SSC Stenographer 2026 PYQ — Question 16

Solution: The correct answer is Entropy always increases in a closed system, providing a direction for time from past to future.
Explanation:
☛ Entropy is a fundamental thermodynamic quantity that measures the degree of disorder, randomness, or unavailable energy in a system.
☛ According to the Second Law of Thermodynamics, the total entropy of an isolated (closed) system can never decrease over time; it can remain constant in ideal reversible processes but must strictly increase in all natural, spontaneous processes.
☛ The British astrophysicist Sir Arthur Eddington introduced the phrase "Arrow of Time" in 1927 to illustrate that the unidirectional increase of entropy distinguishes the past from the future, giving time an irreversible forward direction.
Extra Exam-Relevant Facts:
☛ Mathematical Expression: Entropy change is represented as ΔS = ΔQ / T, where ΔQ is the heat exchanged reversibly and T is absolute temperature.
☛ SI Unit of Entropy: The standard unit of entropy is Joules per Kelvin (J/K).
☛ Boltzmann's Entropy Formula: Austrian physicist Ludwig Boltzmann defined entropy statistically as S = kB ln W, where kB is the Boltzmann constant and W is the number of possible microscopic states.
☛ Irreversibility of Natural Processes: Spontaneous processes such as heat flowing from a hot body to a cold body or the diffusion of gases increase total entropy and cannot reverse on their own.
Important One-Liners for Upcoming Exams:
☛ Who coined the term 'Arrow of Time' in physics? → Sir Arthur Eddington
☛ What is the SI unit of entropy? → Joules per Kelvin (J/K)
☛ Which law of thermodynamics forms the basis of temperature measurement? → Zeroth Law of Thermodynamics
☛ Which law of thermodynamics states that entropy approaches a constant value at absolute zero? → Third Law of Thermodynamics
☛ What is the value of the Boltzmann constant (kB)? → 1.38 × 10⁻²³ J/K