Question

When an ideal diatomic gas is heated at constant pressure, the fraction of the heat energy supplied which increases the internal energy of the gas is

• A. 2/5
• B. 3/5
• C. 3/7
• D. 5/7

Answer 1

Answer: (D)

5/7

Answer 2

When a gas is heated at constant pressure then its one part goes to increase the internal energy and another part for work done against external pressure i.e. $(\Delta Q)_{p} = \Delta U + \Delta W$

⇒ $\mu C_{p}\Delta T = \mu C_{v}\Delta T + P\Delta V$

So fraction of energy that goes to increase the internal energy $\frac{\Delta U}{(\Delta Q)_{p}} = \frac{C_{v}}{C_{p}} = \frac{1}{\gamma} = \frac{5}{7}$ [As $\gamma = \frac{7}{5}$ for diatomic gas]