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AS · Practice questions · Quarks and leptons

What matter is built from.

Six original Cambridge-style questions on quark charges, composition and beta decay.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Fractional charges add up.

01
Analysis
[2 marks]

State the charge on an up quark and on a down quark.

  • Up quark: +2/3 e ✓
  • Down quark: −1/3 e ✓
02
Analysis
[2 marks]

A proton has the quark composition uud. Show that its charge is +1 e.

  • Charge = (+2/3) + (+2/3) + (−1/3) e ✓
  • = (+4/3 − 1/3) e = +1 e ✓
03
Analysis
[2 marks]

A neutron has the quark composition udd. Show that its charge is zero.

  • Charge = (+2/3) + (−1/3) + (−1/3) e ✓
  • = (+2/3 − 2/3) e = 0 ✓
04
Analysis
[2 marks]

State the difference between a baryon and a meson in terms of quark content.

  • A baryon is made of three quarks (qqq) ✓
  • A meson is made of a quark and an antiquark ✓
05
Analysis
[3 marks]

In beta-minus decay a neutron changes into a proton. Describe the quark change involved and show it is consistent with the change in composition.

  • A down quark changes into an up quark ✓
  • udd (neutron) becomes uud (proton), which is the change from down to up in one quark ✓
06
Analysis
[2 marks]

Name two leptons and state what makes them fundamental.

  • For example, the electron and the (electron) neutrino ✓
  • They are fundamental because they are not made of quarks or any smaller particles ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.