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.
In the Bohr model, which of the following represents the radius of the nth orbit in a hydrogen atom?
Answer: A
The radius of nth orbit in hydrogen atom is given by rₙ = n²a₀/Z, where a₀ = 0.53 Å is the Bohr radius and Z is atomic number.
Q.42Medium
A hydrogen atom transitions from n=4 to n=2 state. The wavelength of emitted photon is approximately:
Answer: A
Using Rydberg formula: 1/λ = R(41 - 161) = 3R/16. With R = 1.097×10⁷ m⁻¹, we get λ ≈ 486 nm (H-beta line).
Q.43Medium
The binding energy per nucleon for ⁵⁶Fe is maximum among all nuclei. This suggests that:
Answer: D
Maximum binding energy per nucleon means Fe-56 has highest stability and also highest mass defect. This is the peak of the nuclear stability curve.
Q.44Easy
In beta decay, a neutron converts to a proton. Which particle is emitted?
Answer: B
In β⁻ decay, a neutron converts to proton, emitting electron and antineutrino: n → p + e⁻ + ν̄ₑ
Q.45Medium
A radioactive element has 75% of its original mass remaining after 6 hours. What is its half-life?
Answer: C
Using N = N₀(21)^(t/T₁/₂): 0.75N₀ = N₀(21)^(6/T₁/₂). Taking log: ln(0.75) = (6/T₁/₂)ln(0.5), giving T₁/₂ = 6√3 hours ≈ 10.39 hours.
Q.46Easy
Which of the following statements about alpha particles is correct?
Answer: B
Alpha particles (⁴₂He) are helium nuclei containing 2 protons and 2 neutrons. They have low penetrating power but high ionizing power.
Q.47Medium
The de Broglie wavelength of an electron accelerated through a potential difference of 100 V is approximately:
Answer: A
λ = h/√(2meV). For V = 100V: λ = 12.27/√(2×9.1×10⁻³¹×1.6×10⁻¹⁹×100) ≈ 1.23 Å = 0.123 nm.
Q.48Easy
In photoelectric effect, the stopping potential is 2V. The maximum kinetic energy of ejected electron is:
Answer: A
The stopping potential is related to maximum kinetic energy by: KE_max = eV_s. For V_s = 2V, KE_max = 2 eV.
Q.49Easy
A nucleus undergoes alpha decay. The mass number and atomic number of the daughter nucleus are:
Answer: A
In alpha decay, a nucleus emits ⁴₂He. Parent nucleus (A,Z) → Daughter (A-4, Z-2) + Alpha particle (4,2).
Q.50Medium
In a hydrogen atom, the ground state energy is -13.6 eV. What is the energy required to ionize from n=3?
Answer: A
Energy at n=3: E₃ = -13.96 = -1.51 eV. Ionization energy = 0 - (-1.51) = 1.51 eV.
Q.51Easy
The activity of a radioactive sample is defined as:
Answer: B
Activity A = dN/dt = λN, measured in Becquerels (decays per second) or Curies. It represents the decay rate.
Q.52Hard
An electron in excited state of hydrogen atom has angular momentum 2ℏ. The principal quantum number n is:
Answer: C
Angular momentum L = ℏ√(l(l+1)) = 2ℏ gives l(l+1) = 4, so l = 2. Since l < n, minimum n = 3.
Q.53Medium
When a moving electron is stopped completely in matter, the kinetic energy is converted to:
Answer: C
When high-speed electrons are stopped in matter, their kinetic energy produces both continuous X-ray spectrum (Bremsstrahlung radiation) and heat.
Q.54Hard
The ratio of de Broglie wavelengths of an electron and a proton having same kinetic energy is:
Answer: A
For same KE: λ = h/√(2mKE). λₑ/λₚ = √(mₚ/mₑ) ≈ 42.8 (using mₑ = 9.1×10⁻³¹ kg, mₚ = 1.67×10⁻²⁷ kg).
Q.55Medium
In X-ray production, the minimum wavelength of continuous X-ray spectrum depends on:
Answer: B
Minimum wavelength λ_min = hc/eV, depends only on accelerating voltage V. Higher voltage produces shorter wavelength X-rays.
Q.56Hard
A nucleus ²³⁸₉₂U undergoes two alpha decays and two beta decays. The final nucleus is:
Answer: C
After 2α decays: mass number decreases by 8, atomic number by 4: ²³⁸₉₂U → ²³⁰₈₈Ra. After 2β⁻ decays: atomic number increases by 2: ²³⁰₉₀Th. But rechecking: ²³⁰₈₈Ra is correct intermediate form.
Q.57Hard
The energy released in nuclear fusion of two deuterium nuclei to form ⁴He is approximately:
Answer: A
²₁H + ²₁H → ⁴₂He + energy. Using mass defect and E=mc²: Energy released ≈ 23.8 MeV. This is the basis of thermonuclear fusion.
Q.58Medium
In Compton effect, the wavelength shift depends on:
Answer: D
Compton shift: Δλ = (h/mₑc)(1 - cosθ). It depends on scattering angle θ and electron mass mₑ, not on incident photon energy.
Q.59Easy
Which of the following statements about photoelectric effect is correct?
Answer: C
From Einstein's photoelectric equation: KE_max = hf - φ. As frequency increases, maximum kinetic energy increases proportionally. Stopping potential depends on frequency, KE depends on frequency (not intensity), and threshold frequency is directly related to work function.
Q.60Hard
A free electron at rest absorbs a photon and immediately emits another photon. This process is not possible because:
Answer: B
For a free electron at rest, if it absorbs a photon with energy E and momentum p=E/c, it cannot emit a photon in any direction while conserving both energy and momentum. This is because the electron would need to have kinetic energy, but no emission direction satisfies both conservation laws simultaneously.
