Question

0.08 kg of air is heated at constant volume through 5°C. The specific heat of air at constant volume is 0.17 kcal/kg°C and $J = 4.18$ joule/cal. The change in its internal energy is approximately:

• A. 318 J
• B. 298 J
• C. 284 J
• D. 142 J

Answer 1

Answer: (C)

284 J

Answer 2

Since the process is at constant volume, the change in internal energy $\Delta U$ is given by:

$\Delta U = ms\Delta T$

where $m = 0.08 \, \text{kg}$, $s = 0.17 \, \text{kcal/kg}^\circ\text{C}$, and $\Delta T = 5^\circ \text{C}$.

Convert $s$ from kcal to joules:

$s = 0.17 \times 1000 \times 4.18 \, \text{J/kg}^\circ\text{C}$

Then,

$\Delta U = 0.08 \times (0.17 \times 1000 \times 4.18) \times 5 \approx 284 \, \text{J}$