Quarks, hadrons and leptons Cambridge International AS & A Level Physics revision
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In plain words
Protons and neutrons aren't the end of the story. Each is made of three smaller particles called quarks, held together so tightly that nobody has ever pulled one out on its own. Electrons, on the other hand, really do seem to be fundamental: they belong to a different family, the leptons.
5 things to know
- Quarks are fundamental particles. There are six kinds (flavours): up, down, strange, charm, top and bottom. Up, charm and top have a charge of +⅔e. Down, strange and bottom have −⅓e. Each has an antiquark with the opposite charge.
- Hadrons are particles made of quarks. A baryon is three quarks: the proton is uud and the neutron is udd. A meson is a quark and an antiquark.
- Leptons, such as electrons and neutrinos, are fundamental. They are not made of quarks.
- In β⁻ decay a down quark changes into an up quark, so a neutron (udd) becomes a proton (uud). In β⁺ decay an up quark changes into a down quark, so a proton becomes a neutron.
- Every particle has an antiparticle with the same mass and the opposite charge.
Worked example
Show from its quarks that a proton has a charge of +e.
- A proton is two up quarks and a down quark: uud.
- Charge = (+⅔) + (+⅔) + (−⅓).
- = +1, in units of e.
Tips and tricks
- Protons and neutrons are not fundamental, because they are made of quarks. Electrons and neutrinos are.
- An antiquark has the opposite charge to its quark: an anti-up is −⅔e and an anti-down is +⅓e.
It lands in your notebook with its questions as flashcards.
Quarks, hadrons and leptons: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which particle is a lepton?
Leptons are not made of quarks.
What are the quarks in a proton?
Two up and one down.
What is the charge on a down quark?
So udd has no charge overall.
How does an antiparticle compare with its particle?
A positron is the antiparticle of the electron.
What is a meson made of?
A baryon is the one with three quarks.
Which quark change happens in β⁺ decay?
A proton (uud) becomes a neutron (udd).
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Quarks, hadrons and leptons
Cambridge International AS & A Level Physics 9702 · 8 marks · papermunch.org
Name ______________________________ Date ______________
Write down the quarks in a neutron, and show that its charge is zero.[2]
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udd. (+⅔) + (−⅓) + (−⅓) = 0.
State the difference between a baryon and a meson.[2]
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A baryon is made of three quarks. A meson is made of a quark and an antiquark.
Describe β⁻ decay in terms of quarks.[2]
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A down quark changes into an up quark, so a neutron becomes a proton. An electron and an antineutrino are emitted.
A meson is made of an up quark and an anti-down quark. Find its charge.[2]
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+e. (+⅔) + (+⅓).
Answers: Quarks, hadrons and leptons
- 1. udd. (+⅔) + (−⅓) + (−⅓) = 0.
- 2. A baryon is made of three quarks. A meson is made of a quark and an antiquark.
- 3. A down quark changes into an up quark, so a neutron becomes a proton. An electron and an antineutrino are emitted.
- 4. +e. (+⅔) + (+⅓).