Free Expansion of Ideal Gas Entropy – Rankers Physics
Topic: Thermal Physics
Subtopic: Kinetic Theory of Gases

Free Expansion of Ideal Gas Entropy

Assertion (A): During free expansion of an Ideal gas, entropy is zero.
Reason (R): Internal energy of an ideal gas is zero during free expansion.
 
(1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
(2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(3) (A) is true but (R) is false
(4) Both (A) and (R) are false

Solution:

During free expansion of an ideal gas, no work is done \(W=0\) and no heat is exchanged \(Q=0\). Therefore, change in internal energy \(Delta U = Q - W = 0\). For an ideal gas, \(Delta U = 0\) implies \(Delta T = 0\). Internal energy itself is not zero (it just doesn't change). Free expansion is an irreversible process, so entropy *increases* \(Delta S > 0\), it's not zero. Thus, both (A) and (R) are false.

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