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.
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.62Medium
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)
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.67Medium
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.69Medium
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.70Medium
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.
Q.72Medium
Which process results in zero change of entropy for an ideal gas?
Answer: B
For a reversible adiabatic process, dQ = 0, so dS = dQ/T = 0, hence ΔS = 0. This is also called isentropic process.
Q.73Medium
In a throttling process (Joule-Thomson expansion), which of the following remains constant?
Answer: D
Throttling is an irreversible, adiabatic process where enthalpy remains constant (H_initial = H_final). Temperature and pressure both change, and internal energy remains nearly constant only for ideal gases.
Q.74Medium
Five moles of an ideal monatomic gas are heated at constant volume from 300 K to 600 K. The change in internal energy is:
Two kilograms of water at 100°C is converted to steam at 100°C at 1 atm pressure. The change in entropy is (Latent heat of vaporization = 2.26 × 10⁶ J/kg):
A real gas deviates from ideal behavior. Which condition favors ideal behavior?
Answer: B
At low pressure, molecules are far apart (negligible intermolecular forces), and at high temperature, kinetic energy dominates, making gases behave ideally
Q.79Medium
An open system differs from a closed system in that:
Answer: A
An open system allows both mass and energy exchange with surroundings (e.g., a boiling kettle), while a closed system allows only energy exchange
Q.80Medium
The heat capacity at constant pressure Cp is greater than heat capacity at constant volume Cv for ideal gases because:
Answer: A
At constant P: Q = ΔU + PΔV, so more heat is needed to produce same temperature rise. Relation: Cp - Cv = R
