Not really a problem but more of a confusion with things. (Nuclear Physics) The paragraph on a homework assignment my professor handed out states: "From hadron spectroscopy we learn that the d quark is slightly more massive than the u quark. This accounts for the larger mass of the neutron compared to the proton. We expect that if the masses of the u and d quarks were equal the proton should be more massive than the neutron because it has a net charge..." I'm confused how mass and charge are related to one another. We're using Cottingham as our text and I can't find anything in it that addresses this. Can anyone clear this up for me? Thanks.