of the mixture is p2=(1.00×105N/m2)(240×106m340×106m3)1.40=1.23×106N/m2. From the ideal gas law, the temperature of the mixture after the compression is T2=(p2V2p1V1)T1=(1.23×106N/m2)(40×106m3)(1.00×105N/m2)(240×106m3)293K=600K=328oC.
  2. The work done by the mixture during the compression is W=V2V1pdV. With the adiabatic condition of Equation ???, we may write p as K/Vγ, where K=p1Vγ1=p2Vγ2. The work is therefore W=V2V1KVγdV=K1γ(1Vγ121Vγ11)=11γ(p2Vγ2Vγ12p1Vγ1Vγ11)=11γ(p2V2p1V1)=111.40[(1.23×106N/m2)(40×106m3)(1.00×105N/m2)(240×106m3)]=63J.

Significance

The negative sign on the work done indicates that the piston does work on the gas-air mixture. The engine would not work if the gas-air mixture did work on the piston.


This page titled 3.7: Adiabatic Processes for an Ideal Gas is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by OpenStax via source content that was edited to the style and standards of the LibreTexts platform.

Support Center

How can we help?