Exchange Particles and Feynman Diagrams – A Level Physics



hello today we're continuing in our a level physics revision series looking at exchange particles and Fineman diagrams in an earlier video I set out what's called the standard model which shows all the elementary particles that we're aware of there are six quarks or quarks there are six leptons electrons and neutrinos but there is also a category this column here which are called gauge bosons and they are responsible for the forces that we observe in the world today let's take for example two electrons if we bring them together there will be a repulsive force because like poles or like charges repel but how do these electrons know that the other electron is there there has to be some kind of information exchanged so that they are aware and we say that there is what's called an exchange of particles or exchange bosons gauge bosons in this case it would be the virtual photon that was responsible for exchanging information and is essentially what's called the force carrier now we are aware of four types of forces on in the world one is the strong nuclear force which I described earlier second the electromagnetic is an electromagnetic force which is responsible for the repulsion of two electrons thirdly the weak force which is responsible for keeping the Sun going and finally gravitational force which keeps us on the earth now the gauge bosons responsible for exchanging information are these for the strong nuclear it's the gluon for the electromagnetic force it's the photon for the weak force it's actually the W plus and minus and the Z both of those are responsible for the weak force and in fact we don't know because we've never found what the boson is for gravity we think it's a thing called a graviton but there's no evidence for that and for that reason it doesn't feature in the standard model only hydrants are affected by the gravity by the strong nuclear force hadrons are basically everything made up of quarks but electrons and neutrinos do not feel the strong nuclear force so this only applies to head runs every charged particle will feel the electromagnetic force so if you're neutral you won't feel it but if you've got a charge you will everything is affected by the weak nuclear force and everything is affected by gravity now the larger or the heavier the gauge boson is the short of the range of the force that exists so in the case of a photon that is massless and consequently it has infinite range sorry I just noticed the Sun was distorting the picture so I've taken action to stop that I said that the photon has is massless consequently because it's massless it has infinite range by contrast the W plus minus and the Z bosons are extremely heavy 80 times bigger than a proton so they have a very very short range indeed in an earlier video I explained something about the weak force I said that a neutron can decay into a proton plus an electron plus an anti-electron neutrino but that in fact is not quite what happens what happens is that the neutrino is sorry the neutron Caze into a proton plus a w- boson but that w but- boson very quickly decays in about ten to the minus twenty five of a second into the electron plus the anti electron neutrino and then called richard fineman found a way of expressing this in a much simpler way by pictures on the basis that picture paints a thousand words Fineman diagrams are used to describe part of the particle interactions and the rules of a Fineman diagram are that time always goes upwards so you always start at the bottom with your incoming particles and outgoing particles go out at the top and the interaction takes place in the middle so let's look at exactly what happens when a neutron becomes a proton plus an electron plus an anti-electron neutrino the Fineman diagram says you start off with a neutron coming in you have a proton going out you have a wiggly line bosons always shown by wiggly lines particles are shown by straight lines this is a w- boson which quickly decays into an electron and an anti-electron neutrino and both of those go out so they're going upwards everything goes upwards and that's the Fineman diagram for this reaction in the Sun it's slightly different you have a proton being converted into a neutron plus a positron plus an ordinary electron neutrino so what's the Fineman diagram for that well that will just be that you have a proton coming in you have now a w+ boson you have a neutron going out and then the w+ boson quickly decay into a positron plus an ordinary neutrino and that's the fine band diagram for the proton changing into a neutron how do you know whether it's a w+ or w- well the answer is you have to decide what you've got to do with the charge here you've got proton with a charge of +1 being converted into a neutron with a charge 0 so you've got to get rid of a charge of +1 how do you do that you carry it away with a w+ boson up here you had a neutron which has no charge and a proton leaving so if you've got a positive charge leaving it means you have to balance that with a negative boson which is the w- so you just need to make sure that your charges always match here's another Fineman diagram that you'll need to know this is what's called proton electron capture so we've got two particles coming in a proton and an electron there will be an exchange of information that's the wiggly line and what you will get going out will be a neutron and a neutrino electron neutrino what is this exchange particle here going to be well we've got a positively charged proton coming in and a neutrally charged Neutron going out so we've got to take away a positive charge and that means that a w+ has to go in that direction the w+ then meets up with the negatively charged electron those two will cancel and produce a neutral neutrino and that is in essence the proton plus the electron produces a neutron plus an electron neutrino but there are other interactions here for example is a neutron coming in with an electron neutrino there is then an exchange particle w+ this time moving in this direction and out comes a proton and an electron the principle of Fineman diagrams is pretty much it that anything that can happen does happen you just have to make sure that the charges and barrier numbers are always conserved here's another one a proton and an anti-neutrino this time out goes a neutron and a positron and here is the exchange particle and that's going to be a w+ because the w+ needs to carry the positive charge away from the proton so that now becomes a neutrally charged Neutron it will join up with the neutrally charged neutrino or anti neutrino to produce a positively charged electron which is of course an anti-electron just to write the formula underneath so you can see what's happening in this case a neutron plus a neutrino produce a proton plus and electron in this case it's a proton plus an anti neutrino produces a neutron plus a positron and finally just to demonstrate that anything that can happen does happen you will observe that those two fine word diagrams are simply the flip side of each other here a proton and an electron produces a neutron and a neutrino here a neutron and a neutrino produces a proton and an electron if it can happen it does happen

