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.
The de Broglie wavelength of a neutron at room temperature (T = 300 K) is approximately:
Answer: A
Using λ = h/√(3mkT), where m = 1.67×10⁻²⁷ kg for neutron, h = 6.63×10⁻³⁴ J·s, k = 1.38×10⁻²³ J/K. λ ≈ 0.016 nm
Q.62Easy
In beta decay of ¹⁴₆C, the atomic number of daughter nucleus is:
Answer: B
In beta-minus decay, a neutron converts to a proton, emitting an electron and antineutrino. Atomic number increases by 1: Z = 6 + 1 = 7. The daughter nucleus is ¹⁴₇N.
Q.63Easy
The half-life of a radioactive substance is 10 days. What fraction of the original sample remains after 30 days?
Answer: C
After 30 days = 3 half-lives. Remaining fraction = (21)³ = 81
Q.64Easy
In Bohr's model, the angular momentum of electron in nth orbit is:
Answer: C
Bohr's quantization condition: L = mvr = n(h/2π) = nℏ, where n = 1, 2, 3...
Q.65Medium
An electron transitions from n=3 to n=1 in hydrogen atom. The frequency of emitted photon is (R = 1.097×10⁷ m⁻¹):
The nuclear radius of a nucleus with mass number A is given by R = R₀A^(31), where R₀ ≈ 1.2 fm. For ⁸⁸₃₈Sr, the nuclear radius is approximately:
Answer: B
R = 1.2 × (88)^(31) ≈ 1.2 × 3.65 ≈ 4.4 fm
Q.67Medium
The decay constant λ of a radioactive substance and its half-life t₁/₂ are related by:
Answer: B
From N = N₀e^(-λt), at t = t₁/₂, N = N₀/2, so 21 = e^(-λt₁/₂), giving λ = ln2/t₁/₂
Q.68Medium
An electron and a proton have the same kinetic energy. The ratio of their de Broglie wavelengths is:
Answer: A
λ = h/p. For same KE: p = √(2mKE). Therefore λ ∝ 1/√m. Ratio = λe/λp = √(mp/me)
Q.69Hard
Which decay process increases the neutron to proton ratio?
Answer: C
In β⁻ decay, a neutron converts to proton, but this occurs in daughter nucleus. Actually, β⁻ increases Z (protons) but keeps A constant, so N decreases relatively. In β⁺ decay, proton decreases. Answer reconsideration: Beta-minus decay converts n→p+e⁻+ν̄, effectively decreasing N and increasing Z. The question asks which increases N/Z ratio - that would be β⁻ decay when considering the overall effect on nucleus.
Q.70Medium
In nuclear fission of ²³⁵U, approximately 200 MeV energy is released. The mass defect in this reaction is approximately:
According to Heisenberg's uncertainty principle, if the uncertainty in position of an electron is 0.1 nm, the minimum uncertainty in velocity is approximately:
When fast electrons knock out inner shell (K-shell) electrons, electrons from outer shells fall down, emitting characteristic X-rays specific to the target material.
Q.73Medium
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.74Medium
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.75Easy
Which of the following statements about the photoelectric effect is correct?
Answer: B
Stopping potential Vs = (hf - W)/e, which depends directly on frequency. Intensity affects photocurrent, not maximum KE. Threshold frequency exists. Emission requires photon energy > work function.
Q.76Easy
In Bohr's model of hydrogen atom, the angular momentum of electron in the nth orbit is:
Answer: A
Bohr's quantization condition states that angular momentum L = mvr = nh/(2π), where n is the principal quantum number.
Q.77Medium
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.78Medium
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.79Medium
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.80Easy
If the binding energy per nucleon of ⁵⁶Fe is 8.79 MeV and that of ²³⁵U is 7.59 MeV, which nucleus is more stable?
Answer: A
Higher binding energy per nucleon indicates greater nuclear stability. ⁵⁶Fe (8.79 MeV/nucleon) is more stable than ²³⁵U (7.59 MeV/nucleon).
