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1.8.5Motion, Forces and Energy

Nuclear Fusion — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 88 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Curriculum Equation Key Radioactive Decay & Half-Life FormulasFast Exam Solution
N(t) = N₀ × (1/2)t / t½ | λ = ln(2) / t½ ≈ 0.693 / t½ | E = Δm × c²
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Lesson summary

1.8.5 Nuclear Fusion — IGCSE PHYSICS notes: Describe nuclear fusion as the energy source of the Sun and stars and compare it with fission.Nuclear fusion = the joining of two light nuclei to form a heavier nucleus, releasing energy.It powers the Sun and other stars.In the Sun's co…

  • The helium nucleus has slightly less mass than the nuclei that formed it, and the lost mass is transferred as energy (E
  • Very high temperature
  • Deuterium / tritium
  • Electrostatic repulsion
  • Geothermal
  • Hydroelectric

Part 1 of 6What fusion is

Nuclear fusion
Nuclear fusion the joining of two light nuclei to form a heavier nucleus, releasing energy.
It powers the Sun and other stars.
In the Sun's core hydrogen nuclei (protons) fuse in steps to form helium.
The helium nucleus has slightly less mass than the nuclei that formed it, and the lost mass is transferred as energy (E = mc²).
Fusion releases much more energy per kilogram of fuel than any chemical reaction.

Part 2 of 6Conditions needed

Nuclei are positively charged and repel each other with a strong electrostatic force.
Very high temperature
  • Very high temperature millions of degrees so nuclei move fast enough to overcome the repulsion.
    • (about 15 million °C in the Sun's core)
Very high pressure or density so collisions are frequent; in the Sun gravity from its own mass provides this.
Matter at these temperatures is a plasma, in which electrons are stripped from atoms.
The Sun can keep fusing hydrogen for billions of years.

Part 3 of 6Energy from lost mass

1
Four hydrogen nuclei eventually form one helium nucleus with a mass loss of about 0.7%.
2
This mass multiplied by c² gives a large energy.
3
The Sun converts about 4 million tonnes of mass into energy every second.
4
Outward pressure from the hot plasma balances the inward pull of gravity, so the star is stable until hydrogen runs low.

Part 4 of 6Fusion on Earth

1
Experimental reactors use hydrogen isotopes deuterium and tritium.
2
The plasma must be heated above 100 million °C and held away from the walls by strong magnetic fields.
3
Potential advantages:
  • Deuterium is available in seawater.
  • Very large energy output per kilogram.
  • No carbon dioxide, and the main product, helium, is harmless.
  • No long-lived waste like fission and no runaway chain reaction.
4
Current disadvantages:
  • Conditions are extremely hard to achieve and maintain, with huge costs.
  • Reactors giving more energy than they use are not yet in routine electricity production.
  • Tritium is radioactive and must be produced, and the reactor structure becomes mildly radioactive.

Part 5 of 6Fusion and fission compared

FissionFusion
ProcessLarge nucleus splits into smaller nucleiSmall nuclei join to form a larger nucleus
Typical fuelUranium-235, plutoniumHydrogen isotopes (deuterium, tritium)
ConditionsNeutron absorbed at ordinary temperatureVery high temperature and pressure
WasteLong-lived radioactive wasteMainly helium, little long-lived waste
Used todayYes, in power stationsIn stars; experimental on Earth

Part 6 of 6More from the specification

More from the specification
Renewable vs Non-Renewable Energy Resources
  • Non-renewable fossil fuels (coal, oil,
  • and natural gas) represent finite stores of chemical energy,
  • whereas renewable resources such as solar panels, wind, tidal power,
  • and geothermal energy provide continuous energy supply replenished naturally.
Geothermal and Tidal Energy Resources
  • Geothermal energy harnesses thermal energy stored beneath the Earth's crust to generate steam that drives a turbine,
  • while tidal energy harnesses the gravitational pull of the Moon and Sun on oceanic tides.
Solar Panels and Photovoltaic Cells
  • Solar photovoltaic cells convert radiant sunlight directly into electricity via the photoelectric effect,
  • whereas solar heating panels capture thermal radiation to warm domestic water stores.
Nuclear Fusion as an Ideal Future Energy Supply
  • Thermonuclear fusion in stars joins light hydrogen nuclei to release massive quantities of energy;
    • reproducing fusion on Earth would provide a virtually unlimited, safe energy supply with abundant fuel and zero greenhouse gas emissions.
Tidal and geothermal
  • Tidal and geothermal tidal energy harnesses gravitational pull from the Moon, whereas geothermal utilizes thermal energy stored within the Earth.
Solar panels
  • Solar panels photovoltaic cells convert sunlight directly into electrical energy without burning fossil fuels or emitting greenhouse gases.
Turbine generators
  • Turbine generators steam driven by thermal energy turns a turbine to supply electrical power through electromagnetic induction.
Energy stored
  • W = ½QV = ½CV²
    What the symbols mean: W or E = energy stored (J) · Q = charge (C) · V = p.d. (V) · C = capacitance (F)
Biofuel
  • Fuel made from recently living plant or animal material.
Renewable
  • An energy resource that will not run out.
Fossil fuel
  • Fuel formed over millions of years from dead plants and animals.
Solar panel
  • A device that uses sunlight to heat water.
Solar cell
  • Device that converts light energy directly into electrical energy.
Global warming
  • Rise in the average temperature of the Earth.
Renewable resource
  • A resource that is replenished at the same rate as it is used.
Energy store
  • Where energy is held, such as kinetic or chemical.
Geothermal
  • Energy from hot rocks underground.
Turbine
  • Device turned by steam or a moving fluid to drive a generator.
Hydroelectric
  • Electricity from water flowing down from a high reservoir.

Key terms

TermMeaning
Nuclear fusionJoining of light nuclei to form a heavier nucleus, releasing energy.
PlasmaA hot gas of charged particles (ions and electrons).
Deuterium / tritiumIsotopes of hydrogen used as fusion fuel.
Electrostatic repulsionForce between like charges that keeps nuclei apart.
FissionSplitting of a large nucleus releasing energy.
HeliumProduct of the fusion of hydrogen in stars.

Avoid these mistakes

1
Mixing up fusion (joining) and fission (splitting).
2
Believing fusion occurs at ordinary temperatures in power stations on Earth.
3
Saying the Sun burns like a coal fire — it is nuclear fusion.
4
Saying fusion has no technical difficulties.
5
Thinking no energy is released because mass is lost — the lost mass becomes energy.

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