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.
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.642Hard
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.643Medium
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.646Medium
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.647Medium
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.648Easy
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.649Easy
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.650Medium
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.651Medium
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.652Medium
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.653Easy
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).
Q.654Easy
A ²¹⁰₈₄Po nucleus undergoes alpha decay. The resulting nucleus is:
Answer: A
Alpha decay: A decreases by 4, Z decreases by 2. ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He. The resulting nucleus is Lead-206.
Q.655Medium
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.656Easy
X-rays are produced when high-speed electrons strike a metal target. This is an example of:
Answer: B
Bremsstrahlung (braking radiation) occurs when fast electrons are decelerated by the nucleus, producing continuous X-ray spectrum.
Q.657Medium
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.658Medium
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.659Medium
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.660Medium
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:
