OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(k) balancing of quark transformation equations in terms of charge
Full specification wording
Fundamental particles: (k) balancing of quark transformation equations in terms of charge
Quark transformation equations must balance in charge.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 11 of 12
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Quark transformation equations must balance in charge. Each quark has a fractional charge: up-type quarks have charge +2⁄3 and down-type quarks have charge −1⁄3, while antiquarks have the opposite sign. Leptons such as the electron have charge −1, the positron +1, and neutrinos zero. To balance an equation, add the charges on the left and set them equal to the sum on the right. For example, in d → u + ⁰₋₁e + ν̄ the left side is −1⁄3 and the right side is +2⁄3 + (−1) + 0 = −1⁄3. A systematic method is to write the charge above each symbol, sum each side, and adjust the lepton charges until they match. This check also helps identify whether a proposed transformation is physically possible.
What earns the marks
- States the fractional charges of up-type (+2⁄3) and down-type (−1⁄3) quarks and the opposite charges of their antiquarks.
- States the charges of the electron (−1), positron (+1) and neutrinos (0).
- Adds charges on each side of a quark transformation equation and shows that the totals are equal.
- Uses charge balance to test whether a proposed quark transformation is valid.
- Applies the method to both β⁻ and β⁺ transformations without confusing the sign of the emitted lepton.
Common mistakes and how to put them right
Forgetting that quark charges are fractional: treating an up quark as +1 and a down quark as −1 gives incorrect totals.
Ignoring the charge of the emitted lepton.
Put it right: The electron or positron contributes ±1 and must be included in the sum.
Reversing the sign of the antineutrino or neutrino.
Put it right: Neutrinos have zero charge, so their sign does not affect charge balance, but their particle type still matters for lepton number.
Exam tips
- Write the charge above every symbol before adding, so no term is missed.
- Check both charge and lepton number; charge balance alone does not guarantee a valid equation.
- If the totals do not match, look first at the sign of the emitted lepton and the quark flavour charge.
Try the lesson
The start of the Fundamental particles: (k) balancing of quark transformation equations in terms of charge lesson, made from checked teaching notes for this specification point.
Listen
Quark transformation equations must balance in charge. Each quark has a fractional charge: up-type quarks have charge +2⁄3 and down-type quarks have charge −1⁄3, while antiquarks have the opposite sign. Leptons such as the electron have charge −1, the positron +1, and neutrinos zero. To balance an equation, add the charges on the left and set them equal to the sum on the right. For example, in d → u + ⁰₋₁e + ν̄ the left side is −1⁄3 and the right side is +2⁄3 + (−1) + 0 = −1⁄3. A systematic method is to write the charge above each symbol, sum each side, and adjust the lepton charges until they match. This check also helps identify whether a proposed transformation is physically possible. A strong answer earns marks for these points. States the fractional charges of up-type (+2⁄3) and down-type (−1⁄3) quarks and the opposite charges of their antiquarks. States the charges of the electron (−1), positron (+1) and neutrinos (0). Adds charges on each side of a quark transformation equation and shows that the totals are equal. Uses charge balance to test whether a proposed quark transformation is valid. Exam tip: Write the charge above every symbol before adding, so no term is missed.
See it
Key words
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Fractional quark charge
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Lepton charge
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Equation balancing as a validity test
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See every definition
- Fractional quark charge
- Up-type quarks have charge +2⁄3 and down-type quarks have charge −1⁄3, while antiquarks have the opposite sign.
- Lepton charge
- The electron has charge −1, the positron +1, and neutrinos zero.
- Equation balancing as a validity test
- Adding the charges on each side of a quark transformation equation and showing the totals are equal, which also helps identify whether a proposed transformation is physically possible.
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