Video Transcript
Which of the following is composed
of three quarks in the form uud? (A) A proton, (B) an electron, (C)
beta particles, (D) alpha particles, or (E) a neutron.
Quarks are subatomic particles with
a fractional electric charge and are fundamental constituents of matter. There are six types or flavors of
quark, which are up, down, charm, strange, top, and bottom, and they are given the
symbols u, d, c, s, t, and b. The question asks, “Which of the
options is composed of three quarks in the form uud?” So we’re only interested in two of
the types of quark, up and down.
Option (A) is a proton. Protons and neutrons are composed
of quarks. Protons and neutrons are
exclusively formed of a combination of three up and down quarks. So option (A) a proton or option
(E) a neutron could be the answer to this question.
Option (B) is an electron. An electron is an elementary or
fundamental particle. This means that it’s not made from
other particles. So it cannot be composed of three
quarks. So option (B) an electron is not
the answer to this question. Option (C) is beta particles. Beta particles are high-energy
electrons or positrons. We have already established that an
electron is a fundamental particle, so it cannot be composed of quarks. A positron is an antiparticle to
the electron. It has the same mass as an
electron, but it has the opposite charge to an electron. Thus, it has a plus one charge. Positrons, like electrons, are not
composed of quarks. So we know that option (C) beta
particles is not the answer to this question.
Option (D) is alpha particles. An alpha particle is composed of
two protons and two neutrons, so it’s identical to a helium four nucleus. Since each proton or neutron is
composed of three quarks, an alpha particle must consist of 12 quarks. Since the question asks us “Which
of the following is composed of three quarks?” we know that option (D) alpha
particles cannot be the answer. So we are left to figure out if the
answer to this question is (A) a proton or (E) a neutron.
We can do this by looking at the
charges on the u and d quarks. The charge on the up quark is plus
two-thirds 𝑒, where 𝑒 is the elementary charge and is defined as the electrical
charge carried by a single proton. It has a value of 1.602 times 10 to
the power of minus 19 coulombs. A down quark has a charge of minus
one-third 𝑒. The definition of an elementary
charge tells us that a proton has the charge 𝑒. This is written as plus one 𝑒 or
sometimes just plus one. A neutron, on the other hand, has
no charge. So for quarks in the form uud, the
charge would be plus two-thirds 𝑒 plus two-thirds 𝑒 minus one-third 𝑒.
If we calculate the answer to this
sum, we get plus three over three 𝑒, which is equivalent to plus one 𝑒. This is the same as a charge on a
proton. So a proton is composed of two up
quarks and one down quark. A neutron, on the other hand,
contains one up quark and two down quarks. If we total the charges for one up
quark and two down quarks, we get zero, which is the charge on a neutron. We have used the electric charges
on up and down quarks to show that a proton contains two up quarks and one down
quark.
So the answer to the question
“Which of the following is composed of three quarks in the form uud?” is (A) a
proton.
