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Question
1 litre of an ideal gas (γ = 1.5) at 300 K is suddenly compressed to half its original volume. (a) Find the ratio of the final pressure to the initial pressure. (b) If the original pressure is 100 kPa, find the work done by the gas in the process. (c) What is the change in internal energy? (d) What is the final temperature? (e) The gas is now cooled to 300 K keeping its pressure constant. Calculate the work done during the process. (f) The gas is now expanded isothermally to achieve its original volume of 1 litre. Calculate the work done by the gas. (g) Calculate the total work done in the cycle.
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Solution
Given:
γ = 1.5
T = 300 K
Initial volume of the gas, V1 = 1 L
Final volume, V2 = `1/2` L
(a) The process is adiabatic because volume is suddenly changed; so, no heat exchange is allowed.
P1V1γ = P2V2γ
Or `"P"_2 ="P"_1("V"_1/"V"_2)^gamma = "P"_1(2)^gamma`
`"P"_2/"P"_1 = 2^1.5 = 2 sqrt2`
(b) P1 = 100 kPa = 105 Pa
and P2 = `2sqrt2` × 105 Pa
Work done by an adiabatic process,
`"W" = ("P"_1"V"_1 - "P"_2"V"_2)/(gamma - 1)`
`"W" = (10^5 xx 10^-3 -2sqrt2 xx 10^5 xx 1/2 xx 10^-3)/(1.5 -1)`
W = -82 J
(c) Internal energy,
dQ = 0, as it is an adiabatic process.
⇒ dU = − dW = − (− 82 J) = 82 J
(d)
Also, for an adiabatic process,
T1V1γ−1 = T2V2γ−1
`"T"_2 ="T"_1 ("V"_1/"V"_2)^(gamma -1)`
= 300 × (2)0.5
`= 300 xx sqrt 2 xx = 300 xx 1..4142`
T2= 424 K
(e) The pressure is kept constant.
The process is isobaric; so, work done = PΔV=nRdT.
Here, n = `("P""V")/("R""T") = (10^5 xx 10 ^-3)/("R" xx 300) = 1/(3"R")`
So, work done =`1/(3"R") xx "R" xx (300-424) = -41.4"J"`
As pressure is constant,
`"V"_1/"T"_1 = "V"_2/"T"_2 ... (1)`
`"V" _1 = "V"_2("T"_1)/"T"_2`
(f)Work done in an isothermal process,
`"W" = "n""R""T" "l""n" "V"_2/"V"_1`
= `1/(3"R") xx "R" xx "T" xx ln (2)`
= 100 × ln 2 = 100 × 1.039
= 103 J
(g) Net work done (using first law of thermodynamics)
= − 82 − 41.4 + 103
= − 20.4 J
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