Physics Class 10 Unit 21: Nuclear Physics Notes
Definitions
- Nucleus: central core of the atom with protons and neutrons. Nucleon: a proton or a neutron. Nuclide: a type of atom with a specific number of protons and neutrons.
- Notation: ᴬ_Z X. A = nucleon number = protons + neutrons, Z = proton number, neutrons N = A − Z. Example: uranium-239 has 239 − 92 = 147 neutrons.
- Isotopes: atoms of the same element (same Z) with different numbers of neutrons. They have the same chemical properties (same number of electrons) but different physical properties (mass, stability).
- Radioactivity: spontaneous emission of radiation from unstable nuclei. Nuclei become radioactive because they are unstable (too many neutrons or protons, or too large).
- Half-life: time for half of the original radioactive nuclei in a sample to decay. Stable isotopes have infinite half-life.
- Radioactive dating: finding the age of an object from the remaining radioactivity; carbon-14 dating for once-living things (limit about 50,000 years); needs C-14 which is taken in only while alive.
- Nuclear fission: heavy nucleus splits into smaller nuclei releasing large energy. Nuclear fusion: light nuclei join to form a heavier nucleus releasing large energy (source of the Sun's energy).
- Background radiation: ionising radiation always present (radon gas is the largest natural source, also cosmic rays, rocks, food). Units: becquerel (activity), gray (absorbed dose), sievert (effective dose).
- Dark matter: unseen matter inferred from its gravity (more than 80 % of matter). Falsifiability: a scientific hypothesis must be capable of being disproved.
Decay and formulas
- Alpha decay: ᴬ_Z X to ᴬ⁻⁴_{Z−2} Y + ⁴₂He. Beta decay: ᴬ_Z X to ᴬ_{Z+1} Y + e⁻ (nucleon number unchanged, proton number up by 1). Gamma: no change in A or Z, a photon leaves.
- Half-life: remaining N = N0 (1/2)ⁿ, n = t/T½. After 1, 2, 3 half-lives the fraction left is 1/2, 1/4, 1/8.
- Mass-energy: E = mc², c = 3 × 10⁸ m/s.
- Fission example: ²³⁵U + n to ¹⁴⁴Ba + ⁸⁹Kr + 3n + energy. Fusion: ²H + ³H to ⁴He + n + energy.
Properties of radiations
- Alpha: helium nucleus, charge +2, highly ionising, stopped by paper or a few cm of air. Beta: electron, charge −1, medium ionisation, stopped by a few mm of aluminium, travels farther in air. Gamma: EM wave, no charge, least ionising, most penetrating, needs thick lead or concrete.
- Uses: radiotherapy (cobalt-60 gamma), food sterilisation, smoke alarms, tracers, thickness gauges, dating. Safety: keep distance, limit time, use shielding (lead), store waste safely.
- Controlled fission in a reactor: neutrons keep the chain reaction going; moderator slows neutrons, control rods absorb extra neutrons, coolant removes heat. Uncontrolled fission is a bomb.
- Fusion needs very high temperature (millions of kelvin) and pressure to overcome repulsion between nuclei.
- Alpha particle scattering (Rutherford): most alpha particles passed straight through (atom mostly empty space); a few deflected at large angles (nucleus is small, dense, positive); very few bounced straight back (almost all the mass is in the nucleus).
Past-paper questions with model answers
- What is an isotope? Do isotopes have similar chemical properties? Yes (same electron arrangement) (2+1).
- What is half-life? Explain with an example; why is half-life a characteristic property of a radioisotope (each isotope has a fixed value that is not changed by heat or pressure).
- What is radioactivity? Why do some nuclei become radioactive? (1+2).
- Briefly describe alpha particles with one reaction (2+1), e.g. ²²⁶₈₈Ra to ²²²₈₆Rn + ⁴₂He.
- Evidence from the alpha scattering experiment (any three facts, above).
- Process of nuclear fission with an example reaction; controlled nuclear fission in reactors with diagram (Section C).
- Radioisotopes and their use in carbon dating (1+4).
- Lead-212, 50 mg, half-life 10 h, after 50 h: 5 half-lives, 50/32 = 1.56 mg.
- One-eighth left after 420 days: 3 half-lives, T½ = 140 days. 1 kg to 250 g (quarter) in 2 half-lives of radium = 3200 years.
- E = mc² for 2 μg: 2 × 10⁻⁹ kg × 9 × 10¹⁶ = 1.8 × 10⁸ J = 180 MJ.
Revision tip: for each chapter, first learn the definitions and formulas, then draw the diagrams (wave forms, ray diagrams, circuits, transformer, motor, truth tables), and finally solve the numericals above without looking.