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 nucleus ⁹⁴₃₈Sr undergoes beta-minus decay followed by another beta-minus decay. The final nucleus is:
Answer: A
First β⁻ decay: ⁹⁴₃₈Sr → ⁹⁴₃₉Y + e⁻ + ν̄. Second β⁻ decay: ⁹⁴₃₉Y → ⁹⁴₄₀Zr + e⁻ + ν̄. Final nucleus is ⁹⁴₄₀Zr.
Q.304Medium
A photon of frequency 6 × 10¹⁵ Hz is incident on a metal surface with work function 2 eV. What is the maximum kinetic energy of the ejected photoelectron? (h = 4.14 × 10⁻¹⁵ eV·s)
Answer: B
Energy of photon E = hf = 4.14 × 10⁻¹⁵ × 6 × 10¹⁵ = 24.84 eV. Maximum KE = E - W = 24.84 - 2 = 22.84 eV (approximately 21.84 eV with standard constants).
Q.305Medium
An electron in the first excited state of hydrogen atom (n=2) transitions to ground state (n=1). The wavelength of emitted photon is approximately:
Answer: A
Using Rydberg formula: 1/λ = R(11² - 21²) = R(43). With R = 1.097 × 10⁷ m⁻¹, λ ≈ 121.6 nm (Lyman alpha line).
Q.306Medium
The de Broglie wavelength of an electron with kinetic energy 50 eV is:
Answer: A
λ = h/p = h/√(2mE). For 50 eV electron: λ = 6.63×10⁻³⁴/√(2×9.1×10⁻³¹×50×1.6×10⁻¹⁹) ≈ 0.173 nm.
Q.307Medium
In Compton scattering, a photon collides with a free electron at rest. Which quantity always increases?
Answer: B
In Compton effect, photon transfers energy to electron, losing energy and increasing wavelength. Δλ = (h/m_e c)(1 - cosθ).
Q.308Medium
The activity of a radioactive sample decreases from 8000 Bq to 1000 Bq in 20 hours. The half-life of the sample is:
Answer: A
A = A₀(21)^(t/T₁/₂). 1000 = 8000(21)^(20/T₁/₂). (81) = (21)³, so 20/T₁/₂ = 3, T₁/₂ = 6.67 hours.
Q.309Medium
In a nuclear reactor, control rods are used to absorb neutrons. Which isotope is commonly used in control rods?
Answer: A
Boron-10 and Cadmium have high neutron absorption cross-sections and are used in control rods to regulate chain reactions in nuclear reactors.
Q.310Medium
A radioactive nucleus ₆₀₂₇Co undergoes beta-plus decay. The daughter nucleus is:
Answer: A
In beta-plus decay, Z decreases by 1, A remains constant. ⁶⁰₂₇Co → ⁶⁰₂₆Ni + e⁺ + νₑ.
Q.311Medium
The threshold frequency for photoelectric effect in a metal is 6 × 10¹⁴ Hz. The work function of the metal is:
Answer: A
Work function W = hf₀ = 6.63 × 10⁻³⁴ × 6 × 10¹⁴ = 3.98 × 10⁻¹⁹ J ≈ 2.48 eV.
Q.312Medium
According to the uncertainty principle, if the position of an electron is known with uncertainty Δx = 10⁻¹⁰ m, the minimum uncertainty in momentum is:
The intensity of characteristic X-rays depends on:
Answer: C
Characteristic X-ray intensity depends on the number of inner-shell electrons available (atomic number) and the number of incident electrons (beam current).
Q.314Medium
A photon of wavelength 100 pm strikes a stationary free electron. After Compton scattering at an angle of 60°, the wavelength of the scattered photon is found to be 102.4 pm. What is the kinetic energy of the recoil electron? (Given: h = 6.63 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s, mₑ = 9.1 × 10⁻³¹ kg)
Answer: A
Using Compton scattering formula: λ' - λ = (h/mₑc)(1 - cos θ). With given values, the wavelength shift is 2.4 pm. Using energy conservation, the incident photon energy is E₀ = hc/λ ≈ 12.4 keV. The scattered photon energy E' = hc/λ' ≈ 12.1 keV. The kinetic energy of electron = E₀ - E' ≈ 1.65 keV.
Q.315Medium
In an n-type semiconductor, the Fermi level lies:
Answer: B
In n-type semiconductors, donor levels introduce electrons near the conduction band, shifting the Fermi level upward, making it closer to the conduction band than to the valence band.
Q.316Medium
The conductivity of a semiconductor increases with temperature because:
Answer: D
While mobility decreases with temperature due to increased phonon scattering, the exponential increase in intrinsic carrier concentration dominates, resulting in net increase in conductivity. However, (b) is the primary reason.
Q.317Medium
The intrinsic carrier concentration ni in a semiconductor is given by: ni = √(NcNv)exp(-Eg/2kT). What does Nc represent?
Answer: B
Nc is the effective density of states in the conduction band, which depends on the effective mass of electrons and temperature. Similarly, Nv is for the valence band.
Q.318Medium
In forward biasing of a p-n junction, the depletion region width:
Answer: B
Forward bias reduces the potential barrier at the junction, allowing carriers to move across more easily. This reduces the depletion region width, which is inversely related to the applied voltage.
Q.319Medium
The reverse saturation current in a p-n junction diode depends on:
Answer: B
Reverse saturation current I0 is determined by intrinsic carrier concentration (ni), diffusion coefficient (D), and junction properties. It is independent of reverse voltage but depends strongly on temperature.
Q.320Medium
Zener breakdown in a semiconductor occurs when:
Answer: B
Zener breakdown involves quantum mechanical tunneling of electrons directly from valence band to conduction band under strong reverse electric field. It occurs in heavily doped junctions at lower voltages than avalanche breakdown.
