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  • 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.

Hi. It's Mr. Andersen and this is AP Physics essential video 2. It is on fundamental particles


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B2 中上級

基本粒子 (Fundamental Particles)

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