 In the previous video, we introduced Feynman diagrams and how we can draw diagrams for the electromagnetic interaction. In this video, we will look at drawing diagrams for the weak and strong interactions, which will follow the same rules as we introduced in the last video. Recall there are four key rules for drawing Feynman diagrams. At each vertex or interaction point, there must be an exchange boson. All the different parts of the diagram must be connected. The conservation laws of each interaction must be satisfied at each vertex, and each vertex has exactly one arrow pointing into it and one arrow pointing out of it. Let's apply these rules for the weak interaction. Remember, the weak interaction is mediated by the W+, W- and neutral Z bosons, which are all massive. These are drawn as wiggly lines on Feynman diagrams similar to photons in electromagnetism. Through the weak interaction, particles can change flavor. Leptons can become different, leptons and quarks can become different quarks. For example, a down quark might become an up quark, which is exactly what happens in nuclear beta decay. One of the down quarks in a neutron becomes an up quark through the interaction with a W boson, which then goes on to produce a lepton neutrino pair. There is one thing missing in this diagram, and that is whether we should label the W boson with a plus or a minus. We know that charge must be conserved at each vertex in the weak interaction, so we can look at this first vertex here where the down quark becomes an up quark. On the left we have a down quark, which has a charge of negative third, and on the right we have the charge of the W, which we will call X, and an up quark, which has a charge of positive two-thirds. For charge to be conserved, the left-hand side must equal the right-hand side, so X is in minus one. This means that our exchange boson must be the W minus. Now let's look at the strong interaction. The strong force mediates the interaction between quarks, which have color charges. The exchange boson that does this is called the gluon, and is represented by a curly line in Feynman diagrams. Recall from Liz's videos that the color charge of a quark can be red, green, blue, anti-red, anti-green, or anti-blue. Gluons carry a combination of two colors, a red, green, or blue, and an anti-red, anti-green, or anti-blue. Quarks are then able to change color by exchanging gluons. This makes Feynman diagrams a little trickier to draw, as you will have to make sure both charge and color charge are conserved at each vertex. Drawing diagrams for the weak and strong interaction is a very similar process to drawing diagrams for the electromagnetic interaction, except now we have a wider variety of processes that we can draw. It just takes a little bit more consideration when drawing these diagrams to make sure that we have all the relevant quantities conserved at each interaction point.