Modern Physics
Introduction
Modern physics deals with phenomena at the atomic and nuclear level. In ICSE Class 10 Physics, you study radioactivity, nuclear reactions, and their applications — a topic that connects fundamental science with real-world energy and safety issues.
Radioactivity
Radioactivity is the spontaneous disintegration of unstable atomic nuclei, accompanied by the emission of radiation.
Types of Radiation
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus (2p + 2n) | High-speed electron | Electromagnetic wave |
| Symbol | ⁴₂He | ⁰₋₁e | γ |
| Charge | +2e | −e | 0 |
| Mass | 4 u | Negligible | 0 |
| Speed | Slow (~10⁷ m/s) | Fast (~10⁸ m/s) | Speed of light |
| Ionising power | Highest | Moderate | Lowest |
| Penetrating power | Lowest (stopped by paper) | Moderate (stopped by 3 mm Al) | Highest (stopped by thick lead) |
| Range in air | ~5 cm | ~1 m | ~1 km |
Nuclear Fission
The splitting of a heavy nucleus into two lighter nuclei with the release of a large amount of energy.
Example: Uranium-235 fission
²³⁵₉₂U + ¹₀n → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + 3¹₀n + Energy
Chain Reaction
The neutrons released in fission can cause further fissions, leading to a self-sustaining chain reaction.
- Controlled chain reaction — Used in nuclear reactors (moderator and control rods regulate the reaction).
- Uncontrolled chain reaction — Used in atom bombs (rapid, explosive).
Nuclear Reactor Components
| Component | Function |
|---|---|
| Fuel (U-235, Pu-239) | Fissionable material |
| Moderator (graphite, heavy water) | Slows down fast neutrons |
| Control rods (cadmium, boron) | Absorb excess neutrons |
| Coolant | Removes heat from the reactor core |
Nuclear Fusion
The combining of two light nuclei to form a heavier nucleus with the release of energy.
Example: Hydrogen fusion in the Sun
²₁H + ³₁H → ⁴₂He + ¹₀n + Energy
Fusion requires extremely high temperatures (~10⁷ K) to overcome electrostatic repulsion between nuclei.
Isotopes and Radioisotopes
Isotopes
Atoms of the same element with the same atomic number but different mass numbers.
Example: ¹²₆C, ¹³₆C, ¹⁴₆C
Radioisotopes
Unstable isotopes that undergo radioactive decay.
| Radioisotope | Half-life | Use |
|---|---|---|
| Cobalt-60 | 5.27 years | Cancer therapy (gamma rays) |
| Iodine-131 | 8 days | Thyroid disorders diagnosis |
| Uranium-235 | 7.04 × 10⁸ years | Nuclear reactors |
| Carbon-14 | 5730 years | Carbon dating |
Background Radiation
Background radiation is the low-level radiation present in the environment from natural and artificial sources.
Sources
| Natural Sources | Artificial Sources |
|---|---|
| Cosmic rays from space | Medical X-rays |
| Radioactive elements in soil (U, Th, Ra) | Nuclear fallout |
| Radon gas from the ground | Nuclear waste |
| Potassium-40 in our bodies | Consumer products |
Radiation Safety
Precautions
- Use lead shields or thick concrete walls.
- Handle radioactive materials with remote manipulators (tongs, robotic arms).
- Minimise exposure time.
- Maximise distance from the source.
- Wear dosimeters to track exposure.
- Proper disposal of radioactive waste.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing fission with fusion | Fission = splitting heavy nucleus; Fusion = combining light nuclei |
| Thinking all radiation is harmful | Controlled radiation is useful in medicine, industry |
| Mixing penetrating and ionising power | Higher ionising power = lower penetrating power (inverse relationship) |
| Forgetting that gamma has no mass/charge | Gamma is EM radiation, not a particle |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: properties of α, β, γ radiation (comparison table), nuclear fission vs fusion, isotopes and their uses, background radiation, safety measures.
Marks Blueprint: Properties of α, β, γ — 2 marks, Fission vs Fusion — 2 marks, Background radiation/Safety — 1 mark.
Self-Test Questions
-
Compare alpha, beta, and gamma radiation in terms of nature, charge, ionising power, and penetrating power.
-
What is nuclear fission? Explain with an example.
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Differentiate between nuclear fission and nuclear fusion.
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What are isotopes? Give an example. What are radioisotopes and how are they used in medicine?
-
What is background radiation? List two natural and two artificial sources.
-
State three safety precautions for handling radioactive materials.
