Question

Which of the following processes exhibits the decrease in internal energy of a system?

• A. Isothermal expansion of an ideal gas.
• B. Isothrmal compression of an ideal gas.
• C. Adiabatic expansion of an ideal gas.
• D. Adiabatic compression of an ideal gas

Hint:

In an adiabatic expansion no heat enters, so the gas does work at the cost of its own energy.

Answer 1

The Correct Option is C

Step 1: Idea
For an ideal gas, internal energy is tied to temperature alone. So ask in which process the temperature drops.

Step 2: Go through the four processes
Isothermal means constant $T$, so the energy of the gas does not change in (A) or (B). In an adiabatic process the gas is insulated, so no heat flows in or out.

Step 3: Adiabatic cases
When an insulated gas expands, it must supply the work of pushing the surroundings from its own store of energy, so its temperature falls and $U$ decreases. When it is compressed, work is added to it, so $T$ and $U$ rise.

Step 4: Conclusion
Only the adiabatic expansion lowers the internal energy, which is option (C).

Final Answer:
Only the adiabatic expansion of an ideal gas lowers its internal energy, option (C). \[ \boxed{\text{Adiabatic expansion}} \]