字幕列表 影片播放 列印英文字幕 Hi. It's Mr. Andersen and this is AP Physics essential video 2. It is on fundamental particles which are particles that have no internal structure. So if I were to ask you the following question, this is a sample of pure iron, if I were to say is that a fundamental particle? Hopefully you would say no. You know that iron is made up of atoms. Now is the atom a fundamental particle? No. You know that an atom is made up of an atomic nucleus surrounded by these negative electrons. And so are those fundamental? Well you know that the nucleus itself is made up of protons and neutrons. And if we look into those protons and neutrons, you may not know it but they are made up of quarks themselves. A proton is made up of two ups and one down quark. And a neutron is made of two downs and one up quark. And so when we finally get to the level of quarks and electrons we are at the level of fundamental particles. In other words particles in physics that have no internal structure. And so atoms are made up of subatomic particles, electrons, protons and neutrons. And not all of these are fundamental particles. And so an electron is because it has no internal structure. But protons and neutrons are made up of quarks. And those quarks themselves have no internal structure. And so they are fundamental but the protons and neutrons are not. Now when we get to the level of the quarks we will find that they have properties like mass and spin and charge. And we can sum up those charges and it tells us a lot about the overall charge of the protons and the neutrons. What are some other fundamental particles? Photons are going to be little quanta of light or electromagnetic radiation. Neutrinos are another type of fundamental particle. And so what makes up matter at the smallest level are these fundamental particles. And so you can think of that cube of purified iron as a system made up of other objects. And we have to dig into that system to find the most fundamental of those objects. We know that in a sample of iron there are going to be 26 protons. And then we are going to have about 30 neutrons in a typical nucleus. As we zoom into that we finally get our first fundamental particle, the electron. But we have to dig farther into the proton and the neutron to find the fundamental particles, those quarks on the inside. And so here is a data set on an up quark and a down quark. You can see what their mass is, what their charge is and then what their spin is. And so if I want to figure out what is the charge of a proton, I am going to have to add up all three of these fundamental particles. And so if I take the charge of two up quarks, two-thirds times two, that is four-thirds. And then I am going to subtract the charge of one down quark, which is going to be four-thirds minus one-third, I get three-thirds or a positive one charge of a proton. It is coming from those fundamental particles. Likewise with the neutron, we have got one up, which is two-thirds positive charge minus two one-third charges for the two down quarks. And so a neutron is going to have no charge. And is if you are trying to ask a question about the behavior of protons and neutrons at this really really tiny level we have to understand what they are made up of. We have to understand these fundamental particles. And so physicists have come up with this standard model of understanding matter at the subatomic level. And it has spawned all these fundamental particles. Some you are totally familiar with. And so we have an electron for example or a photon. Some you have just heard a little bit about like these six types of quarks that make up matter. But some are brand new to you and are brand new to science. For example the top quark was discovered in 1995. The tau neutrino is 2000. And if you have been reading the paper, the Higgs boson was discovered in 2013 using the Large Hadron Collider. And so you do not have to memorize all of the charges and masses or names of these fundamental particles. But you should understand this, the difference between a fundamental particle such as a quark, and a system composed of fundamental particles like an atom itself. And I hope that was helpful.