2020 A/L Physics Past Paper · Question 06 (English Medium)

Answer: (4) uud, udd

What the question asks

We must find: which three quarks make up a proton and a neutron — "respectively" means the proton first, then the neutron.

Relevant theory

the proton and neutron are a combination of three quarks.

An

important property of quark particles is that the magnitude of their charge is 1/3 or 2/3 of

the charge of an electron.

The proton is made up of two u quark particles of charge +2/3 and one d quark particles

of charge -1/3. So, the charge of the portion is +1.

The neutron is made up one u quark particle and two ൽ quark particles. Therefore, the

charge of the neutron is zero.

According to this, most of the matter in the universe are

made up of u and d quarks and electrons. All the other fundamental particles can be

observed only in the collisions between cosmic rays and high energy particles.

Reading the options

Checking by charge ($u = +\tfrac{2}{3}e$, $d = -\tfrac{1}{3}e$): $uud$: $\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$ — proton ✓; $udd$: $\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$ — neutron ✓. $The\,order\,(p,\; n) =$ ($uud$, $udd$) — option (4).

Why the other options fail

s quarks do not occur in ordinary matter.

$udd$ is the neutron (charge 0); $uus$ contains s.

$ssd$ contains s; $uud$ is the proton, not the neutron.

The order is swapped: $udd$ is not the proton.

Conclusion

A proton $= uud$ (charge $+1$), a neutron $= udd$ (charge $0$) — the answer is (4).

The quark content can be checked by charge: a proton's charge should come to $+1$, a neutron's to $0$.

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