Question

During an isothermal expansion of an ideal gas

• A. its internal energy decreases
• B. its internal energy does not change
• C. the work done by the gas is equal to the quantity of heat absorbed by it
• D. both (b) and (c) are correct

Answer 1

Answer: (D)

Key Idea: Temperature remains constant in an isothermal process. In an ideal gas, the internal energy depends only upon the temperature of the gas. When an ideal gas undergoes an isothermal change, there is no change in its internal energy $(\Delta U=0)$ . From first law of thermodynamics $\Delta U=Q-W$ For isothermal change $\Delta U=0$ $\therefore$ $Q=W$ Hence, in an isothermal process in an ideal gas the heat absorbed by the gas is entirely used in the work done by the gas. Note: This is true for an ideal gas only and not for real systems.