21 thoughts on “Exchange Particles and Feynman Diagrams – A Level Physics

  1. 1:55 I can't figure what symbol he's using to represent the force of gravity…
    Can't make out the handwriting.
    Is that an S or an integral?…

  2. Any theory that suggests that our universe is composed of particles is silly. Particles are discrete objects that can be separated from each other and then they can be recombined. This simple subtraction and addition assumption imposed on matter is simple minded. Because of this simple mindedness, physicists are now defining more and more particles to try to link some so-called particles they defined in the past. But how are the newly defined particles linked together?
    This problem is easy to understand. Physicists should not assume that every matter is composed of particles in the first place. Simply because you can kick a particles out of a matter, it doesn't automatically imply that that matter is "composed" of particles. It is like cutting an arm from your body. That arm is no longer the same thing. So, you can kick an electron out of an atom, but that thing called electron has lost its property when it is inside an atom. You can't even call it an electron when it is inside an atom.

  3. Question, when you have a field and say you have a static situation where there is no motion or its in equilibrium, do you need a photon being exchanged to feel the electrostatic force? I mean, protons are everywhere and they all contribute to the final field, but that does not mean you need a photon from every proton to establish the field. There are photons but there is also the field. So isnt it true that you can have a force from the field but no photon exchanges required. Is there not a field and a photon?

  4. how the anti-neutrino in Beta minus and the positron in the Beta plus decay diagrams go up ? isn't anti particle suppose to go backwards in time?? the arrows are confusing …

  5. On Amazon the best reviews on products are usually from users who've had the product for at least a year. Seen this video a year ago and have gone farther into the subject. Must say this video is remarkable and "stays with you". This instructor knows what is relevant, applying a unique approach focusing on the provocative things scientific minds appreciate. The video's value is that it is introductory -yes but at the same time provides artful preparation for advanced learning. Having seen other videos, the instructor's approach stands out by focusing on forces as a means to understanding particles. It is a very effective approach helpful in understanding Feynman diagrams.
    Great Video, Sir!

  6. About the electron capture: as the weak interaction only works at very short ranges and as electrons must be at a certain distance from the nucleus due to the uncertainty principle (and also because they cannot have less energy than the allowed by quantization), how can the weak interaction between the proton and electron occur? from that, I think the proton and the electron couldn't come close enough so they are within the range needed for the weak interaction to take place… also, the interaction couldn't occur from afar because the boson would decay before it met the electron due to the boson's lifespan and thus would not turn the electron in a neutrino…

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