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Nuclear Fission and Fusion — Revision Notes

NIOS · Class 12 · Physics

Nuclear Fission and Fusion revision notes for NIOS Class 12 Physics: 4 topics in quick points. Part of the NIOS Class 12 Physics syllabus.

45 questions32 flashcards4 formulas & key relations5 concepts

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A diagram illustrating the nuclear fusion process, where two light nuclei combine under extreme conditions to form a heavier, more stable nucleus, releasing a large amount of energy.
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Key Topics to Revise

1

Chemical Reactions vs Nuclear Reactions

  • In chemical reactions, only the valence (outermost) electrons of atoms rearrange. The nucleus remains completely unaffected.
  • Energy involved in chemical reactions is of the order of a few eV (electron volts). For example, burning one carbon atom releases 4.08 eV.
  • In nuclear reactions, the nuclei of atoms interact and new elements are formed. This process is called transmutation of nuclei.
2

Conservation Laws for Nuclear Reactions

  • Law 1 – Conservation of Mass Number: The total mass number (A) of all reactants equals the total mass number of all products.
  • Law 2 – Conservation of Atomic Number (Charge): The total atomic number (Z) of all reactants equals the total atomic number of all products.
  • Law 3 – Conservation of Mass-Energy: The total input kinetic energy plus the rest mass energy of reactants equals the total output kinetic energy plus the rest mass energy of products.
3

Nuclear Fission – Discovery, Mechanism and Energy

  • Nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann, who found that bombarding uranium with slow neutrons produced barium (intermediate mass element), not a transuranic element.
  • Lise Meitner and Otto Frisch explained the results using the liquid drop model of the nucleus and coined the term 'nuclear fission' (analogy with biological cell division).
  • Bohr and Wheeler developed the complete theory and predicted that ²³⁵U is more fissile (easily undergoes fission) than ²³⁸U.
4

Nuclear Chain Reaction and Nuclear Reactor

  • A nuclear chain reaction occurs when neutrons produced in one fission event trigger further fission events, sustaining or multiplying the reaction.
  • Each fission of ²³⁵U releases 2–3 neutrons (average 2.54) which can each cause further fissions, leading to an exponential increase in reactions.
  • When the rate of neutron production equals the rate of neutron loss, the reaction is self-sustained.

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Full Notes

Key Concepts

In a chemical reactionAll nuclear reactions must obey fourNuclear fission is the processA fissile material is oneWhen U

Frequently Asked Questions

What are the important topics in Nuclear Fission and Fusion for NIOS Class 12 Physics?
Key topics in Nuclear Fission and Fusion include Chemical Reactions vs Nuclear Reactions, Conservation Laws for Nuclear Reactions, Nuclear Fission – Discovery, Mechanism and Energy, Nuclear Chain Reaction and Nuclear Reactor. Study these first, then practise questions on each for the NIOS Class 12 board exam.
How should I revise Nuclear Fission and Fusion for the NIOS Class 12 board exam?
Learn the core ideas first, then work through the 45 practice questions on Nuclear Fission and Fusion. Revise definitions regularly and use flashcards for quick recall before the exam.

Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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