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.