Question
It is now believed that protons and neutrons (which constitute nuclei of ordinary matter) are themselves built out of more elementary units called quarks. A proton and a neutron consist of three quarks each. Two types of quarks, the so called ‘up’ quark (denoted by u) of charge + (2/3) e, and the ‘down’ quark (denoted by d) of charge (-1/3) e, together with electrons build up ordinary matter. (Quarks of other types have also been found which give rise to different unusual varieties of matter.) Suggest a possible quark composition of a proton and a neutron.
Solution
Charge on 'up' quark,
Charge on 'down' quark,
Charge on a proton = e
Charge on a neutron = 0
Let a proton contains x ‘up’ quarks and (3 - x) ‘down’ quarks. Then total charge on a proton is
x = 2
and 3 - x = 3 - 2 = 1
i.e., proton contains 2 ‘up’ quarks and 1 ‘down’ quark. Its quark composition should be uud.
Let a neutron contains y ‘up’ quarks and (3 – y) ‘down’ quarks. Then total charge on a neutron is
and 3 – y = 3 – 1 = 2
i.e., neutrons contain 1 ‘up’ quark and 2 ‘down’ quarks. Its quark composition should be udd.
