Describe quark flavours and charges
Recall the six quark flavours, fractional charges and the opposite charge of each antiquark.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 11. These original particle models and equations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use atomic number, nucleon number, electric charge and conservation with signed values.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the charge of a down quark?
What you will learn
- Recall up, down, strange, charm, top and bottom quarks
- Use +2e/3 for up-type and −e/3 for down-type quark charges
- Determine the opposite charge of an antiquark
The six quark flavours form two charge groups
The six quark flavours are up, down, strange, charm, top and bottom. Up, charm and top each have charge +2e/3. Down, strange and bottom each have charge −e/3.
The syllabus requires these charge values and flavour names; no knowledge of other quark properties is required here.
Antiquark charge has the opposite sign
Each quark flavour has a corresponding antiquark with the same flavour label and opposite charge. For example, an up antiquark has charge −2e/3 and an anti-down quark has charge +e/3.
When adding quark charges in a hadron, include each constituent's signed charge. Do not assume that every antiquark is negatively charged; the sign depends on its quark partner.
Pause and explain
What is the charge of an anti-up quark?
Worked example
Find the total charge of an up quark and an anti-down quark.
Show the worked solution
- Up quark charge is +2e/3.
- Down quark charge is −e/3, so anti-down is +e/3.
- Add the signed charges.
Answer +e.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
List the six quark flavours and group them by charge.
Give me a hint
Use the two charge groups: up-type and down-type.
Compare my reasoning
- Up, charm and top have charge +2e/3.
- Down, strange and bottom have charge −e/3.
- Check all six flavours are included.
Up, down, strange, charm, top and bottom; u/c/t are +2e/3 and d/s/b are −e/3.
Look for these in your work
- Six flavours stated
- Groups correct
- Charges correct
Find the total charge of a strange quark and anti-strange quark pair.
Give me a hint
An antiquark has the opposite charge to the same-flavour quark.
Compare my reasoning
- s has charge −e/3.
- Anti-s has charge +e/3.
- The total is zero.
0.
Look for these in your work
- Quark charge used
- Antiquark sign reversed
- Charges summed
A proposed three-quark combination is udb. Find its total charge in units of e and explain whether this charge alone makes it a proton.
Give me a hint
Add the signed constituent charges, then distinguish charge from full composition.
Compare my reasoning
- u +2e/3, d −e/3 and b −e/3.
- Total = 2e/3 − e/3 − e/3 = 0.
- A proton is specifically uud; matching a charge alone is not enough to identify a proton.
Total charge is zero. It is not a proton because the proton composition is uud; particle identity also depends on the quark combination.
Look for these in your work
- Each charge signed
- Total calculated
- Composition criterion stated
Common mistakes
- Assigning all quarks the same charge
- Forgetting to reverse charge for antiquarks
- Confusing flavour name with electric charge sign
What can you explain now?
What is the charge of a down quark?
Compare with the explanation
−e/3
Down-type flavours have negative one-third charge.
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Make it stick
Why is an anti-down quark positively charged even though it is an antiquark?
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