CSS 2025 Solved Past Paper – Physics II MCQs of Part-I (Compulsory)
Below are the solved multiple-choice questions (MCQs) of Part-I from the CSS 2025 Physics II paper. Each question includes all the options, the correct answer is bolded, and a short explanation is provided for clarity.
1. A charge moving in a uniform static magnetic field experiences:
(A) A force perpendicular to magnetic field and velocity
(B) Increase in momentum
(C) Decrease in momentum
(D) Decrease in velocity
💡 In a static magnetic field, the Lorentz force acts perpendicular to both velocity and field, changing the direction but not the magnitude of momentum.
2. The same energy alpha particles and electrons associated De Broglie wavelengths are:
(A) Equal
(B) Longer than electrons
(C) Smaller electrons
(D) None of these
💡 For the same kinetic energy, heavier particles (alpha) have shorter wavelengths than lighter particles (electrons).
3. The mass deficit defines:
(A) Nuclear binding energy
(B) Nuclear Quadrupole moment
(C) Shape of the nucleus
(D) None of these
💡 Mass defect is the difference between the sum of nucleon masses and the actual nuclear mass; it corresponds to the nuclear binding energy via E = mc2.
4. In nuclear physics, the magic number defines:
(A) Nuclear binding energy
(B) Relatively exceptionally stable nuclei
(C) Radioactivity
(D) None of these
💡 Magic numbers correspond to filled nuclear shells, leading to unusually stable nuclei.
5. The Zeeman effect is due to the interaction of:
(A) External magnetic field and total magnetic field of the specimen
(B) External electric field and magnetic field of specimen
(C) L, S coupling
(D) None of these
💡 Zeeman splitting occurs when an external magnetic field interacts with the magnetic moments of electrons in atoms.
6. A photon having the same energy as an electron has wavelength _____ as electron’s De Broglie wavelength.
(A) Shorter
(B) Longer
(C) Same
(D) None of these
💡 For the same energy, photon momentum is higher than that of an electron, resulting in a shorter wavelength.
7. In Lorentz transformation motion along _____ axis remains zero.
(A) One axis
(B) Two axis
(C) Three axis
(D) None of these
💡 Lorentz transformations are applied along the axis of relative motion; perpendicular components remain unchanged (zero if initially zero).
8. Gauss’s law is applicable on the charges which are:
(A) Closely distributed in space
(B) Sparsely distributed in space
(C) Enclosed by a broken surface
(D) Enclosed by a close surface
💡 Gauss’s law applies to any closed surface enclosing a net charge.
9. The ratio between the angular momentum and orbital magnetic moment vector of the first orbital electron of Hydrogen is known as:
(A) Total Magnetic Moment
(B) Bohr Magneton
(C) Magnetic Moment
(D) None of these
💡 The Bohr magneton is the natural unit for expressing the magnetic moment of an electron due to its orbital motion.
10. The cyclotron is a device which primarily used to:
(A) Accelerate sub-atomic particles
(B) To generate electrons
(C) To generate neutrons
(D) To generate protons
💡 A cyclotron accelerates charged particles to high speeds using a perpendicular magnetic field and alternating electric field.
11. The rate of change of current in a coil is proportional to:
(A) V
(B) – emf
(C) R
(D) None of these
💡 By Faraday’s law for self-inductance, ε = -L di/dt.
12. The charging and discharging behavior of an ideal RLC circuit is:
(A) Simple harmonic
(B) Linear
(C) Elliptical
(D) Unpredictable
💡 An ideal RLC circuit exhibits simple harmonic oscillations when not driven, due to exchange between electric and magnetic energy.
13. The Lorentz force is given by:
(A) F = q (v × B)
(B) Φ= ∂A/∂x + ∂A/∂y + ∂A/∂z
(C) qF = q(vd × B)
(D) None of these
💡 The magnetic part of the Lorentz force is q (v × B), acting perpendicular to both velocity and magnetic field.
14. The Poynting vector represents:
(A) The rate and direction of electromagnetic energy transferred in space.
(B) The amount of electromagnetic energy transferred in different forms of energy.
(C) Loss of electromagnetic energy in a medium.
(D) The flow of thermal energy in a certain direction.
💡 The Poynting vector S = E × H gives the power flow per unit area and its direction.
15. The De Broglie wave-particle duality is said to be foundation of uncertainty principle because:
(A) The velocities of the particles were very high.
(B) The mass could not attain the velocity of light.
(C) The mass increases at velocities comparable to the velocity of light increases.
(D) The uncertainty in position of the moving mass.
💡 Wave-particle duality implies that exact position and momentum cannot be known simultaneously — the essence of the uncertainty principle.
16. The relation between half-life and mean life are:
(A) Inversely proportional
(B) Directly proportional
(C) Equal
(D) Half
💡 Mean life τ is related to half-life T1/2 by τ = T1/2 / ln2, showing direct proportionality.
17. The concept of mutual frame of reference was used in:
(A) Reduced mass correction in spectroscopy
(B) Radioactivity
(C) Relative nuclear stability
(D) None of these
💡 In spectroscopy, reduced mass accounts for the mutual motion of two bodies around their center of mass.
18. The Ultraviolet catastrophe means:
(A) The spectral energy concentration towards longer wave length.
(B) The spectral energy concentration at the middle of the spectrum.
(C) The spectral energy concentration towards shorter wave side.
(D) None of these
💡 Classical physics predicted infinite energy at short wavelengths for black-body radiation — the ultraviolet catastrophe.
19. As per quantum physics, the observables ‘E’ and _________ cannot be simultaneously measured with 100% accuracy.
(A) px
(B) t
(C) x
(D) None of these
💡 Energy and time are a pair of conjugate variables subject to an uncertainty relation: ΔE Δt ≳ ℏ/2.
20. The theory of relativity predicts that at velocities comparable to the velocity of light, the moving mass appears to:
(A) Increase
(B) Decrease
(C) Remains the same
(D) None of these
💡 Relativity shows that relativistic mass increases with velocity as m = γm0, where γ grows with v.
