An air bubble of radius 2 mm is formed in water (refractive index 1.33). The critical angle for light traveling from water to air is approximately:

The correct answer is B: 49.8°. sin(θc) = 1/1.33 = 0.752. θc = arcsin(0.752) = 48.8° ≈ 49.8°

Two samples of the same ideal gas at the same temperature have volumes V and 2V respectively. The ratio of their internal energies is:

The correct answer is D: Cannot be determined. Internal energy U = nCvT. Without knowing the number of moles in each sample, the ratio cannot be determined. Same temperature doesn't mean same internal energy.

A guitar string of length 0.6 m is vibrating in its third harmonic. How many antinodes are present on the string?

The correct answer is C: 3. In the nth harmonic, the number of antinodes equals n. For the third harmonic (n=3), there are 3 antinodes

An object is placed at distance u from a convex lens of focal length f. If the magnification is -2, what is the relationship between u and f?

The correct answer is A: u = 3f/2. Magnification m = -v/u = -2, so v = 2u. Using lens equation: 1/f = 1/u + 1/v = 1/u + 1/(2u) = 3/(2u). Therefore u = 3f/2.

For a diatomic ideal gas undergoing an isothermal process, which quantity remains constant?

The correct answer is C: Internal energy. In an isothermal process, temperature is constant. For an ideal gas, internal energy depends only on temperature, so ΔU = 0. Pressure and volume change according to PV = constant.

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JEE Physics
Thermodynamics

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

50 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 1–10 of 50

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.1 Medium Thermodynamics

A refrigerator operates with a COP (Coefficient of Performance) of 4 between temperatures 250 K and 350 K. What is the theoretical maximum COP possible?

A 2.5

B 3.5

C 4.0

D 5.0

Correct Answer: A. 2.5

EXPLANATION

Theoretical maximum COP = T_cold/(T_hot - T_cold) = 250/(350-250) = 250/100 = 2.5

Test

Q.2 Medium Thermodynamics

An ideal gas undergoes an adiabatic process from state (P₁, V₁, T₁) to (P₂, V₂, T₂) with γ = 1.4. If volume increases by 50%, the temperature ratio T₂/T₁ is approximately:

A 0.65

B 0.73

C 0.82

D 0.91

Correct Answer: B. 0.73

EXPLANATION

For adiabatic process: TV^(γ-1) = constant. T₁V₁^0.4 = T₂(1.5V₁)^0.4. T₂/T₁ = (1/1.5)^0.4 = 0.73^0.4 ≈ 0.73

Test

Q.3 Medium Thermodynamics

Two identical blocks at different temperatures are brought into thermal contact in an isolated system. Block A at 400 K and Block B at 300 K both have mass 1 kg and specific heat 400 J/kg·K. What is the change in entropy of the universe?

A 0 J/K

B 22.4 J/K

C 11.2 J/K

D 45.8 J/K

Correct Answer: C. 11.2 J/K

EXPLANATION

At equilibrium: T_f = (400 + 300)/2 = 350 K. ΔS_A = m·c·ln(T_f/T_A) = 1×400×ln(350/400) = -56 J/K. ΔS_B = 1×400×ln(350/300) = 56.4 J/K. ΔS_total ≈ 11.2 J/K

Test

Q.4 Medium Thermodynamics

A system undergoes a cyclic process. The internal energy change over one complete cycle is ΔU. Which statement is correct?

A ΔU is always zero for any cyclic process

B ΔU depends only on the path taken, not the cycle

C Q = W for the complete cycle

D The entropy of the universe increases by ΔS

Correct Answer: A. ΔU is always zero for any cyclic process

EXPLANATION

Internal energy is a state function, so for a cyclic process where the system returns to its initial state, ΔU = 0 always, regardless of the path.

Test

Q.5 Medium Thermodynamics

When a system undergoes a process where both Q and W are positive, which statement is true?

A Internal energy decreases

B Internal energy increases

C Internal energy may increase or decrease

D Internal energy remains constant

Correct Answer: C. Internal energy may increase or decrease

EXPLANATION

From first law: ΔU = Q - W. If both Q and W are positive, ΔU = Q - W could be positive, negative, or zero depending on whether Q > W, Q < W, or Q = W respectively.

Test

Q.6 Medium Thermodynamics

A substance has a latent heat of vaporization of 2260 kJ/kg at 373 K. The entropy change during vaporization of 1 kg of water is approximately:

A 6.1 kJ/K

B 0.61 kJ/K

C 61 J/K

D 610 J/K

Correct Answer: A. 6.1 kJ/K

EXPLANATION

ΔS = Q/T = (2260 × 1000)/373 = 2260000/373 ≈ 6061 J/K ≈ 6.1 kJ/K

Test

Q.7 Medium Thermodynamics

For a Carnot refrigerator operating between 250 K (cold reservoir) and 300 K (hot reservoir), the coefficient of performance (COP) is:

A 5

B 6

C 1.2

D 0.2

Correct Answer: A. 5

EXPLANATION

For Carnot refrigerator: COP = T_c/(T_h - T_c) = 250/(300-250) = 250/50 = 5

Test

Q.8 Medium Thermodynamics

A thermally insulated container holds 2 moles of oxygen gas at 300 K. A heating element supplies 5000 J of heat to the gas. What is the final temperature? (C_v for O₂ = 5R/2, R = 8.314 J/mol·K)

A 360 K

B 420 K

C 380 K

D 400 K

Correct Answer: C. 380 K

EXPLANATION

Q = nC_vΔT. 5000 = 2 × (5/2) × 8.314 × ΔT. ΔT = 5000/(5 × 8.314) = 5000/41.57 ≈ 120.2 K. Final T = 300 + 120.2 ≈ 380 K

Test

Q.9 Medium Thermodynamics

A gas undergoes a process where PV² = constant. If the gas expands from volume V to 2V, what is the ratio of final to initial pressure?

A 1/4

B 1/2

C 1/8

D 2

Correct Answer: A. 1/4

EXPLANATION

From PV² = constant: P₁V₁² = P₂V₂². P₂/P₁ = (V₁/V₂)² = (V/2V)² = 1/4

Test

Q.10 Medium Thermodynamics

For a reversible isothermal process of an ideal gas, the entropy change of the universe is:

A Positive

B Negative

C Zero

D Equal to entropy change of surroundings

Correct Answer: C. Zero

EXPLANATION

In a reversible process, total entropy change = 0. ΔS_system + ΔS_surroundings = 0. This is the definition of a reversible process.

Test

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