How It Works: Why Mass Converts to Energy

⚙️ 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.