Question

The expansion of gas having no opposing force is called as

• A. free expansion
• B. reversible expansion
• C. adiabatic expansion
• D. isothermal expansion

Hint:

"No opposing force" means the environment is a total vacuum ($P_{\text{ext}} = 0$). Expanding into a vacuum means the gas can move "freely" without pushing against anything, which is why it is called a free expansion.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the specific thermodynamic term used to describe a gas expansion process that encounters absolutely zero external resistance or opposing force.

Step 2: Key Formula or Approach:
Thermodynamic work done ($W$) during the expansion of a gas against an external opposing pressure ($P_{\text{ext}}$) is given by the integral formula:
$$W = -\int P_{\text{ext}} \, dV$$ When there is no opposing force, the external pressure is zero ($P_{\text{ext}} = 0$).

Step 3: Detailed Explanation:
Let's analyze what happens when a gas expands into a completely empty space (a vacuum):
Because a vacuum contains no particles, it cannot exert an opposing force or containment pressure against the boundary of the expanding gas volume.
Since $P_{\text{ext}} = 0$, substituting this into our work equation gives:
$$W = - (0) \cdot \Delta V = 0$$ This special scenario where a gas expands spontaneously into an unrestricted vacuum without doing any mechanical work is called a free expansion.
Other choices like reversible expansion (requires $P_{\text{ext}} \approx P_{\text{int}}$ at all points), adiabatic expansion (no heat exchange), and isothermal expansion (constant temperature) all typically happen against an active opposing force.

Step 4: Final Answer:
The expansion of a gas against no opposing force is classified as free expansion, which corresponds to option (A).