Properties of Quarks

The charge of a hadron is determined by the sum of the charges of its quarks
Each flavour of quark has a certain relative charge:

Relative charge of quarks

Each flavour of quark has a charge of either +⅔e or -⅓e

For example, a proton is made up of two up quarks and a down quark. Adding up their charges gives the charge of a proton:

$uud = p$

$+ \frac{2}{3} + \frac{2}{3} - \frac{1}{3} = + 1 e$

The equivalent antiparticle of the quark is the anti-quark
These are identical to quarks except with opposite relative charges

Anti-quarks

Charge of anti-quarks, downloadable AS & A Level Physics revision notes

Each flavour of anti-quark has a charge of either -⅔e or +⅓e. The quark composition of anti-protons and anti-neutrons changes to anti-quarks

Worked example

Particles are made up of a combination of three quarks or two quarks. Which quark combination would not give a particle a charge of -1 or 0?

A. up, strange, strange

B. charm, charm, down

C. top, anti-up

D. anti-up, anti-up, anti-strange

Answer: B

Add up the charge of the individual quarks
- Charm, charm, down = $\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = + 1$
The other quark combinations add up to either 0 or −1
- A: Up, strange, strange = $\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0$
- C: Top, anti-up = $+ \frac{2}{3} - \frac{2}{3} = 0$
- D: Anti-up, anti-up, anti-strange
  - Remember that anti-particles have the opposite signs

Examiner Tip

You will be expected to remember the charge of each quark. However, instead of memorising the charges of anti-quarks too, just remember they are identical but with opposite signs.

Properties of Quarks (CIE AS Physics)

Revision Note