Question

Which of the following process is reversible :

• A. A block moving on a rough surface comes to rest after same time.
• B. Two gases are mixed in a container
• C. A block kept on a smooth surface is connected to a fixed spring and is oscillating horizontally.
• D. The water is heated on a stove.

Answer 1

Answer: (C)

C

Answer 2

Here's the analysis of each process:

(A) A block moving on a rough surface comes to rest after some time. This process involves friction, which is a dissipative force. Friction converts kinetic energy into heat, which is then dissipated into the surroundings. This process increases the entropy of the universe and cannot be reversed without leaving a change in the surroundings. Hence, it is irreversible.

(B) Two gases are mixed in a container. Mixing of gases is a spontaneous process that increases the entropy of the system (and the universe). The mixed gases will not spontaneously separate. Separating them requires external work and specialized processes, which would leave a net change in the surroundings. Hence, it is irreversible.

(C) A block kept on a smooth surface is connected to a fixed spring and is oscillating horizontally. Assuming the surface is perfectly smooth (no friction) and there is no air resistance, the oscillation is ideal simple harmonic motion. In this ideal case, mechanical energy is conserved. The process can be reversed at any point by reversing the direction of motion (e.g., by applying an infinitesimal force). If reversed, the system will retrace its path, and there are no dissipative losses. In the context of thermodynamics, ideal, frictionless processes are considered reversible.

(D) The water is heated on a stove. This process involves heat transfer from the stove (at a higher temperature) to the water (at a lower temperature) across a finite temperature difference. Heat flow across a finite temperature difference is an irreversible process. A reversible heat transfer would occur quasi-statically across an infinitesimal temperature difference.

Among the given options, the oscillation of a block on a smooth surface connected to a spring (under ideal conditions implied by "smooth surface") is the only process that can be considered reversible.