2024 A/L Physics Past Paper · Question 06 (English Medium)

Answer: (3) only (A) and (C) are true.

What the question asks

We want: which of the statements (A), (B) and (C) are true for a muon.

What each letter means:

$\mu^-$: the muon, with a charge of $-1$.

$e^-$: the electron.

$\text{MeV/c}^2$: the unit used for the mass of a particle.

Relevant theory

There are two types of fermions. They are leptons and quarks. The electron belongs to

the fermion category while the proton and neutron are a combination of three quarks.

e (electron, 0.511 MeV/c², -1), μ (muon, 106 MeV/c², -1)

Reading the options

The first quote says that there are two types of fermions, leptons and quarks, and that a proton or a neutron is a combination of three quarks. The second quote is from the table of fundamental particles: the electron has a mass of $0.511\ \text{MeV/c}^2$, and the muon, a lepton like the electron, has a mass of $106\ \text{MeV/c}^2$.

For example, think of a box of building blocks. Some blocks are single pieces that cannot be pulled apart. Others are built from three small pieces snapped together.

The electron and the muon are single pieces. The proton is three pieces snapped together.

Reasoning and calculation

It is a lepton.

In the table of fundamental particles the muon $\mu$ stands in the leptons, next to the electron, with the same charge $-1$. So the muon is a lepton.

A

It is made out of three quarks.

Leptons and quarks are the two types of fermions. A lepton is itself a fundamental particle, not made of quarks. Three quarks make a proton or a neutron. So the muon is not made of three quarks.

B

Its mass is larger than that of an electron.

The muon has a mass of $106\ \text{MeV/c}^2$ and the electron $0.511\ \text{MeV/c}^2$. $106 \div 0.511$ is about $207$, so the muon is about $207$ times as heavy as the electron.

C

Statement (A) is true, because the muon is in the lepton row of the table.

Statement (B) is false, because a lepton is not made of quarks.

Statement (C) is true, because $106\ \text{MeV/c}^2$ is larger than $0.511\ \text{MeV/c}^2$.

So just (A) and (C) are true.

Why the other options fail

(C) is also true: the muon, $106\ \text{MeV/c}^2$, is heavier than the electron, $0.511\ \text{MeV/c}^2$.

(B) is false: three quarks make a proton or a neutron, not a lepton.

(A) is true: the muon is a lepton, so it is not made of three quarks.

(B) is false, so the three are not true together. A lepton is not made of quarks.

Conclusion

The muon is a lepton, it is not made of quarks, and it is heavier than the electron. (A) and (C) are true.

The answer is (3).

The muon is like a heavy electron: same charge, same family (leptons), about $207$ times the mass.

Three quarks make protons and neutrons, never leptons.

Exam tip