What is the charge of an up quark and the charge of a down quark? How many quarks make up a proton and a neutron?
Protons and neutrons are each composed of three quarks. Protons are made up of two 'up' quarks and one 'down' quark while neutrons are made up of two 'down' quarks and one 'up' quark.
Quarks carry fractional electrical charges. An 'up' quark has a charge of +2/3 and a 'down' quark has a charge of -1/3. Is this consistent with what we know about protons and neutrons? Remember that protons carry an electrical charge of +1 while neutrons carry no electrical charge. We said earlier that a proton has two 'up' quarks and one 'down' quark, so it has a total charge of (+2/3) + (+2/3) + (-1/3) = +1. We also said that a neutron has two 'down' quarks and one 'up' quark, so it has a total charge of (-1/3) + (-1/3) + (+ 2/3) = 0. Happily, both the proton and the neutron 'end up' with the charge they should have.
Steve Gagnon, Science Education Specialist (Other answers by Steve Gagnon)