UKC

I was thinking about electric charge and realised I did not know the answers to some very basic questions.

Firstly, why are there two types of charge? I assume at some level it boils down to maths.

Secondly, why do opposite charges attract one another and like charges repel one another?

Gravity appears to have only one type of charge and these (like) charges attract.

FWIW, I have a degree in physics.

In reply to lowersharpnose:

Why do magnets not exist as monopoles?

How come inertial mass and gravitational mass are the same?

why does the speed of light not vary in inertial frames?

I think these questions and yours stop being physics and move into philosophy.

In reply to lowersharpnose:

Interesting question... the lack of apparent symmetry between electric charge (which can be negative or postive) and mass (which can only be positive as far as we know) is the kind of things which might be perturbing if you look for symmetry in things, which physicists generally do.

The question of attraction and repulsion is a fairly simply one (well, kind of), any charged object produces an electric field (which if you like you can think of as being mediated by the exchange of photons, or you can just think of as a mathematical construct). The existence of these fields means that like-charge particles reduce their total potential energy by moving apart and opposite-charge particles do so by moving apart, therefore they do so.

In reply to Oujmik:

How does an exchange of photons between two unlike charges bring them closer together?

It is easier to see photon exchange as mechanism for repulsion.

This is hard to grasp.

In reply to lowersharpnose:

Interesting question, it's made my head hurt.
As to why there are two charges, I don't know, something to do with quarks?
The fact the same charges repel and opposite attract, is about achieving equilibrium, the natural state is for most system to be in a neutral state ( block of ice in a warm room will warm up to the same temperature as the room). Is this the same effect within the given system (wire) the voltage difference (abundance and lack of electrons at either end) will cause the electrons to flow between atoms in an attempt to equalize the system.

Not sure if this helps.

In reply to lowersharpnose:

Oh hell. Charge conservation boils down to some horribly complex consequences of group/representation theory. I haven't done this for about 15 years, so forgive me if I'm vague or wrong

Symmetry with respect to phase transformations results in a numerical value we call "charge" being conserved. There's no particularly good reason for the number to be positive or negative so it can be either, so long as nothing changes the overall amount.

As for the attraction vs repulsion I'm going to handwave and mutter something about virtual photons. I'm sure I used to understand it

In reply to BIgYeti86:


Supposedly they do, but they're just very rare because "magnetic monopoles" are very heavy and thus de-coupled from the rest of matter early in the expansion of the universe.


Because gravity and acceleration are the same thing!


Because the laws of physics are invariant with different inertial frames, and thus Maxwell's equations (and thus c) are invariant.


No they don't! Even if they're currently unanswerable they are still physics. Nothing ever stops being physics.

In reply to lowersharpnose:


I think the word you might be looking for is 'fundamental'; has slightly less of a trivial air about it than 'basic'

Certainly, the question of 'how does electrostatic attraction work' is a horrendously difficult one. Exchange of virtual particles doesn't really explain the 'why' behind repulsion either, unless you visualise photons as little snooker balls which seems a little silly.

As to why there are two kinds of charge... yeah, it probably does just boil down to the maths. That's not an explanation, but more of an invocation of the anthropic principle. Why do we have 3 dimensions of space and one of time? Cos if we didn't, atomic structures couldn't form. Etc. It isn't clear that it is a question to which it is possible to obtain a concrete answer for.

I'd say we know too little about gravity to even begin to talk about that. But it does seem fair to say that comparing electric charge with mass is definitely apples to oranges. The strong force has 3 charges. Doesn't make much sense to compare electromagnetism to that, either.

In reply to lowersharpnose:


You're thinking of it as thought the photon can only carry positive momentum, and thus bump into and push away the charge-carrying particle.

However, the photons being exchanged are virtual, and thus don't have the "right" energy and momentum (don't obey E^2 = c^2p^2 + m_o^2 c^4), and thus the exchange can result in a pull as much as a push.

In reply to Coel Hellier:


Ta. Intuitively, I could see that photon exchange could result in a 'pull', but seemed a bit suspicious. Didn't know that virtual photons can 'cheat', so to speak.

In reply to lowersharpnose:


I think the answer to this is along the lines that the mediator of the electromagnetic force is a spin-1 particle (the photon) whereas the mediator of the gravitional force (the graviton) is a spin-2 particle.

In Quantum Field Theory, if you have mediators with spin or 0, 2, 4 etc, like-charges attract. If the mediators have spin 1, 3, etc, like charges repel. This follows from the maths of QFT.

In reply to Coel Hellier:


That sounds a little like a circular argument though... 'like electromagnetic charges repel because the mathematics we have put together to describe it say it will'

In reply to Coel Hellier:


So why didn't electric charge do the same thing?


why? (Yes I know relativity but why does that describe space time, and not a different set of rules?)


you have answered with because it doesn't, so why doesn't it vary?


Yes they do, maybe with future physics they will get answered but there will be still a 'why?' underneath. Why do photons have no charge yet gluons have colour? Why are things made of particles and not a continuous substrate? Why is entropy increasing?

In reply to Coel Hellier:


Why can virtual photons can dis-obey they above, but real ones can't?

(I did QED in my 3rd year as an undergrad so get it, but its not why, its how)

In reply to BIgYeti86:


Because the electric-charge-carrying particles are a lot lighter so can be produced at vastly lower temperatures (= much later in the universe's expansion).


And answering those "why" questions is still physics. Nothing ever stops being physics.

In reply to BIgYeti86:


The answer is the uncertainty principle. A particle can have the "wrong" energy (as in, not obeying the above equation), but only for a limited time, given by Delta-E.Delta-t = h-bar. Similarly it can have the "wrong" momentum but only for a limited distance, given by Delta-p.Delta-x = h-bar.

A short-lived particle that is disobeying that equation is called a "virtual" particle. A long-lived particle has to obey it and is called a "real" particle.