AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Properties of quarks and antiquarks: charge, baryon number and strangeness.
Full specification wording
Quarks and antiquarks: Properties of quarks and antiquarks: charge, baryon number and strangeness.
Quarks are the fundamental constituents of hadrons, and each quark has a set of conserved or bookkeeping properties that determine how particles interact and decay.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Quarks are the fundamental constituents of hadrons, and each quark has a set of conserved or bookkeeping properties that determine how particles interact and decay. The up quark carries charge +2/3, the down and strange quarks each carry charge −1/3; antiquarks carry the opposite charge. Baryon number is +1/3 for every quark and −1/3 for every antiquark, so three quarks give a baryon number of 1. Strangeness is 0 for up and down quarks, −1 for a strange quark and +1 for an antistrange quark. For example, a proton contains uud, giving charge +2/3 +2/3 −1/3 = +1 and baryon number 1/3 +1/3 +1/3 = 1. These properties are combined arithmetically when identifying particles or checking whether a reaction is allowed.
What earns the marks
- State the charge of each quark: up +2/3, down −1/3, strange −1/3, with antiquarks having equal magnitude and opposite sign.
- State that every quark has baryon number +1/3 and every antiquark has baryon number −1/3.
- State that strangeness is 0 for up and down quarks, −1 for the strange quark and +1 for the antistrange quark.
- Combine quark properties by addition to find the total charge, baryon number and strangeness of a hadron.
- Recognise that antiquarks reverse the sign of charge, baryon number and strangeness relative to their corresponding quarks.
- Use conservation of charge and baryon number, and the behaviour of strangeness, when checking particle interactions.
Common mistakes and how to put them right
Giving the down quark a positive charge.
Put it right: The down quark has charge −1/3, while the up quark has charge +2/3.
Treating baryon number as a whole number for individual quarks.
Put it right: Each quark has baryon number +1/3, not 1.
Forgetting that the antiquark of a strange quark has strangeness +1 rather than −1, because strangeness changes sign for antiparticles.
Exam tips
- Write out the quark composition and then add charge, baryon number and strangeness in separate columns to avoid mixing the quantities.
- Check that the total charge of a proposed hadron is an integer and that its baryon number is 0, 1 or −1 for the particles required by the specification.
- When a question gives a decay or reaction, test charge and baryon number first, then use strangeness to decide whether the process is allowed.
Try the lesson
The start of the Quarks and antiquarks: Properties of quarks and antiquarks: charge, baryon number and strangeness. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following quarks or antiquarks has a charge of +2/3 and a strangeness of 0?
Listen
Quarks are the fundamental constituents of hadrons, and each quark has a set of conserved or bookkeeping properties that determine how particles interact and decay. The up quark carries charge +2/3, the down and strange quarks each carry charge −1/3; antiquarks carry the opposite charge. Baryon number is +1/3 for every quark and −1/3 for every antiquark, so three quarks give a baryon number of 1. Strangeness is 0 for up and down quarks, −1 for a strange quark and +1 for an antistrange quark. For example, a proton contains uud, giving charge +2/3 +2/3 −1/3 = +1 and baryon number 1/3 +1/3 +1/3 = 1. These properties are combined arithmetically when identifying particles or checking whether a reaction is allowed. A strong answer earns marks for these points. State the charge of each quark: up +2/3, down −1/3, strange −1/3, with antiquarks having equal magnitude and opposite sign. State that every quark has baryon number +1/3 and every antiquark has baryon number −1/3. State that strangeness is 0 for up and down quarks, −1 for the strange quark and +1 for the antistrange quark. Combine quark properties by addition to find the total charge, baryon number and strangeness of a hadron. Exam tip: Write out the quark composition and then add charge, baryon number and strangeness in separate columns to avoid mixing the quantities.
See it
Key words
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Baryon number
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Strangeness
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Antiquarks
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Quark properties
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See every definition
- Baryon number
- A bookkeeping property that is +1/3 for every quark and −1/3 for every antiquark, so three quarks give a baryon number of 1.
- Strangeness
- A property that is 0 for up and down quarks, −1 for a strange quark and +1 for an antistrange quark.
- Antiquarks
- Particles that carry the opposite charge, and reverse the sign of baryon number and strangeness, relative to their corresponding quarks.
- Quark properties
- Charge, baryon number and strangeness, which are combined arithmetically when identifying particles or checking whether a reaction is allowed.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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