Video: Relative Charge of Subatomic Particles

Which of the following particles have a relative charge of +2/3? [A] Top antiquark [B] Down quark [C] Top quark [D] Up quark [E] Electron [F] Strange antiquark [G] Charm quark


Video Transcript

Which of the following particles have a relative charge of positive two-thirds? Top antiquark, down quark, top quark, up quark, electron, strange antiquark, charm quark.

Okay, one of the first things we can notice here is that we’re looking for particles that have a fractional relative charge, a relative charge that’s not a whole number. This is the hallmark of a quark. And we see that our list is composed entirely of quarks or antiquarks, with one exception. An electron is also listed here. But we can recall that an electron has a relative charge of negative one. And so, right away, we can cross out this option because an electron does not have the relative charge we’re looking for. That leaves us with the quark and antiquark types listed.

At this point, let’s recall that there are six types of quark: up and down, charm and strange, and top and bottom. We’ve arranged these types in such a way that all three in the top row have the same relative charge, positive two-thirds, and all three in the bottom row also have the same relative charge of negative one-third. This helps us answer this question because we see that an up, a charm, or a top quark all will have the relative charge we’re looking for. And in fact, on our list of possible answers, we see all three of these quark types. And then from this table, we can also say that if we find down, strange, or bottom quarks in our list of answer options, we can cross those out because they don’t have the relative charge we want. And we indeed see that the down quark was an option, but we’ll cross that out, knowing now what we know about it.

And this leaves us with these two antiquarks. One property of the antiquark of a given quark type is that it has the opposite relative charge, that is the same magnitude but the opposite sign. This means that the relative charge of the up antiquark, the charm antiquark, and the top antiquark is negative two-thirds and then that of the other three antiquark types is positive one-third. All this to say there are no antiquarks that have the relative charge we’re looking for. None of them have a relative charge of positive two-thirds, so we can cancel these options off of our list. Our answer then is that only the top quark, the up quark, and the charm quark have a relative charge of positive two-thirds.

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