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 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.622Easy
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.623Medium
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.624Easy
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.625Hard
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.626Medium
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.627Hard
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.628Medium
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.629Hard
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.630Hard
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.631Medium
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.632Easy
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.633Hard
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.
Q.634Medium
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.635Easy
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.636Easy
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.637Easy
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.638Medium
An electron transitions from n=3 to n=1 in hydrogen atom. The frequency of emitted photon is (R = 1.097×10⁷ m⁻¹):
