GCSE · Physics · Edexcel · Spec 1PH0

Nuclear fusion

Smash two tiny nuclei together fast enough and a sliver of their mass becomes energy. That's what powers the Sun.

One fusion event, start to finish

Stage 1 of 6: Two light nuclei. paused

Two light nuclei · 1/6scrub the collision →

Two very light nuclei, such as hydrogen nuclei, head towards each other. Each one contains protons, so each one is positively charged.

Drag along the path to follow two nuclei from pushing apart to joining. Keep an eye on the last two stops.

Check the count

Does anything go missing? Count and see.

Hydrogen-2 and hydrogen-1 fuse to give helium-3. Check whether the number of nucleons and the number of protons are the same before and after.

The number after each name is the number of nucleons: the protons and neutrons in that nucleus. So hydrogen-2 has 2 nucleons, hydrogen-1 has 1 and helium-3 has 3.Nucleon just means a particle in the nucleus: a proton or a neutron.
The element tells you the protons: every hydrogen nucleus has 1 proton and every helium nucleus has 2.
missing step

Which line is step 3?

Nucleons before: 2 + 1 = 3Nucleons before: 1 + 1 = 2Nucleons before: 2 × 1 = 2

So where does the energy come from?

Same count, less mass

You have just checked it: 3 nucleons and 2 protons go in, and 3 nucleons and 2 protons come out. Yet fusion releases energy.

Where does that energy come from? Pick the idea closest to what you think right now.

The hydrogen burns, like fuel in a fire.Some nucleons get destroyed, and that is the energy.Mass can't change, so the energy must come from somewhere else.A tiny bit of mass turns into energy, so the products are slightly lighter.

How sure are you?

I'm sureI think soI'm guessing

The hydrogen burns, like fuel in a fire.

This is a common misconception

Tempting: it is hydrogen, and it is extremely hot. But nothing is burning. Burning is a chemical reaction, and a chemical reaction never turns one element into another. In fusion the nuclei themselves join: hydrogen nuclei become a helium nucleus.

Some nucleons get destroyed, and that is the energy.

This is a common misconception

Look back at your count: 3 nucleons in, 3 out; 2 protons in, 2 out. No nucleon is destroyed. Whatever changes in fusion, it is not the number of particles.

Mass can't change, so the energy must come from somewhere else.

This is a common misconception

This is exactly the idea fusion breaks. The number of particles stays the same, but the mass does not: the products weigh slightly less than what went in. That missing mass is where the energy comes from.

A tiny bit of mass turns into energy, so the products are slightly lighter.

This matches the evidence

Yes. The products have slightly less mass than the original nuclei, and that mass difference is converted into the energy released. E = mc² is the equation that links the mass lost to the energy released.

Same count, less mass. Every particle is still there after fusion, but a sliver of mass has become energy.

Find the slip

Hydrogen-2 + hydrogen-3: what else comes out?

Hydrogen-2 and hydrogen-3 fuse to give helium-4 and one other particle. Use the number of nucleons and the number of protons to work out what the other particle is.

A student's answer — which line goes wrong?

Nucleons before: 2 + 3 = 5
Protons before: 1 + 1 = 2
Helium-4 has 4 nucleons and 2 protons.
So the other particle has 5 − 4 = 1 nucleon.
One nucleon is left over, so it must be a proton.
Answer: hydrogen-2 + hydrogen-3 → helium-4 + a proton.

Predict, then check

Fusion needs nuclei moving at very high speeds, which means very high temperatures.

Where in nature would you expect fusion to be happening all the time, on a huge scale?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How two tiny nuclei become one, and where the missing mass goes.

What you need to know

Nuclear fusion is two light nuclei joining to form a heavier nucleus, with energy released.
Nuclei repel each other strongly because protons are positive, so fusion needs the nuclei to move at very high speeds, which means very high temperatures.
The products have slightly less mass than the original nuclei. That mass difference is converted into the energy released (E = mc²).
The number of nucleons and the number of protons are the same before and after a fusion reaction.
Fusion is the energy source of stars: in the core of a star like the Sun, the temperature and pressure are so high that hydrogen nuclei fuse.

The big picture

Nuclear fusion is when two light nuclei join to form a heavier nucleus. Nuclei are positively charged and repel each other strongly, so they only get close enough to join if they are moving at very high speeds, which needs very high temperatures. The products have slightly less mass than the nuclei that went in, even though the numbers of nucleons and protons stay the same, and that lost mass is released as energy (E = mc²). Fusion is the energy source of stars like the Sun.

Key points

1Fusion: light nuclei join to make a heavier one.

2The obstacle is repulsion: every nucleus is positively charged.

3Very hot means very fast, and only very fast nuclei get close enough to join.

4Same count, less mass: the lost mass is released as energy.

5Stars run on fusion: hydrogen nuclei fuse in their hot, high-pressure cores.

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Worked example

Problem

Explain why nuclear fusion needs very high temperatures.

⚠ Watch out

Saying fusion destroys some nucleons or protons. Both counts are the same before and after; what goes down is the mass.

🧠

Memory hook

The count stays, the mass pays. After fusion every nucleon and every proton is still there, but a sliver of mass has been paid out as energy.

✓

Check yourself

A friend says the Sun is 'basically a giant bonfire of hydrogen'. Using what you now know about fusion, what would you tell them is wrong with that?

Flashcards

(12)

What is nuclear fusion?

Two light nuclei joining to form a heavier nucleus, releasing energy.

Why is it hard to make two nuclei fuse?

Both are positively charged, so they repel strongly. Only nuclei moving at very high speeds (very high temperatures) get close enough to join.

How does the mass of the products compare with the mass of the nuclei that fused?

It is slightly less.

What happens to the mass that is 'missing' after fusion?

It is converted into energy, which is released. E = mc² links the mass to the energy.

Which two numbers are the same before and after a fusion reaction?

The number of nucleons and the number of protons.

What is a nucleon?

A particle in the nucleus: a proton or a neutron.

Hydrogen-2 + hydrogen-1 fuse to give…?

Helium-3.

Hydrogen-2 + hydrogen-3 fuse to give…?

Helium-4 and a neutron.

An unknown product has 1 nucleon and 0 protons. What is it?

A neutron.

Is fusion the same as burning hydrogen?

No. In fusion, nuclei join to make a new, heavier nucleus (hydrogen becomes helium), and the energy comes from mass being converted.

What is the energy source of stars?

Nuclear fusion.

Why can hydrogen nuclei fuse in the core of a star like the Sun?

The temperature and pressure there are so high that hydrogen nuclei are forced together and fuse, releasing energy.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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One fusion event, start to finish

Does anything go missing? Count and see.

Same count, less mass

Hydrogen-2 + hydrogen-3: what else comes out?

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