15 extra multiple-choice questions for Atomic and Nuclear physics (12th Standard Physics, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
In J.J. Thomson's experiment to determine the specific charge of an electron, what is the configuration of the applied fields?
- A. Parallel electric and magnetic fields
- B. Only a high-voltage electric field
- C. Only a uniform magnetic field
- D. Perpendicular electric and magnetic fieldsCorrect
Explanation. Thomson's experiment determines specific charge by balancing the electric and magnetic forces. This requires fields to be perpendicular to each other and the electron's velocity for straight-line motion during velocity selection.
Q2
In Millikan's oil drop experiment, the electric charge on any drop was found to be an integral multiple of which fundamental value?
- A. \(1.6 \times 10^{-19}\) CCorrect
- B. \(3.2 \times 10^{-19}\) C
- C. \(9.1 \times 10^{-31}\) C
- D. \(1.7 \times 10^{11}\) C
Explanation. Millikan calculated the charges on numerous oil droplets and observed they were all integer multiples of a basic unit. This constant value is identified as the magnitude of the electronic charge.
Q3
According to Rutherford's scattering experiment, what is the relationship between the impact parameter and the scattering angle?
- A. As impact parameter increases, scattering angle increases
- B. Scattering angle is independent of the impact parameter
- C. As impact parameter increases, scattering angle decreasesCorrect
- D. Scattering angle first increases then decreases
Explanation. The impact parameter is the perpendicular distance between the nucleus and the particle's path. A larger distance results in a weaker interaction with the nucleus, causing a smaller deflection or scattering angle.
Q4
In the Bohr atom model, the angular momentum of an electron revolving in a stationary orbit is quantized as an integral multiple of:
- A. \(h\)
- B. \(h/\pi\)
- C. \(h/2\pi\)Correct
- D. \(2\pi/h\)
Explanation. Bohr's second postulate states that stable orbits only exist where angular momentum is quantized. This quantization requires the momentum to be an integer multiple of the reduced Planck constant.
Q5
How does the radius \(r_n\) of the \(n^{th}\) orbit of a hydrogen atom vary with the principal quantum number \(n\)?
- A. \(r_n \propto n\)
- B. \(r_n \propto n^2\)Correct
- C. \(r_n \propto 1/n\)
- D. \(r_n \propto \sqrt{n}\)
Explanation. Bohr's theory derives that the orbital radius of an electron is proportional to the square of the principal quantum number. Higher energy states correspond to orbits significantly further from the nucleus.
Q6
Which spectral series of the hydrogen atom lies in the visible region of the electromagnetic spectrum?
- A. Lyman series
- B. Balmer seriesCorrect
- C. Paschen series
- D. Pfund series
Explanation. The Balmer series consists of transitions where electrons drop to the second energy level. The resulting photon emissions have wavelengths that fall within the range visible to the human eye.
Q7
In the standard nuclear notation \(_Z^A X\), what does the value of \(A - Z\) represent?
- A. The number of protons
- B. The number of electrons
- C. The number of neutronsCorrect
- D. The total number of nucleons
Explanation. In nuclear notation, \(A\) is the mass number (total nucleons) and \(Z\) is the atomic number (total protons). Subtracting the protons from the total gives the count of neutrons in the nucleus.
Q8
Atoms of different chemical elements that possess the same number of neutrons are known as:
- A. Isotopes
- B. Isobars
- C. IsotonesCorrect
- D. Isomers
Explanation. Isotones are nuclei that have an identical neutron count despite having different atomic numbers. While they belong to different elements, their internal structures contain the same number of neutral nucleons.
Q9
What is a characteristic property of nuclear density for nuclei with mass number \(A > 10\)?
- A. It increases linearly with \(A\)
- B. It is inversely proportional to \(A\)
- C. It is approximately constant for all nucleiCorrect
- D. It depends on the number of orbiting electrons
Explanation. Since the nuclear radius scales with the cube root of the mass number, the volume is directly proportional to \(A\). Thus, mass per unit volume remains nearly the same for all nuclei.
Q10
Based on mass-energy equivalence, one atomic mass unit (u) is equivalent to approximately how much energy?
- A. 13.6 eV
- B. 931 MeVCorrect
- C. 200 MeV
- D. 1.02 MeV
Explanation. Applying Einstein's formula to the mass of one atomic unit yields an energy value of approximately 931 MeV. This conversion factor is fundamental for calculating binding energy from mass defects.
Q11
In a spontaneous alpha decay process, by how many units does the mass number of the parent nucleus decrease?
- A. 1
- B. 2
- C. 3
- D. 4Correct
Explanation. Alpha decay involves the emission of a helium nucleus, which contains two protons and two neutrons. The loss of these four nucleons results in the parent nucleus reducing its mass number by four.
Q12
Which particle was proposed by Pauli to explain the continuous energy range of particles observed in beta decay?
- A. Proton
- B. Positron
- C. NeutrinoCorrect
- D. Gamma ray
Explanation. Beta particles were observed to have varying energies, seemingly violating conservation laws. Pauli proposed a neutral, low-mass particle called the neutrino to carry away the missing energy and momentum in the decay.
Q13
What is the relationship between the decay constant \(\lambda\) and the half-life \(T_{1/2}\) of a radioactive sample?
- A. \(T_{1/2} = \lambda / 0.6931\)
- B. \(T_{1/2} = 0.6931 \times \lambda\)
- C. \(T_{1/2} = 0.6931 / \lambda\)Correct
- D. \(T_{1/2} = 1 / \lambda\)
Explanation. The half-life is defined as the time needed for half of a sample to disintegrate. It is inversely proportional to the decay constant, representing a slower decay process for smaller constant values.
Q14
In a nuclear reactor, what is the primary function of a moderator like heavy water?
- A. To absorb excess neutrons and stop the reaction
- B. To slow down fast neutrons to thermal energiesCorrect
- C. To increase the kinetic energy of emitted neutrons
- D. To shield the environment from radiation
Explanation. Fast neutrons produced in fission are unlikely to cause further fission in uranium-235. Moderators use collisions with light nuclei to reduce neutron speed, making them efficient 'thermal' neutrons for a sustained chain reaction.
Q15
Which fusion cycle is the most dominant energy source for the Sun according to Hans Bethe?
- A. Carbon-nitrogen cycle
- B. Proton-proton cycleCorrect
- C. Helium-helium cycle
- D. Deuterium-tritium cycle
Explanation. The Sun generates energy primarily through the proton-proton cycle. This series of thermonuclear reactions fuses hydrogen nuclei into helium, releasing enormous energy due to the mass difference between the starting and final particles.
