Question

The average degrees of freedom per molecule for a gas is 6. The gas performs 25 J of work when it expands at constant pressure. The heat absorbed by gas is

• A. 75 J
• B. 100 J
• C. 150 J
• D. 125 J

Answer 1

Answer: (B)

100 J

Answer 2

As f = 6 (given) ∴ $\gamma = 1 + \frac{2}{f} = 1 + \frac{2}{6} = \frac{4}{3}$

Fraction of energy given for external work $\frac{\Delta W}{\Delta Q} = \left( 1 - \frac{1}{\gamma} \right)$

⇒ $\frac{25}{\Delta Q} = \left( 1 - \frac{1}{4/3} \right)$ $= 1 - \frac{3}{4} = \frac{1}{4}$

⇒ $\Delta Q = 25 \times 4 = 100Joule$