[paul cotter (OP)]

A follow up to eternal student's question re does charge contribute to mass. What exactly is charge? The whole edifice of electrical engineering and much of physics is concerned with the behaviour of charge whether it be static or dynamic for which we have good understanding. Why does an electron repulse another electron and attract a suitable quark trio? Having worked in e.e. I have always taken charge for granted and never questioned the underlying causation. For a non-physicist like myself the concept that the electromagnetic phenomena are mediated by virtual photons is not a satisfactory answer but it is probably the only one I will get.

[evan_au]

As an Electrical Engineer, the OP assumes "charge" to be "electrical Charge".

However, when adding up conserved quantities in quantum theory, there are several types of charge, including color charge and isospin charge.

https://en.wikipedia.org/wiki/Charge_(physics)

[paul cotter (OP)]

I should have been more specific, electrical charge is what concerns me.

[Origin]

I have always taken charge for granted and never questioned the underlying causation

If you are asking why like charges repel and opposite charges attract, I doubt you will get an answer you like.  It is like asking why does mass warp space.  The answer to both question I believe is 'they just do'.

[Eternal Student]

Hi.

What is charge?
   The conventional approach would suggest it is a fundamental characteristic that particles have.   That's it... no deeper explanation than that.
   People would then direct you to quantum field theory and the standard model of particle physics.   In this there are several fundamental characteristics that particles can have (e.g. "Spin").  "Electric charge" is just one of those fundamental characteristics a particle can have.   They might then suggest that electric charge has effects that are well explained by interactions due to the electromagnetic force, which again seems to be a fundamental interaction that exists in the standard model.   All in all,  I think it boils down to the idea that it works and that's probably all that matters:   The standard model works well , it makes predictions that can be tested and in that model we do have and do require an electric charge.  I mean, you could change its name and possibly ascribe some more complicated mathematics to it - like make it a vector valued thing and develop some rules for manipulating it - but it would be just the electric charge and the models we have now in disguise.   
   So, overall, the electric charge might have some deeper explanation but it doesn't really matter at the moment.   No better theory has come along that seems capable of explaining more or explaining the same things but with less assumptions.

Best Wishes.

[paul cotter (OP)]

Yea, that's pretty much as expected, thank you(s).

[varsigma]

I think this kind of question, and the answers, point to something Feynman said about energy--nobody understands what it is.

But you can't help but notice that, after saying this, he goes on to say quite a lot about energy and work, which suggests that whether anybody knows what it "really" is, we get to use it anyway.

What he actually says is: "it's important to understand that in physics today, nobody understands what energy is", then he more or less adds "but it doesn't matter . . ."

But if you take his warning to heart, then you should be wary of someone saying they do understand what energy is.
In the same lecture, Feynman says energy is just a number. How can he know that? I'm still processing it, and he said it a while ago. Isn't he "just" saying it's a way to think about what it is, since "nobody knows what a number is . . ." ??

[Eternal Student]

Hi.

   That's a good set of points, @varsigma .    You're not going to get many replies because it's a topic that is often discussed and I'm fairly sure was discussed again only just recently.    However, it is a good set of points, well done for paying attention and reading some of Feynman  etc.

In the same lecture, Feynman says energy is just a number. How can he know that?

    I think the essence of what he was said is that energy is just a number, "A conserved quantity", in the examples he gave.   If I recall correctly he started with an example of a boy putting toy bricks into a toy box etc. and his mother noticing that the total number of bricks in his bedroom was a conserved quantity (or something like that).   Feynman then invites you to believe that energy is a conserved quantity in all other areas of science but certainly doesn't prove it in that lecture.  More significantly he advises you to do exactly as you have done - be aware that no one is well aware of what it is in any greater or deeper sense then just being a conserved quantity.

Best Wishes.

[alancalverd]

I find it easier to start from "what physics is": the business of constructing mathematical models of our observations, to help us predict what will happen next.

So we observe that rubbing ebonite with cat fur induces an attractive property to both materials, and we ascribe that property to something we call charge. The choice of substances, incidentally, is not a random one: positive charge is actually defined as what accumulates on the cat fur, and negative charge is defined as what resides on the ebonite! Yes, folks, the entire edifice of quantum electrodynamics and electrical engineering all depends on our being able to recognise a cat and a piece of vulcanised rubber!

Anyway it then turns out that the smallest object that can carry a negative charge is a particle with mass, and that all charge is quantised in units of one electron charge (or one missing electron charge), and from this we construct the whole of electrostatic and electromagnetic physics, from atoms to power stations.

So what charge is, is the property of an electron to repel another electron or attract an object that is deficient of electrons.

Then along comes the positron: same mass, spin, etc but (a) a precisely opposite charge quantum and (b) an unexplained scarcity in the observed universe.   

Thus in answer to the OP you can either quote Feynman as"nobody knows but it doesn't matter" or Calverd as "it's what does what we see, and it matters a great deal". The two statements are equivalent.

[Eternal Student]

Hi.

  Crumbs...  is ebonite a rubber compound?   Looks like it is on a Google search.    I always thought it was a rock or natural mineral they found.  Still, nevermind, at least I understood what a cat was.

Anyway it then turns out that the smallest object that can carry a negative charge is a particle with mass, and that all charge is quantised in units of one electron charge (or one missing electron charge) 

    Yes that's OK and true enough.    Classical physics ignores quarks that have 1/3 the charge of an electron, or "quasiparticles" that appear as emergent properties from an ensemble of particles and almost everyone ignores string theory where fractional charges are sometimes used.
     Overall, charge probably is quantised although it seems to be personal choice if you consider it to be in quanta of  e  or  1/3 e,   or smaller if and when some new theory appears.

LATE EDITING:   Are you sure it wasn't a piece of Amber they rubbed the cat on?

Best Wishes.

[alancalverd]

Amber is known as ελεκτροη in Greek, and it was indeed Greek yarn spinners who are credited with the first recorded observations of electrostatics, though I've never understood what part of the spindle was made from amber. Interesting that the Institute of Physics gets its corporate underwear in a klein bottle over sex equality and diversity when the whole basis of electrical engineering and electronics was discovered by women, though sadly 3000 years before Nobel. 

[paul cotter (OP)]

Sometimes you don't need amber or ebonite. One of our cats gives my wife static shocks just by stroking. It doesn't happen with me and the cat seems oblivious to this phenomenon. Sometimes I will hear the crack followed by "ouch" Maybe the electron should be dropped and replaced with the felion.

[alancalverd]

My favorite static anecdote concerned a mammographic x-ray unit we installed in a London hospital. The radiographer insisted that the floor had a pure wool carpet. The accountants ordered a nylon carpet. Few things distress a patient more than someone walking over a nylon carpet in rubber shoes and then touching her nipple. At great expense the entire room was stripped out and refitted with a wool carpet with antistatic treatment.