JEE Main · 2014 · 06 Apr 2014 · Offline · Heat & Thermodynamics

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One mole of diatomic ideal gas undergoes a cyclic process ABC as shown in figure. The process BC is adiabatic. The temperatures at A, B and C are 400 K, 800 K and 600 K respectively. Choose the correct statement:

(A)The change in internal energy in the process AB is $- 350 R$ .
(B)The change in internal energy in the process BC is $- 500 R$ .✓ Correct
(C)The change in internal energy in whole cyclic process is 250R.
(D)The change in internal energy in the process CA is 700R.

Answer: (B)

Solution

For a diatomic gas

C_{V} = \frac{5 R}{2}

, and internal energy depends only on temperature,

Δ U = n C_{V} Δ T

. Along BC the temperature falls from

800

K to

600

K, so

Δ U_{B C} = \frac{5 R}{2} (600 - 800) = - 500 R

. For comparison AB gives

+ 1000 R

and CA gives

- 500 R

, and over the whole cycle the changes cancel to zero. The change in internal energy in process BC is

- 500 R

JEE Main · 2014 · 06 Apr 2014 · Offline · Single correct option

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JEE Main · 2014 · 06 Apr 2014 · Offline · Heat & Thermodynamics

Solution

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