2023 A/L Physics Past Paper · Question 08 (English Medium)

Answer: (2) $udd$

What the question asks

We want: which quarks make up a neutron.

What each letter means:

$u$: the up quark, charge $+\tfrac{2}{3}$ of the unit charge $e$.

$d$: the down quark, charge $-\tfrac{1}{3}$ of the unit charge $e$.

$\bar{u}$, $\bar{d}$: the anti-quark of $u$ and of $d$.

Relevant theory

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

of charge -1/3.

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

charge of the neutron is zero.

Reading the options

For example, think of three coins that you put together to make a total.

Two coins of $+2$ and one coin of $-1$ make a total of $+3$.

One coin of $+2$ and two coins of $-1$ make a total of $0$.

A quark is like a coin, with its own charge. The total charge tells you which mix of quarks you have.

The proton has the first mix, and so its charge is not zero. The neutron has the second mix, and so its charge is zero.

The quote gives the neutron as one $u$ quark and two $d$ quarks.

Test each option against the quote and the charge.

(1) $uud$: two $u$ and one $d$. This is the proton, with charge $+1$.

(2) $udd$: one $u$ and two $d$. This matches the quote.

$$\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$$

(3) $uuu$: charge $+2$.

(4) $\bar{u}\bar{u}\bar{u}$: anti-quarks, charge $-2$.

(5) $\bar{d}\bar{d}\bar{d}$: anti-quarks, charge $+1$.

Only option (2) matches.

Why the other options fail

$uud$ is two $u$ quarks and one $d$ quark. That is the proton, with charge $+1$.

The neutron has one $u$ quark and two $d$ quarks. Three $u$ quarks would give charge $+2$.

The neutron has one $u$ quark and two $d$ quarks, not three anti-up quarks.

The neutron has one $u$ quark and two $d$ quarks, not three anti-down quarks.

Conclusion

$udd$

A neutron is one $u$ quark and two $d$ quarks.

The answer is (2).

Add the quark charges to check. $u$ is $+\tfrac{2}{3}$ and $d$ is $-\tfrac{1}{3}$.

$uud$ gives $+1$, the proton. $udd$ gives $0$, the neutron.

Exam tip