I have seen many Feynman diagrams, but I have only seen interactions between electrons, and anti-positrons between themselves and each other. However, I have never seen one done for Protons, is there a specific reason for this?

Also, I have a little trouble understanding the maxwell tensor (aka field strength tensor, electromagnetic tensor, faraday's tensor, etc.). Is it supposed to work similiar to the stress tensor (I mean on paper, when I draw it out, not in theory).