2022 A/L Physics Past Paper · Question 07 (English Medium)

Answer: (4) $-\dfrac{2}{3}e$

What the question asks

We want: the charge of the $\bar{u}$ quark, the anti-quark of the $u$ quark.

What each letter means:

$e$ ($\text{C}$): the elementary charge, the size of the charge of an electron.

$q_u$ ($e$): charge of a $u$ quark.

$q_{\bar{u}}$ ($e$): charge of a $\bar{u}$ quark.

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.

For each of these particles there is an anti-

particle which is completely opposite to it.

Reading the options

For example, think of money in your pocket and a debt of the same size.

The debt is the exact opposite of the money. If the money is $+10$, the debt is $-10$.

An anti-particle is like the debt. Every property that can be plus or minus, such as charge, flips its sign.

$$q_{\bar{u}} = -\,q_u$$

In words: the charge of the anti-quark is the opposite of the charge of the quark.

The book gives $q_u = +\dfrac{2}{3}e$. So the anti-quark has the opposite sign.

Put in the charge of the $u$ quark:

$$q_{\bar{u}} = -\left(+\dfrac{2}{3}e\right) = -\dfrac{2}{3}e$$

This is option (4).

Why the other options fail

An anti-particle is the opposite of its particle, so the charge is $-\dfrac{2}{3}e$, not zero.

$+\dfrac{1}{3}e$ is the charge of an anti-$d$ quark. The $\bar{u}$ quark is the opposite of $u$.

$+\dfrac{2}{3}e$ is the charge of the $u$ quark. The anti-quark has the opposite sign.

$-\dfrac{1}{3}e$ is the charge of the $d$ quark, which is a different quark.

Conclusion

The $u$ quark has charge $+\dfrac{2}{3}e$. The anti-quark is the exact opposite.

$$q_{\bar{u}} = -\dfrac{2}{3}e$$

The answer is (4).

An anti-particle is completely opposite to its particle, so its charge has the opposite sign.

Find the charge of the quark first, then flip the sign.

Exam tip