2019 A/L Physics Past Paper · Question 04 (English Medium)

Answer: (5) 6 quarks and 6 leptons.

What the question asks

Matter is made of fundamental particles.

They have no smaller parts inside.

They come in two types.

Find how many there are of each type.

Relevant theory

The two types are quarks and leptons.

The charge of a quark is $\tfrac{1}{3}$ or $\tfrac{2}{3}$ of the electron charge.

Quarks feel the strong force.

Leptons do not.

The electron is a lepton.

A proton and a neutron are each made of three quarks.

There are two types of fermions. They are leptons and quarks.

Reading the options

Sort each pair by its properties.

A fractional charge means a quark.

If not, it is a lepton.

Up ($u$), down ($d$)

Quark: generation 1

Charges $+\tfrac{2}{3}e$, $-\tfrac{1}{3}e$

Charm ($c$), strange ($s$)

Quark: generation 2

Top ($t$), bottom ($b$)

Quark: generation 3

Electron ($e^-$), its neutrino ($\nu_e$)

Lepton: generation 1

No strong interaction

Muon ($\mu^-$), its neutrino ($\nu_\mu$)

Lepton: generation 2

Tau ($\tau^-$), its neutrino ($\nu_\tau$)

Lepton: generation 3

Count the quarks: $3 \times 2 = 6$.

Count the leptons: $3 \times 2 = 6$.

Now compare with the options.

(1) and (2) have only one type.

(3) has 4 of each.

(4) has only 4 leptons.

Only (5) has 6 of each.

Why the other options fail

This option has no leptons.

But the electron in every atom is a lepton.

This option has no quarks.

But a proton and a neutron are made of quarks.

This option counts only two generations.

There are three. Each has 2 quarks and 2 leptons.

This option leaves out some leptons.

Each of the three charged leptons has its own neutrino.

That makes 6 leptons.

Conclusion

There are 3 generations.

Each has 2 quarks and 2 leptons.

So there are 6 quarks and 6 leptons. The answer is (5).

Remember the three generations.

Quarks: ($u$, $d$), ($c$, $s$), ($t$, $b$).

Leptons: ($e$, $\nu_e$), ($\mu$, $\nu_\mu$), ($\tau$, $\nu_\tau$).

Exam tip