Practice <strong>NEET UG Physics</strong> MCQ questions covering Mechanics, Thermodynamics, Waves & Optics, Modern Physics, Electricity & Magnetism, and Electromagnetic Induction. Aligned with NCERT Class 11 & 12 Physics syllabus.
Which statement about entropy is correct for an isolated system?
Answer: B
Second law of thermodynamics: For an isolated system, ΔS ≥ 0 (entropy increases for irreversible processes, constant for reversible).
Q.63Medium
In an isothermal process of an ideal gas, the work done by the gas is:
Answer: B
For isothermal process: W = nRT ln(V_f/V_i) = nRT ln(P_i/P_f). Since ΔU = 0, q = W.
Q.64Medium
A gas undergoes compression where both pressure and temperature increase. This process is most likely:
Answer: D
In adiabatic compression (q=0), work is done on gas, increasing both P and T without heat exchange. Other processes would have different P-T relationships.
Q.65Medium
The coefficient of performance (COP) of a refrigerator is 5. If 100 J of work is done on it, how much heat is removed from the cold reservoir?
In a throttling process (Joule-Thomson expansion), which quantity remains constant?
Answer: C
In throttling, enthalpy H remains constant (isenthalpic process). For ideal gases, temperature also remains constant, but for real gases, it can change.
Q.75Easy
The standard molar entropy of a substance at 0 K according to the third law of thermodynamics is:
Answer: B
Third law of thermodynamics: The entropy of a perfect crystal at absolute zero is zero. S(0K) = 0 for perfect crystals.
Q.76Hard
A composite Carnot engine and refrigerator system operates between three reservoirs. If the engine operates between 500 K and 300 K, and the refrigerator between 300 K and 200 K, what is the theoretical relationship for reversible operation?
Answer: A
For reversible processes on the middle reservoir (300K), entropy change must be zero: Q_h,engine/500 = (Q_h,engine - W_engine)/300 and similar relations apply to refrigerator at 300K.
Q.77Easy
A gas expands isothermally from volume V₁ to V₂. The work done by the gas is W and heat absorbed is Q. What is the relationship between W and Q for an ideal gas?
Answer: A
In an isothermal process, ΔU = 0. By first law: Q = ΔU + W, therefore Q = W for an ideal gas.
Q.78Easy
A reversible heat engine operates between two reservoirs at temperatures T₁ = 400 K and T₂ = 300 K. What is the maximum possible efficiency of this engine?
