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Standard XII

Physics

2nd Law of Thermodynamics

An ideal gas ...

Question

An ideal gas undergoing isenthalpic expansion from 6 bar to 12 bar 450 K. What is the entropy change during the expansion?

(Take $c_{p} = 1 k J / k g K, R = 0.30 k J / k g K$ )

0 kJ/kgK

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-0.482 kJ/kgK

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0.482 kJ/kgK

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None of these

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Solution

The correct option is C 0.482 kJ/kgK
For an ideal gas
h = f(T)
As h is constant in isenthalpic expansion, temperature is also constant.
$∴$ $T_{1} = T_{2} = 450$ K
$Δ s = C_{p} I n \frac{T_{2}}{T_{1}} - R I n \frac{P_{2}}{P_{1}}$
= $1 l n \frac{450}{450} - 0.30 l n \frac{1.2}{6} = 0.482 k J / k g K$

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An ideal gas undergoing isenthalpic expansion from 6 bar to 12 bar 450 K. What is the entropy change during the expansion? (Take cp=1 kJ/kgK, R=0.30 kJ/kgK)

The correct option is C 0.482 kJ/kgKFor an ideal gas h = f(T) As h is constant in isenthalpic expansion, temperature is also constant. ∴ T1=T2=450K Δs=CpInT2T1−RInP2P1 =1 ln450450−0.30ln1.26=0.482 kJ/kgK

An ideal gas undergoing isenthalpic expansion from 6 bar to 12 bar 450 K. What is the entropy change during the expansion?

(Take $c_{p} = 1 k J / k g K, R = 0.30 k J / k g K$ )