Relationship between charge and mass

[mateomy]

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.

 

[Dick]

Think about what happens if you assemble the Earth from the parts that made it up. They all fall together because they all attract each other. In the end you have a hot Earth because they've all fallen together. Then the Earth cools and radiates the energy away. So the Earth has lost energy and hence lost mass. The Earth is lighter than the parts that made it up. The proton is the opposite. The parts that make it up in the net repel each other. You need to put extra energy into make a proton from the parts to account for the electrostatic repulsion.