E.5.3—Fusion conditions

Use high temperature

Nuclei are positively charged and repel electrically. High core temperature gives them enough kinetic energy and speed to approach despite this repulsion.

Use high density

High density puts more nuclei into a given volume, increasing the collision frequency and the probability of close encounters.

Reach the strong-force range

Fusion requires nuclei to approach closely enough for the attractive strong interaction to act and for a bound product to form.

Common trap

Do not use surface temperature or star size as the direct fusion condition. The relevant evidence is high core temperature and density.

How it is assessed

Questions explain why fusion occurs in stellar cores or identify which solar features make fusion possible.

Command terms

Explain / State

What earns marks

Mention both temperature and density, then link each to a distinct physical role.

Watch for

Giving only high temperature or citing surface temperature instead of core conditions.

Retrieve stellar balance

Fusion releases energy, outward thermal/radiation pressure balances inward gravity, and high temperature and density allow fusion in the core.

Retrieve stellar inference

Mass controls evolution; HR regions classify stars; parallax gives distance; and $L=4\pi R^2\sigma T^4$ gives stellar radius from luminosity and temperature.

E.5.3—Fusion conditions

Explain Fusion Conditions

E.5.3 Exam Analysis

Assessment in practice

Retrieve the Stellar Model