Hi. It’s Mr. Andersen and this is AP Physics essentials video 57. It is on the strong nuclear
force. Remember there are four fundamental forces in the universe. We have gravity and
electromagnetism. They both operate at all scales with gravity dominating at the large
and electromagnetism dominating in the small. But we also have the strong and the weak nuclear
force. And it took scientists awhile to figure that out just because we do not live at the
scale of a nucleus. And so how is a strong nuclear force different than the others? Well
unlike gravity and electromagnetism, it only dominates at the very small scale. It is way
stronger than all of the other forces and it is what is holding the nucleus together
and then the components of the protons and neutrons inside the nucleus as well. And so
the fundamental problems scientists saw right away with the nucleus is that if you have
two positive charges and protons are going to be positive charges next to each other,
according to electromagnetic forces they should be pushing themselves apart. We should have
repulsion. But they do not just go flying apart. They are held together. And so there
must be a force, and we call that the strong nuclear force holding it together. It is the
greatest of all of the fundamental forces. Something like 130 times that of electromagnetism.
And so once we get to this really small scale, and that is where strong forces are going
to operate, it is going to take over. Now how small do I mean. On the order of a fentometer,
which is 1 times 10 to the negative 15 meters. And so once we get to the level of a nucleus
there is going to be a strong nuclear force and this is a force between all of the components
of the nucleus, both the protons and the neutrons, and it is holding it together. And even at
a smaller level, if we look at the components of those nucleons, the quarks, it is holding
those together to make the protons and the neutrons. And so how does a strong nuclear
force work? What we think is going on is that mesons are going to be exchanged between these
two protons. And so a meson is a quark and antiquark and it will kind of bounce between
the two, almost like a ping pong ball. And it holds them together. So we have this strong
force holding them together. Not only protons, but neutrons as well. And you know this, that
if we zoom into a proton itself it is made up of all these quarks. And what is holding
that together? We have these gluons that are literally gluing the proton together. And
so the strong nuclear force is even greater when we get to this small scale. And so we
really have two worlds at play here. If we have 2 protons that are far enough apart,
electromagnetism is going to push them apart. But once we get to the level of the strong
nuclear force it is going to pull them together. And how big is this circle here? It is about
two fentometers apart. Or it is about 2 and a half diameters of a proton apart. And so
what happens is as we push this proton close, electromagnetism is going to be that repulsion
you see, but once we get inside that barrier, that strong nuclear force is going to pull
them together. And so where is an example of us seeing that in science? Well you could
look inside the nucleus itself. And so if we are going with an atom that we are familiar
with, like hydrogen, hydrogen has one proton, and so it is going to sit inside this binding
area of the nucleus. What is binding it? It is going to be the strong nuclear force. But
let’s move to something like helium. Helium is going to have 4 nucleons here. And so what
happens when you have 4 nucleons? We have greater force, strong force between all of
these. And so you can see that the binding energy, if we are looking at helium is going
to increase. If we increase the number of nucleons, if we are looking at helium for
example, there is still a small nucleus because it is pulling it all together, but what eventually
happens is once we get nucleons inside there that are starting to move outside of this
scale, and you can see on this graph where that occurs, once we go past iron, now the
strong nuclear forces are not great enough to hold that together. Now electromagnetism
starts to take over and that is why we start to have radiation occur. These are not stable
anymore and so we are starting to lose bits of that nucleus. And so did you learn to identify
the strong force as the force that holds not only the nucleus together but the components
of the nucleus together? I hope so. And I hope that was helpful.