7 · IGCSE Physics
Fission and fusion
Watch a neutron split a uranium-235 nucleus, or two hydrogen nuclei fuse into helium, and see how the small loss of mass becomes released energy through E = Δm c².
Key equation: E = Δm c²
c = 3.00 × 10⁸ m s⁻¹, 1 u = 1.661 × 10⁻²⁷ kg
Reaction:
c = 3.00 × 10⁸ m s⁻¹, 1 u = 1.661 × 10⁻²⁷ kg
Reaction:
Mass before:
Mass after:
Mass lost Δm:
Energy released E:
Energy per kg of fuel:
Try this
- In fission, compare the two product pairs. Which pair has the larger mass loss Δm, and how does that change the energy released?
- In fusion, lower the temperature step by step. Find the lowest temperature at which the nuclei still fuse in this model and explain why the Sun’s core must be so hot.
- Compare the energy per kilogram of fuel for fission and fusion. Which releases more per kilogram, and by roughly what factor?
Exam tip: when explaining fusion, say that nuclei are positively charged and repel, so very high temperature (and pressure) is needed to give them enough kinetic energy to get close enough to join.