A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Properties of Quarks and Antiquarks — mark scheme explained
The short answer
In AQA A-Level Physics, understanding the properties of quarks and antiquarks is crucial for grasping particle physics. This section focuses on charge, baryon number, and strangeness, as well as how these properties combine to form baryons, antibaryons, and mesons. Quark Properties Quarks are fundamental particles that make up protons, neutrons, and other hadrons.
The question
Determine the total charge, baryon number, and strangeness of a particle composed of two up quarks and one strange quark (uus).
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For questions involving calculations, show all steps clearly. For conceptual questions, provide concise and accurate answers. Always check your units and ensure that your final answer is in the correct form.
What the command words demand
- Determine
- Calculate or find the value of a specific property, such as charge or baryon number.
- Identify
- Recognize and name the components or properties of particles.
- Explain
- Provide a clear and concise description of a process or concept, including relevant details.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 2-3 minutes per mark for this topic to ensure you have enough time to think through each step and double-check your work.
- Identify the properties of each quark:0 marks
- - Up quark (u): Charge = + 2 / 3 , Baryon number = + 1 / 3 , Strangeness = 00 marks
- - Strange quark (s): Charge = - 1 / 3 , Baryon number = + 1 / 3 , Strangeness = -10 marks
- Calculate the total charge:0 marks
- - Total charge = 2 × (+ 2 / 3 ) + (- 1 / 3 ) = 4 / 3 - 1 / 3 = +11 mark
- Calculate the total baryon number:0 marks
- - Total baryon number = 2 × (+ 1 / 3 ) + (+ 1 / 3 ) = 2 / 3 + 1 / 3 = 11 mark
- Calculate the total strangeness:0 marks
- - Total strangeness = 0 + (-1) = -11 mark
Final answer: Total charge: +1, Total baryon number: 1, Total strangeness: -1
Work through every step correctly and you earn all 3 marks.
Another worked example
A particle is composed of one up quark and two anti-down quarks (u d̅d̅ ). Determine its total charge, baryon number, and strangeness.
- Identify the properties of each quark/antiquark:0 marks
- - Up quark (u): Charge = + 2 / 3 , Baryon number = + 1 / 3 , Strangeness = 00 marks
- - Anti-down quark ( d̅ ): Charge = + 1 / 3 , Baryon number = - 1 / 3 , Strangeness = 00 marks
- Calculate the total charge:0 marks
- - Total charge = (+ 2 / 3 ) + 2 × (+ 1 / 3 ) = 2 / 3 + 2 / 3 = + 4 / 31 mark
- Calculate the total baryon number:0 marks
- - Total baryon number = (+ 1 / 3 ) + 2 × (- 1 / 3 ) = 1 / 3 - 2 / 3 = - 1 / 31 mark
- Calculate the total strangeness:0 marks
- - Total strangeness = 0 + 0 + 0 = 01 mark
Final answer: Total charge: + 4 / 3 , Total baryon number: - 1 / 3 , Total strangeness: 0
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing the charge of up and down quarks
Why it happens: Students often mix up the charges of up (+ 2 / 3 ) and down (- 1 / 3 ) quarks.
Fix: Memorize the charge values: up quark (u) has + 2 / 3 , and down quark (d) has - 1 / 3 .
Incorrectly calculating the total charge of a particle
Why it happens: Students sometimes add or subtract charges incorrectly, leading to wrong totals.
Fix: Double-check your arithmetic when adding or subtracting fractional charges. Use a step-by-step approach to ensure accuracy.
Forgetting the baryon number of quarks and antiquarks
Why it happens: Students may overlook the fact that all quarks have a baryon number of + 1 / 3 and all antiquarks have - 1 / 3 .
Fix: Remember that each quark contributes + 1 / 3 to the baryon number, and each antiquark contributes - 1 / 3 .
Misunderstanding strangeness conservation
Why it happens: Students may think that strangeness is always conserved, but it is only conserved in strong and electromagnetic interactions, not weak interactions.
Fix: Understand that strangeness is conserved in strong and electromagnetic interactions but can change in weak interactions. Review examples of weak decays to reinforce this concept.
Confusing the components of mesons
Why it happens: Students may mix up the quark-antiquark pairs that form different mesons, such as pions and kaons.
Fix: Memorize the specific combinations: π + = u d̅ , π - = d u̅ , K + = u s̅ , and K - = s u̅ .
Incorrectly identifying the decay products of a neutron
Why it happens: Students may forget that a neutron decays into a proton, an electron, and an antineutrino, or they may confuse the roles of quarks in this process.
Fix: Review the decay process: n (udd) → p (uud) + e - + ν̅ e . Focus on how the down quark changes to an up quark and the emission of a W - boson.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Quark Properties | Add quark charges, baryon numbers, and strangeness to find a particle's overall properties. | 3 |
| Quark Property Calculation | Combine quark properties to find a particle's charge, baryon number, and strangeness. | 3 |
| Conservation Laws Check | Check each conservation law to identify which the particle decay obeys and violates. | 4 |
| Quark Composition Change | State the quark structure of the neutron and proton before and after beta-minus decay. | 4 |
| Total across these question types | 14 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.