Nuclear Physics — Structure, Radioactivity, Fission & Fusion Explained | Chapter 43 of University Physics

Chapter 43 delves into the heart of the atom—its structure, the forces binding protons and neutrons, how nuclei decay, and how we harness nuclear reactions for energy. This guide provides a clear, concise overview of concepts from binding energy to controlled fission and stellar fusion.

Watch the full video summary on YouTube for animated diagrams and real-world examples.

Nuclear Properties & Structure

• Composition: Nuclei contain Z protons and N neutrons (A = Z + N).
• Size & Density: Radii scale as R = R₀·A^1/3; nuclei are extremely dense (~10¹⁷ kg/m³).
• Isotopes: Same Z, different N; magnetic moments of nucleons underlie NMR and MRI.

Binding Energy & Nuclear Models

Binding energy (E_B) equals the mass defect times c² and peaks near A≈60, indicating maximum stability. Models include:

• Liquid-Drop Model: Uses volume, surface, Coulomb, asymmetry & pairing terms to estimate E_B.
• Shell Model: Explains “magic numbers” (2, 8, 20, 28, 50, 82, 126) where nuclei exhibit extra stability.

Radioactive Decay & Stability

• Alpha (α): Emission of ⁴₂He, reduces A by 4, Z by 2.
• Beta (β⁻): Neutron → proton + e⁻ + ν̄; increases Z by 1.
• Beta (β⁺) & Electron Capture: Proton → neutron + e⁺/ν, decreases Z by 1.
• Gamma (γ): Emission of photon from excited nucleus, no change in Z or A.

Decay chains transform heavy, unstable isotopes into stable end products.

Decay Rates, Activity & Radiation Safety

Decay is exponential: N(t)=N₀e^–λt, with half-life T_½=0.693/λ. Activity A=λN (Bq or Ci). Biological effects depend on absorbed dose (Gy, rad) and quality factor (Sv, rem), guiding medical imaging and radiation therapy protocols.

Nuclear Reactions & Energy

Induced reactions obey conservation of mass-energy, charge and nucleon number. Reaction Q-value: Q=(m_initial–m_final)c². Exoergic (Q>0) reactions release energy; endoergic (Q<0) require input.

Nuclear Fission & Chain Reactions

• Process: Heavy nucleus (e.g., ²³⁵U) + n → fission fragments + ~200 MeV + 2–3 n.
• Chain Reaction: Neutrons from fission sustain further reactions; controlled in reactors via moderators (water, graphite) and control rods (B, Cd).
• Applications: Power generation, nuclear submarines; safety demands rigorous shielding and regulation.

Nuclear Fusion & Stellar Reactions

Fusion of light nuclei (like the proton–proton chain in stars) releases energy when combining to form helium. Overcoming the Coulomb barrier requires extreme temperatures/pressures. Experimental approaches include magnetic confinement (tokamaks) and inertial confinement; superconducting magnets and advanced lasers push these frontiers.

Conclusion

Chapter 43 unveils the immense power locked within the nucleus—from the forces binding nucleons to the harnessing of fission and fusion for energy. Understanding these principles is key to applications in power generation, medical diagnostics, and astrophysics.

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