What mistakes do students make with Fundamental particles?

Writing the neutrino as an antineutrino. The β⁺ product is the electron neutrino ν, which balances lepton number against the positron. Using an electron instead of a positron. β⁺ decay emits a positron, the antiparticle of the electron. Balancing charge incorrectly by treating the up quark as negative. The up quark charge is +2⁄3, not −2⁄3.

What are the exam tips for Fundamental particles in Physics OCR A-Level?

Write the quark-level equation and check charge on both sides before choosing an option. Check lepton number as well as charge; the neutrino type must match the emitted lepton. Remember that β⁺ decay reduces the proton number by one and increases the neutron number by one.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

(j) β⁺ decay in terms of a quark model; u → d + ⁰₊₁e + ν

Full specification wording

Fundamental particles: (j) β⁺ decay in terms of a quark model; u → d + ⁰₊₁e + ν

Beta-plus decay occurs in proton-rich nuclei when a proton converts into a neutron.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 10 of 12

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Identifies that in β⁺ decay an up quark transforms into a down quark, changing a proton into a neutron.
States the transformation u → d + ⁰₊₁e + ν with the positron and electron neutrino as products.
Confirms charge conservation: +2⁄3 = −1⁄3 + (+1) + 0.
Recognises that the positron and neutrino are leptons, not quarks, and that the weak interaction mediates the change.
Distinguishes β⁺ decay from β⁻ decay by the direction of quark flavour change and the emitted particles.

Common mistakes and how to put them right

Writing the neutrino as an antineutrino.
Put it right: The β⁺ product is the electron neutrino ν, which balances lepton number against the positron.
Using an electron instead of a positron.
Put it right: β⁺ decay emits a positron, the antiparticle of the electron.
Balancing charge incorrectly by treating the up quark as negative.
Put it right: The up quark charge is +2⁄3, not −2⁄3.

Exam tips

Write the quark-level equation and check charge on both sides before choosing an option.
Check lepton number as well as charge; the neutrino type must match the emitted lepton.
Remember that β⁺ decay reduces the proton number by one and increases the neutron number by one.

Try the lesson

The start of the Fundamental particles: (j) β⁺ decay in terms of a quark model; u → d + ⁰₊₁e + ν lesson, made from checked teaching notes for this specification point.

Listen

Beta-plus decay occurs in proton-rich nuclei when a proton converts into a neutron. In quark terms the proton is uud and the neutron is udd, so an up quark changes into a down quark. The transformation is u → d + ⁰₊₁e + ν, where ⁰₊₁e is the positron (the antiparticle of the electron) and ν is the electron neutrino. Charge balance is essential: the up quark has charge +2⁄3, while the down quark (−1⁄3), the positron (+1) and the neutrino (0) sum to +2⁄3. Lepton number balances because the positron has lepton number −1 and the neutrino has +1. The positron and neutrino are leptons emitted by the weak interaction, not quarks. A reliable method is to write the parent and daughter quark contents and confirm that exactly one quark flavour changes. A strong answer earns marks for these points. Identifies that in β⁺ decay an up quark transforms into a down quark, changing a proton into a neutron. States the transformation u → d + ⁰₊₁e + ν with the positron and electron neutrino as products. Confirms charge conservation: +2⁄3 = −1⁄3 + (+1) + 0. Recognises that the positron and neutrino are leptons, not quarks, and that the weak interaction mediates the change. Exam tip: Write the quark-level equation and check charge on both sides before choosing an option.

See it

A top-down flowchart. It starts with a node labelled 'Proton-rich nucleus', which leads to 'Proton (uud) converts to neutron (udd)', then to 'Up quark changes into down quark', then to the transformation 'u → d + ⁰₊₁e + ν'. From this transformation node, four branches lead to: 'Positron ⁰₊₁e emitted', 'Electron neutrino ν emitted', 'Charge balance: +2⁄3 = −1⁄3 + (+1) + 0', and 'Lepton number balance: positron −1, neutrino +1'. The two emission nodes both lead to a final node 'Positron and neutrino are leptons, not quarks'. — Shows how β⁺ decay proceeds from a proton-rich nucleus through the quark flavour change u → d with the emission of a positron and electron neutrino, including charge and lepton number balance.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. It starts with a node labelled 'Proton-rich nucleus', which leads to 'Proton (uud) converts to neutron (udd)', then to 'Up quark changes into down quark', then to the transformation 'u → d + ⁰₊₁e + ν'. From this transformation node, four branches lead to: 'Positron ⁰₊₁e emitted', 'Electron neutrino ν emitted', 'Charge balance: +2⁄3 = −1⁄3 + (+1) + 0', and 'Lepton number balance: positron −1, neutrino +1'. The two emission nodes both lead to a final node 'Positron and neutrino are leptons, not quarks'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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