50. Which thermodynamic law provides the basis for temperature measurement and thermal equilibrium?
[SSC JE Mechanical, June 2024]
(A) Zeroth law
(B) First law
(C) Second law
(D) Third law
Answer: (A) Zeroth law
Explanation: The zeroth law states that if two systems are each in thermal equilibrium with a third system, they are in equilibrium with each other. It makes temperature a consistent property and justifies using thermometers to compare thermal states.
49. According to the second law of thermodynamics, how are work and heat commonly classified?
[SSC JE Mechanical, June 2024]
(A) Work is high-grade energy and heat is low-grade energy
(B) Heat is high-grade and work is low-grade
(C) Both are always high-grade
(D) Both are always low-grade
Answer: (A) Work is high-grade energy and heat is low-grade energy
Explanation: Mechanical work can, in principle, be converted completely into heat. A heat engine, however, cannot convert all heat obtained from a single reservoir into useful net work. This limitation leads engineers to describe work as higher-grade energy than heat.
48. For a good electrical heating element, which are desirable?
I. Low resistivity;
II. High melting point;
III. Small temperature coefficient of resistance;
IV. High specific heat.
[SSC JE Electrical, June 2024]
(A) II, III and IV
(B) I, II and IV
(C) Only II and III
(D) I, II and III
Answer: (C) Only II and III
Explanation: Heating elements should resist melting during operation and maintain relatively stable electrical resistance as temperature changes. High resistivity is generally desirable; low resistivity is not. Excessively high heat capacity delays warming, so only statements II and III are preferred.
47. Who established the quantitative mechanical equivalent of heat through experiments?
[SSC CPO, June 2024]
(A) William Henry
(B) James Prescott Joule
(C) Hermann von Helmholtz
(D) John Dalton
Answer: (B) James Prescott Joule
Explanation: James Prescott Joule demonstrated that mechanical work could raise water temperature, linking work and heat quantitatively. His paddle-wheel experiments supported energy conservation. The SI unit of energy, the joule, honours his contribution to thermodynamics.
46. Which of these is NOT merely a physical change?
[SSC CGL Tier-I, September 2024]
(A) Heating iron until red-hot
(B) Conversion of milk into curd
(C) Evaporation of diesel
(D) Sublimation of ammonium chloride
Answer: (B) Conversion of milk into curd
Explanation: Curdling involves biochemical conversion of milk components and formation of new products, so it is treated as a chemical change. Heating, evaporation and sublimation can occur without changing chemical identity, although the substance’s temperature or physical state changes.
45. Which scientist introduced the term zeroth law of thermodynamics?
[SSC CGL Tier-I, July 2023]
(A) James Clerk Maxwell
(B) Max Planck
(C) Josiah Willard Gibbs
(D) Ralph H. Fowler
Answer: (D) Ralph H. Fowler
Explanation: Ralph H. Fowler formally introduced the expression zeroth law in the twentieth century. It describes transitivity of thermal equilibrium and underlies meaningful temperature comparison. Although named after the first three laws, it is logically more fundamental to thermometry.
44. A thermometer works on which thermodynamic law?
[SSC JE Mechanical, October 2023]
(A) Second law
(B) Joule’s law
(C) Third law
(D) Zeroth law
Answer: (D) Zeroth law
Explanation: A thermometer reaches thermal equilibrium with an object whose temperature is being measured. By the zeroth law, systems in equilibrium with the same thermometer have the same temperature. This lets a calibrated thermometer indicate the thermal state of another body.
43. Which law rules out a heat engine operating at 100 percent thermal efficiency between ordinary reservoirs?
[SSC JE Mechanical, December 2023]
(A) Third law
(B) First law
(C) Second law
(D) Zeroth law
Answer: (C) Second law
Explanation: The second law requires an engine operating cyclically between heat reservoirs to reject some heat rather than convert all absorbed heat into work. The first law addresses energy conservation; it does not by itself prohibit a perfectly efficient heat engine.
42. Which claim about heat and work in thermodynamics is incorrect?
[SSC JE Mechanical, December 2023]
(A) A system possesses heat and work as stored properties
(B) Heat and work are path-dependent quantities
(C) Both are associated with processes rather than states
(D) Heat and work cross the system boundary
Answer: (A) A system possesses heat and work as stored properties
Explanation: A system stores internal energy, not heat or work. Heat and work describe two ways energy crosses the system boundary while a process occurs. Their amounts depend on the path followed, whereas internal energy is a state function.
41. Which transition does NOT describe latent heat of vaporisation?
[SSC JE Mechanical, October 2023]
(A) Saturated liquid turning to vapour
(B) Ice melting into liquid water
(C) Water becoming steam at its boiling point
(D) Liquid becoming gas without temperature change
Answer: (B) Ice melting into liquid water
Explanation: Latent heat of vaporisation applies to the liquid-to-gas transition at the saturation temperature. Melting ice requires latent heat of fusion, which concerns a solid-liquid phase change. During either ideal phase transition, supplied energy does not raise the temperature.
