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.
