In a p-n junction diode at thermal equilibrium, the direction of the electric field in the depletion region is:

The correct answer is B: From p-region to n-region. At thermal equilibrium, the built-in electric field points from the p-region (positive) to n-region (negative) to oppose further diffusion of carriers.

When a semiconductor is exposed to light with photon energy greater than band gap energy, the phenomenon is:

The correct answer is B: Photoconductivity. Photoconductivity is the increase in conductivity when photons with E > Eg create electron-hole pairs, increasing charge carrier concentration.

Two parallel wires carrying currents I₁ and I₂ in the same direction, separated by distance d. The force per unit length between them is:

The correct answer is C: F/L = μ₀I₁I₂/(2πd). Magnetic force per unit length between parallel wires: F/L = μ₀I₁I₂/(2πd), attractive for same direction currents

The magnetic susceptibility of a paramagnetic material is:

The correct answer is B: Positive and small. Paramagnetic materials have positive but small magnetic susceptibility (χ > 0, typically 10⁻⁵ to 10⁻³). Diamagnetic materials have small negative susceptibility, and ferromagnetic materials have large positive susceptibility.

The specific contact resistance of a metal-semiconductor junction is proportional to:

The correct answer is B: exp(-φB/kT). Contact resistance ρc ∝ exp(φB/kT)/Nc where φB is barrier height. Higher barrier leads to exponentially higher resistance following Thermionic emission theory.

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JEE Physics
Thermodynamics

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

27 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 1–10 of 27

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 Easy Thermodynamics

A gas expands isothermally from volume V₁ to 3V₁ at temperature T. If the initial pressure is P₁, what is the work done by the gas?

A P₁V₁ ln(3)

B 3P₁V₁

C P₁V₁/3

D P₁V₁

Correct Answer: A. P₁V₁ ln(3)

EXPLANATION

For isothermal process: W = nRT ln(V_f/V_i) = P₁V₁ ln(3V₁/V₁) = P₁V₁ ln(3)

Test

Q.2 Easy Thermodynamics

Three moles of an ideal diatomic gas are heated at constant pressure from 300 K to 600 K. Calculate the heat absorbed by the gas. (R = 8.314 J/mol·K)

A 74,826 J

B 49,884 J

C 24,942 J

D 99,768 J

Correct Answer: A. 74,826 J

EXPLANATION

For constant pressure: Q = n·Cp·ΔT. For diatomic gas, Cp = (7/2)R. Q = 3 × (7/2) × 8.314 × (600-300) = 3 × 3.5 × 8.314 × 300 = 26,194 × 2.85 ≈ 74,826 J

Test

Q.3 Easy Thermodynamics

A heat engine operates between two reservoirs at temperatures 500 K and 300 K. What is the maximum theoretical efficiency of this engine?

A 40%

B 60%

C 75%

D 85%

Correct Answer: A. 40%

EXPLANATION

Maximum efficiency (Carnot efficiency) = 1 - (T_cold/T_hot) = 1 - (300/500) = 1 - 0.6 = 0.4 = 40%

Test

Q.4 Easy Thermodynamics

A system absorbs 8000 J of heat and its internal energy increases by 5000 J. According to the first law of thermodynamics, what work is done by the system?

A 3000 J

B 13000 J

C 5000 J

D -3000 J

Correct Answer: A. 3000 J

EXPLANATION

First law: ΔU = Q - W, so W = Q - ΔU = 8000 - 5000 = 3000 J (work done by the system)

Test

Q.5 Easy Thermodynamics

A thermodynamic system does 600 J of work while 2400 J of heat is supplied to it. What is the change in internal energy?

A 1800 J

B 2400 J

C 3000 J

D 600 J

Correct Answer: A. 1800 J

EXPLANATION

By first law of thermodynamics: ΔU = Q - W = 2400 - 600 = 1800 J (where W is work done by system)

Test

Q.6 Easy Thermodynamics

The internal energy of an ideal gas depends only on:

A Temperature and pressure

B Temperature and volume

C Temperature only

D Pressure and volume

Correct Answer: C. Temperature only

EXPLANATION

For an ideal gas, internal energy U depends only on temperature (U = nC_vT). This is independent of pressure or volume, as shown by kinetic theory.

Test

Q.7 Easy Thermodynamics

A gas expands adiabatically from state (P₁, V₁, T₁) to state (P₂, V₂, T₂). Which relation is correct for this process?

A P₁V₁^γ = P₂V₂^γ

B T₁V₁^(γ-1) = T₂V₂^(γ-1)

C P₁/T₁ = P₂/T₂

D All of the above

Correct Answer: D. All of the above

EXPLANATION

For adiabatic process: PV^γ = constant, TV^(γ-1) = constant, and T^γ P^(1-γ) = constant. All three relations are equivalent and correct.

Test

Q.8 Easy Thermodynamics

A system absorbs 4000 J of heat and does 1500 J of work. Change in internal energy is:

A 2500 J

B 5500 J

C 1500 J

D -2500 J

Correct Answer: A. 2500 J

EXPLANATION

First law: ΔU = Q - W = 4000 - 1500 = 2500 J (where W is work done by system)

Test

Q.9 Easy Thermodynamics

A Carnot engine operates between 400 K and 300 K. Its maximum efficiency is:

A 25%

B 33.33%

C 50%

D 75%

Correct Answer: A. 25%

EXPLANATION

Carnot efficiency η = 1 - T_cold/T_hot = 1 - 300/400 = 0.25 = 25%

Test

Q.10 Easy Thermodynamics

The internal energy of an ideal gas depends on:

A Both temperature and volume

B Both temperature and pressure

C Only temperature

D Only pressure

Correct Answer: C. Only temperature

EXPLANATION

Internal energy U of ideal gas is U = nCᵥT, which depends only on temperature, not on pressure or volume individually

Test

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