How It Works: Why Mass Converts to Energy
Part of Nuclear Fission & Fusion — GCSE Physics
This how it works covers How It Works: Why Mass Converts to Energy within Nuclear Fission & Fusion for GCSE Physics. Revise Nuclear Fission & Fusion in Atomic Structure for GCSE Physics with 13 exam-style questions and 25 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 11 of 18 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 11 of 18
Practice
13 questions
Recall
25 flashcards
⚙️ How It Works: Why Mass Converts to Energy
In both fission and fusion, the total mass of the products is slightly less than the total mass of the reactants. This "missing mass" is called the mass defect. According to E = mc², this mass is converted entirely to energy.
In fission of U-235: reactant mass ≈ 235.044 u, product mass ≈ 234.909 u. Mass defect ≈ 0.135 u = 2.24 × 10⁻²⁸ kg. Energy released: E = mc² = 2.24 × 10⁻²⁸ × (3×10⁸)² ≈ 2 × 10⁻¹¹ J per fission. Tiny! But with ~10²⁴ fissions per gram of uranium, this adds up to hundreds of megajoules.
The reason fusion produces more energy per kg is that the binding energy per nucleon is higher for helium than for hydrogen — there's a larger energy "gap" to fill, meaning more energy is released per reaction relative to the mass of fuel used.