Transcription
In the next three talks, we're going to be looking at three of the four fundamental forces or fundamental interactions. And we'll start with the strong force before moving on to the weak and the electromagnetic force. And we're going to go at a level that is applicable for our Radiology physics syllabus. So let's start with the strong force.
Now, as we looked at in our previous talk, when we separated particles into matter and force, we said that the majority of matter in our universe is made up of the up and down quarks. And our hadrons, our protons and our neutrons, are comprised of up and down quarks. Now, it's the strong force that allows these up and down quarks to sit tightly packed together, making up the proton and the neutron respectively. And we said that the boson, the gluon, is the force carrier for that strong force. So that strong force allows protons to stay together as protons and neutrons to stay together as neutrons. We'll look at the same force, a residual force, that allows protons and neutrons to sit so tightly packed together within the nucleus of an atom. You may have wondered how positively charged protons can sit so closely next to one another and not repel away with the electromagnetic force. And it's a strong force of the nuclear force that allows this to happen.
So let's have a closer look at our hadrons, our protons and our neutrons. We see our proton is made of two up and one down quark. And our neutron is made up of one up and two down quarks. If we look at the proton closer, I said it's the gluon that is a force carrier that allows the quarks to sit so closely together. Now, the strong force is best thought of as an elastic band. The longer the elastic band gets, the taller the elastic band gets, the stronger that force gets. So the further the up and down quarks get away from each other, the stronger that strong force is keeping them together. It's not like magnetism where the closer we get, the harder or stronger the force gets. Here, the further we get, the stronger the force gets as we stretch that elastic band or resistance band. That force gets stronger and stronger.
Now, in modern physics, we assign properties to particles. Some of them you would have heard of before, such as charge or mass or spin. And another property we give to subatomic particles is what's known as color. Now, color is beyond the scope of this talk, but it's relevant for the strong force. Now, what the gluon does is it oscillates between the up and down quarks and changes that fundamental property of each quark known as color. Now, think of color as something such as charge or spin and mass. It's a fundamental property. If we try and describe charge in our head, it's very difficult to say what charge actually is. It's a property of that subatomic particle, the same as color.
Now, as that gluon moves between the various up and down quarks within the proton, it causes the color of those quarks to change. And as those quarks separate from one another more, that gluon brings those closer and closer together. It's an extremely strong force keeping these quarks together.
Now, I mentioned that the gluon keeps the quarks together within the proton and the neutron respectively. But that force is so strong that there's a residual force that extends outside the bounds of the hadrons. Now, we call that the nuclear force. It's still the strong force, but it's a residual force that extends beyond the bounds of the neutron and the proton respectively. And that is what allows protons to sit so tightly packed to one another because the strong force is so much stronger than that electromagnetic force of repulsion between those positively charged protons. So here we've got our gluons keeping our neutrons and protons together. And that residual strong force, now known as the nuclear force, is what's keeping the nucleus together.
Now, the nuclear force is interesting because it not only keeps the nucleus closely packed together, but it also actually gives the nucleus some mass. When the neutrons and protons get too close to one another, that nuclear force actually repels those protons and neutrons away, giving the nucleus some form of mass. So we can actually represent that graphically. We see force represented on our y-axis here. Anything that's positive on the y-axis is a repulsive force. And anything that's negative on the y-axis is an attractive force. And here is the distance between hadrons. You see when the distance is really small, when the neutrons and protons get too close together, there's a strong force of repulsion between them, pushing them away from one another. But as we get to this distance between 10 to the minus 16 and 10 to the minus 15 meters, that force then becomes attractive. So it keeps the protons and neutrons within a specific distance, about 10 to the minus 16 meters apart from one another. And that's a really strong force of attraction between those two hadrons, known as the nuclear force. It allows those protons to be so close to one another. That force is much stronger than the electromagnetic force of repulsion between the two. And you'll see when we look at nuclear stability later on, when we add more and more neutrons and we increase that distance, our nucleus then starts to become unstable.
So that's about all we really need to know about the strong force. Now, again, I'm making this content available here on YouTube. If you want to test yourself with actual past paper questions, the first line in the description will be a link to my question bank course where I go through the questions, answering them like this. All these questions have come up in multiple exams before. Otherwise, I'll see you in the next talk where we're going to talk about the weak force. I'll see you there.