AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students should be able to explain how fusion processes lead to the…
Full specification wording
The life cycle of a star: Students should be able to explain how fusion processes lead to the formation of new elements.
An element is defined by the number of protons in its nucleus, so any process that changes that number makes a new element, and that is exactly what fusion does.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An element is defined by the number of protons in its nucleus, so any process that changes that number makes a new element, and that is exactly what fusion does. Two light nuclei are forced together into a single heavier nucleus, and the protons of both end up in the same nucleus, so the atomic number goes up and the element changes. Getting them together is the hard part, because both nuclei are positively charged and repel one another, so only the enormous temperature and pressure in a star's core give them enough kinetic energy to collide and join. The new nucleus has slightly less mass than the parts that made it, and that missing mass is released as energy, which keeps the core hot enough for the next reaction.
What earns the marks
- Two light nuclei joining to form one heavier nucleus.
- The number of protons in the nucleus increasing, so a different element is formed.
- The very high temperature and pressure needed to overcome the repulsion between the nuclei.
- Some of the mass being converted into energy, which is then released.
Common mistakes and how to put them right
Describing whole atoms joining rather than nuclei, so the role of the protons is lost.
Saying fusion makes isotopes rather than new elements, when it is the proton number that changes.
Leaving out the repulsion between the positive nuclei, so there is no reason why extreme conditions are needed.
Saying that energy is created in fusion rather than released from the mass of the nuclei.
Exam tips
- Use the word nuclei throughout, because atoms fuse loses the mark in several mark schemes.
- Quote both conditions, high temperature and high pressure, when explaining why fusion needs a star.
- Remember that an element is fixed by its proton number when a question asks why a new element appears.
Try the lesson
The start of the The life cycle of a star: Students should be able to explain how fusion processes lead to the formation of new elements. lesson, made from checked teaching notes for this specification point.
Have a go first
In a main sequence star, hydrogen nuclei join together in fusion reactions. What element is formed directly by this fusion of hydrogen nuclei?
Listen
An element is defined by the number of protons in its nucleus, so any process that changes that number makes a new element, and that is exactly what fusion does. Two light nuclei are forced together into a single heavier nucleus, and the protons of both end up in the same nucleus, so the atomic number goes up and the element changes. Getting them together is the hard part, because both nuclei are positively charged and repel one another, so only the enormous temperature and pressure in a star's core give them enough kinetic energy to collide and join. The new nucleus has slightly less mass than the parts that made it, and that missing mass is released as energy, which keeps the core hot enough for the next reaction. A strong answer earns marks for these points. two light nuclei joining to form one heavier nucleus. the number of protons in the nucleus increasing, so a different element is formed. the very high temperature and pressure needed to overcome the repulsion between the nuclei. some of the mass being converted into energy, which is then released. Exam tip: Use the word nuclei throughout, because atoms fuse loses the mark in several mark schemes.
Key words
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how fusion works
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proton number defines the element
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repulsion between nuclei
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extreme temperature and pressure
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See every definition
- how fusion works
- Two light nuclei are forced together into a single heavier nucleus, and the protons of both end up in the same nucleus.
- proton number defines the element
- An element is defined by the number of protons in its nucleus, so any process that changes that number makes a new element.
- repulsion between nuclei
- Both nuclei are positively charged and repel one another, so they need enough kinetic energy to collide and join.
- extreme temperature and pressure
- Only the enormous temperature and pressure in a star's core give the nuclei enough kinetic energy to collide and join.
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