Physics is where JEE ranks are usually won or lost, since a single conceptual slip changes the whole answer. Questions here span mechanics, rotational motion, thermodynamics, waves and oscillations, electrostatics, current electricity, magnetism, optics, and modern physics. Numerical problems carry full derivations so you can see which step you skipped, not just which option was right.
The entropy change for a reversible isothermal process is given by:
Answer: C
For reversible isothermal process: ΔS = Q/T = nR ln(Vf/Vi) since Q = nRT ln(Vf/Vi) for ideal gas isothermal expansion.
Q.122Medium
Which process would result in the maximum work output from an ideal gas expansion between two fixed pressures?
Answer: A
For expansion between fixed initial and final states, isothermal process yields maximum work because the gas maintains maximum pressure throughout the expansion compared to adiabatic or other processes.
Q.123Medium
A cylinder with a movable piston contains 2 moles of ideal gas at 300 K and 1 atm. When heated at constant pressure to 600 K, what is the work done by the gas? (R = 8.314 J/mol·K)
If a reversible process occurs in an isolated system, the entropy of the system:
Answer: C
For a reversible process in an isolated system, ΔS_total = ΔS_sys + ΔS_surr = 0 (no heat exchange with surroundings). Therefore entropy remains constant at its initial value.
Q.127Hard
An ideal gas undergoes a process where PV^n = constant. If n = 1, this process is _____ and if n = γ, this process is _____
Answer: A
When n = 1: PV = constant (isothermal). When n = γ = Cp/Cv: PV^γ = constant (adiabatic process).
Q.128Hard
Consider a system where entropy decreases by 50 J/K. Which statement must be true?
Answer: B
By second law, ΔS_total ≥ 0. If ΔS_sys = -50 J/K, then ΔS_surr ≥ +50 J/K to maintain ΔS_total ≥ 0.
Q.129Hard
A polytropic process with polytropic index n = 1.3 is performed on 1 mole of air (diatomic). The work done when volume changes from 1 m³ to 0.5 m³ at initial pressure 100 kPa is:
Answer: A
For polytropic process: W = [P₁V₁ - P₂V₂]/(n-1). Using PVⁿ = const: P₂ = 100 × (01.5)^1.3 ≈ 245.7 kPa. W = [100×1 - 245.7×0.5]/0.3 ≈ 81.2 kJ
Q.130Hard
For a van der Waals gas with equation (P + a/V²m)(Vm - b) = RT, at the critical point, which relationship is valid?
Answer: A
At the critical point, both first and second derivatives of pressure with respect to volume (at constant T) are zero: (∂P/∂V)T = 0 and (∂²P/∂V²)T = 0. This defines the critical point.
Q.131Easy
A system absorbs 500 J of heat and performs 200 J of work on the surroundings. What is the change in internal energy of the system?
Answer: A
By first law of thermodynamics: ΔU = Q − W = 500 − 200 = 300 J
Q.132Easy
An ideal gas undergoes an adiabatic process. If the gas is compressed, which of the following is true?
Answer: B
In adiabatic compression (Q = 0), work is done on the gas (W < 0). By first law: ΔU = Q − W = 0 − W > 0. Since ΔU ∝ ΔT for ideal gas, temperature increases.
Q.133Easy
What is the efficiency of a Carnot engine operating between 400 K and 300 K?
Two identical blocks of metal at different temperatures are brought into thermal contact in an isolated system. The process is:
Answer: C
Heat transfer between objects at different temperatures is irreversible. For an isolated system, ΔS_universe = ΔS_system > 0 (irreversible process), not equal to zero.
Q.136Medium
A gas expands from 1 L to 5 L against a constant external pressure of 2 atm. The work done by the gas is approximately:
The molar heat capacity of a diatomic ideal gas at constant pressure is (R = 8.314 J/mol·K):
Answer: B
For diatomic gas: Cv = (25)R, and Cp = Cv + R = (25)R + R = (27)R. This is at room temperature where vibration is not excited.
Q.138Medium
A cyclic process ABCA is shown on a P-V diagram where AB is isothermal expansion, BC is adiabatic compression, and CA is isochoric process. Which statement is correct?
Answer: A
In a complete cycle returning to initial state, ΔU = 0, so Q = W. For expansion-dominated processes in a typical cycle, W > 0 and Q > 0.
Q.139Medium
A heat engine absorbs 1000 J from a hot reservoir and rejects 600 J to a cold reservoir in one cycle. Its efficiency is:
A reversible heat engine operates between two thermal reservoirs. If the temperature of the cold reservoir decreases while hot reservoir temperature remains constant, the maximum efficiency:
Answer: A
Carnot efficiency η = 1 − (T_c/T_h). As T_c decreases while T_h remains constant, the ratio T_c/T_h decreases, so η increases.
