Which subatomic particle has a positive charge, consists of two up quarks and one down quark, and is linked to the so-called spin crisis?

• A. Protons
• B. Neutrons
• C. Electrons
• D. Quarks

Answer: (A)

Think about what makes up a proton and how its charge arises from its quarks. A proton contains two up quarks and one down quark. Up quarks carry +2/3 of a fundamental charge each, and a down quark carries −1/3. So the total charge is +2/3 + +2/3 + (−1/3) = +1, making the proton positively charged. The spin crisis refers to the discovery that the spins of the quarks inside the proton account for only a portion of the proton’s overall spin, implying that gluon spin and the orbital motion of quarks and gluons also contribute. So the particle described is the proton. Neutrons have a different quark composition (udd) and are neutral, electrons are not made of quarks, and quarks are the fundamental constituents rather than a single particle.