Chapter 6 — Nuclear Physics: Multiple Choice Questions
Instructions: Each question carries 1 mark. Choose the best answer.
1. Radioactivity was discovered by ______.
Answer
(a) Marie Curie (b) Ernest Rutherford (c) Henri Becquerel (d) J.J. Thomson
✅ Answer: (c) Henri Becquerel
ExplanationHenri Becquerel discovered radioactivity in 1896 when he found that uranium compounds emitted radiation that could expose photographic plates. Marie Curie later discovered the radioactive elements polonium and radium and coined the term "radioactivity."
2. Alpha particles are identical to ______.
Answer
(a) Protons (b) Electrons (c) Helium nuclei (d) Neutrons
✅ Answer: (c) Helium nuclei
ExplanationAn alpha particle (α) consists of 2 protons and 2 neutrons — identical to the nucleus of a helium-4 atom (⁴₂He). It has a charge of +2, is relatively heavy, and has low penetrating power (stopped by a sheet of paper or a few cm of air).
3. Which type of radiation has the highest penetrating power?
Answer
(a) Alpha (b) Beta (c) Gamma (d) All are equal
✅ Answer: (c) Gamma
ExplanationPenetrating power order: γ > β > α
- Alpha: Stopped by a sheet of paper or 3–5 cm of air
- Beta: Stopped by a few mm of aluminium
- Gamma: Requires several cm of lead or thick concrete to be significantly reduced
Gamma rays are high-energy electromagnetic radiation (photons) with no mass or charge.
4. The process of splitting a heavy nucleus into lighter nuclei is called ______.
Answer
(a) Nuclear fusion (b) Nuclear fission (c) Radioactive decay (d) Beta emission
✅ Answer: (b) Nuclear fission
ExplanationNuclear fission: Heavy nucleus (e.g., ²³⁵U) + neutron → two medium nuclei + 2–3 neutrons + enormous energy. Used in nuclear reactors and atomic bombs.
Nuclear fusion: Light nuclei (like hydrogen isotopes) combine to form a heavier nucleus + energy. Occurs in the Sun; used in hydrogen bombs (not yet commercially viable for power).
5. The fuel used in a nuclear reactor is ______.
Answer
(a) Coal (b) Uranium-235 (c) Hydrogen (d) Helium
✅ Answer: (b) Uranium-235
ExplanationNuclear reactors use Uranium-235 (²³⁵U) as fuel. When ²³⁵U absorbs a slow neutron, it undergoes fission, releasing about 200 MeV of energy per fission. The energy heats water to produce steam, which drives turbines to generate electricity.
6. Which radiation is not deflected by electric or magnetic fields?
Answer
(a) Alpha (b) Beta (c) Gamma (d) Both alpha and beta
✅ Answer: (c) Gamma
ExplanationGamma rays are electromagnetic radiation with no charge and no mass, so they are not deflected by electric or magnetic fields. Alpha (+2 charge) and beta (−1 charge) are deflected because they are charged particles.
7. The half-life of a radioactive substance is the time required to ______.
Answer
(a) completely decay (b) decay to half its original amount (c) double its activity (d) increase its mass
✅ Answer: (b) decay to half its original amount
ExplanationHalf-life (t₁/₂) is the time taken for half the atoms in a radioactive sample to decay. After one half-life, 50% remains. After two half-lives, 25%. After three, 12.5%, and so on. Each element has a unique half-life (from fractions of a second to billions of years).
8. Nuclear fusion occurs in ______.
Answer
(a) Nuclear reactors on Earth (b) The Sun (c) Radioactive substances (d) Coal power plants
✅ Answer: (b) The Sun
ExplanationThe Sun's energy comes from nuclear fusion — hydrogen nuclei (protons) fuse to form helium, releasing enormous energy. Temperatures of about 15 million °C are needed for fusion. On Earth, fusion has been achieved (hydrogen bombs) but controlled fusion for power is still under development.
9. The unit of radioactivity is ______.
Answer
(a) Watt (b) Becquerel (c) Hertz (d) Roentgen
✅ Answer: (b) Becquerel
ExplanationThe SI unit of radioactivity is the Becquerel (Bq), named after Henri Becquerel. 1 Bq = 1 decay per second. An older unit is the Curie (Ci); 1 Ci = 3.7 × 10¹⁰ Bq. The Roentgen measures radiation exposure in air.
10. Which of the following is a peaceful use of nuclear energy?
Answer
(a) Atomic bomb (b) Hydrogen bomb (c) Nuclear power plant (d) Nuclear warhead
✅ Answer: (c) Nuclear power plant
ExplanationNuclear power plants use controlled nuclear fission to generate electricity — a peaceful, commercial use. They produce no greenhouse gases during operation. Atomic and hydrogen bombs are weapons of destruction. Nuclear power provides about 10% of the world's electricity.
Quick Revision — Key Nuclear Physics Concepts
| Concept | Key Fact |
|---|
| Radioactivity discovered by | Henri Becquerel (1896) |
| Alpha particle | ⁴₂He (2p + 2n), +2 charge, lowest penetration |
| Beta particle | Electron (e⁻), −1 charge, medium penetration |
| Gamma ray | Electromagnetic wave, no charge, highest penetration |
| Nuclear fission | Heavy nucleus splits, e.g., ²³⁵U, used in reactors |
| Nuclear fusion | Light nuclei combine, e.g., H → He, occurs in Sun |
| Half-life | Time for half the sample to decay |
| Unit of radioactivity | Becquerel (Bq) |
| Nuclear reactor fuel | Uranium-235 |
| Moderator in reactor | Heavy water or graphite (slows neutrons) |
Common MCQ Traps
- Fission vs Fusion: Fission = splitting (reactor/bomb); Fusion = joining (Sun/H-bomb)
- Gamma penetration: Gamma is highest, not alpha
- Becquerel vs Curie: Both are units of radioactivity — Becquerel is SI unit
- Half-life: Substance never fully decays to zero mathematically; it approaches zero asymptotically
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