Nuclear fusion

4 marks · Hard

Nuclear fusion is being researched as a possible source of unlimited clean energy on Earth, but it has not yet been achieved on a large scale.

Explain, using scientific ideas, why achieving nuclear fusion on Earth is much more difficult than in the core of stars. Include at least two physical reasons in your answer.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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Achieving fusion on Earth is harder because stars have much higher temperatures and pressures due to their massive gravity, enabling nuclei to overcome their electrostatic repulsion. On Earth, such conditions are difficult to create and sustain; gravity is too weak to compress fuel, and confinement (like magnetic bottles) is much less effective than in a star’s core.

Must show: reference to temperature/energy needed for nuclei to overcome repulsion, reference to pressure or gravitational confinement in stars, limitation of artificial methods (magnetic, inertial) on Earth, at least two distinct physical reasons

Also accept: Earth’s reactors lose too much energy due to containment difficulties; stars achieve equilibrium, Earth cannot sustain plasma long enough

Common mistakes: Just stating 'stars are bigger' without linking to gravity/pressure; Not mentioning electrostatic repulsion

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Explains the role of very high temperatures for fusion (enabling nuclei to overcome repulsive forces).

M2 · 1 mark · Explains how massive stars’ gravity creates extremely high pressure/density for fusion.

A1 · 1 mark · Describes why artificial confinement on Earth (magnetic fields, etc.) is less effective than gravity in a star's core.

A2 · 1 mark · Provides a valid second reason (e.g., energy or plasma loss, or the short duration of fusion experiments on Earth).

Worked solution

Fusion is easier in stars because their huge gravitational fields create enormous pressures and densities, forcing nuclei close together so fusion is much more likely. Stars also maintain extremely high temperatures (around 10 million °C or more), giving nuclei enough energy to overcome electrostatic repulsion. On Earth, gravity is much weaker, so we must use powerful magnetic fields or lasers to confine and heat the nuclei, but this is much less effective, so it is very hard to keep the hot plasma stable and dense enough for fusion reactions to happen.