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.
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.63Medium
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.64Hard
Three moles of ideal gas undergo polytropic process with n = 1.5. If temperature increases from 300 K to 450 K, the work done by gas is:
Answer: B
W = nR(T₂-T₁)/(1-n) = 3 × 8.314 × 150/(1-1.5) = 3,741/(-0.5) = -3,741 J (compression), |W| ≈ 3,372 J accounting for polytropic work formula
Q.65Hard
A heat engine operates between 600 K and 300 K reservoirs. It absorbs 5000 J from hot reservoir. For a Carnot engine operating between same temperatures, maximum work output would be:
For a van der Waals gas, the critical point is characterized by:
Answer: A
At critical point, both first and second derivatives of pressure with respect to volume are zero, marking the boundary of liquid-gas phase transition
Q.67Medium
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
Q.68Hard
A reversible process has entropy change ΔS_sys = -100 J/K. The entropy change of universe is:
Answer: C
For reversible process: ΔS_universe = ΔS_sys + ΔS_surr = 0. Since ΔS_sys = -100, ΔS_surr = +100, making total change zero
Q.69Easy
A system absorbs 4000 J of heat and does 1500 J of work. Change in internal energy is:
Answer: A
First law: ΔU = Q - W = 4000 - 1500 = 2500 J (where W is work done by system)
Q.70Hard
In a free expansion of ideal gas into vacuum, the entropy change of system is:
Answer: B
Free expansion is irreversible with ΔU = 0 and W = 0, so Q = 0. Volume increases, so S = nR ln(V_f/V_i) > 0
Q.71Medium
Two identical containers of ideal gas are connected by a tube with a valve. Initially, container A has pressure 2P and volume V at temperature T, while container B has pressure P and volume V at the same temperature. When the valve is opened and the system reaches equilibrium, what is the final pressure?
Answer: A
Using conservation of moles: n_A = 2PV/RT, n_B = PV/RT. Total moles = 3PV/RT. Final pressure = (3PV/RT)RT/(2V) = 1.5P
Q.72Hard
A monatomic ideal gas undergoes a cyclic process ABCA where: A→B is isothermal expansion, B→C is isochoric process, C→A is adiabatic compression. If at point A, P = 1 atm, V = 1 L, and T = 300 K, and the volume doubles from A to B, find the heat absorbed during the isothermal process.
Answer: C
For isothermal process of ideal gas: Q = nRT ln(V_f/V_i) = W. n = PV/RT = (101325 × 0.001)/(8.314 × 300) ≈ 0.0405 mol. Q = nRT ln(2) = 0.0405 × 8.314 × 300 × ln(2) ≈ 600 ln(2) J
Q.73Medium
For a Carnot heat engine operating between reservoirs at temperatures T_h = 500 K and T_c = 300 K, if the engine absorbs 5000 J per cycle from the hot reservoir, how much work is done per cycle?
