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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the properties of each quark:
- S2
- Up quark (u): Charge = + 2 / 3 , Baryon number = + 1 / 3 , Strangeness = 0
- S3
- Strange quark (s): Charge = - 1 / 3 , Baryon number = + 1 / 3 , Strangeness = -1
- S4
Calculate the total charge:
- S5
- Total charge = 2 × (+ 2 / 3 ) + (- 1 / 3 ) = 4 / 3 - 1 / 3 = +1
- S6
Calculate the total baryon number:
- S7
- Total baryon number = 2 × (+ 1 / 3 ) + (+ 1 / 3 ) = 2 / 3 + 1 / 3 = 1
- S8
Calculate the total strangeness:
- S9
- Total strangeness = 0 + (-1) = -1
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the properties of each quark:
- S2
- Up quark (u): Charge = + 2 / 3 , Baryon number = + 1 / 3 , Strangeness = 0
- S3
- Strange quark (s): Charge = - 1 / 3 , Baryon number = + 1 / 3 , Strangeness = -1
- S4
Calculate the total charge:
- S5
- Total charge = 2 × (+ 2 / 3 ) + (- 1 / 3 ) = 4 / 3 - 1 / 3 = +1
- S6
Calculate the total baryon number:
- S7
- Total baryon number = 2 × (+ 1 / 3 ) + (+ 1 / 3 ) = 2 / 3 + 1 / 3 = 1
- S8
Calculate the total strangeness:
- S9
- Total strangeness = 0 + (-1) = -1
Final answer: Total charge: +1, Total baryon number: 1, Total strangeness: -1
Common mistakes
- Confusing the charge of up and down quarks — 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 — 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 — Remember that each quark contributes + 1 / 3 to the baryon number, and each antiquark contributes - 1 / 3 .
- Misunderstanding strangeness conservation — 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 — 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 — 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
- Full worked solution (all marking points)3 marks