# Mass and Electromagnetics, someone please shed light

1. Sep 17, 2014

Okay so I've recently found an insane interest in physics and am becoming familiar with simple things.
However the vast amount of knowledge I have not yet acquired is pretty troubling me with understanding other things.

I understand that gravity is a field influenced by a mass of an object, correct me if I am wrong, but let's say that the earth rotates around the sun because it is within the field of the suns gravitation, meaning that the amount of mass that the sun will push down on the fabric of space-time creates the opportunity for the earth to orbit around it? (once again correct me if I'm wrong)

How does an electron rotating around an atom nucleus differ from this effect? I mean I have heard that electromagnetic force and gravitation force are not similar or the same.

2. Sep 17, 2014

### Staff: Mentor

The sun does not push "down" - down relative to what?
It does change the spacetime around it, right.

You cannot use the same model to describe the electromagnetic force - as an example, you have negative and positive charges, something gravity and its description do not have. You also have a general separation between charge (here: electric charge) generating the force, and (inertial) mass resisting changes in velocity. For gravity, both are the same.

3. Sep 17, 2014

okay sorry I worded that horribly, as the sun would sit on the fabric of space-time, it would create a warp which would have entities orbiting it (to say if they entered that vicinity)

It's still a little bit hazy for me but I think I'm getting it. It just seemed that it seemed a bit too coincidental that two forces which differed created a somewhat relative outcome... In a sense, and from what I understand about physics is that coincidences don't 'just' happen (Just my view based on the amount of knowledge I currently have)

4. Sep 18, 2014

### Bandersnatch

Sarmad, you're thinking of space-time "fabric" being deformed by mass like in the following video:
This is not really how gravity works in General Relativity. The analogy arguably has got some pedagogical value, but you'll find plenty of people(myself included) who think it's really more misleading than helpful.

The main problem with it is that it doesn't show what it purports to show. There is no time dimension to curve in the analogy, so at best it represents curvature of space, or gravitational potential well around masses.

This one is more accurate:
The problem with it is that it doesn't lend itself to classroom presentation so well.

As for the differences with atoms, you've got the already mentioned problem of charges - in the first type of presentation the gravity well is either a dimple or a bump depending on what the charge of a particle that "sees" it; in the second visualisation the geodesic(straight line) curves in or out. Since there can be two kinds of charges present, both would need to happen at the same time.
Then there's the quantum effects in the micro world - all orbits are constrained to specific radii, and due to Heisenberg uncertainty principle you can't really say where e.g. an electron actually is. After seeing Bohr's planetary model of an atom one might get the impression that it's just a small solar system, but it's by no means accurate representation of the quantum world. Such simplified model quickly breaks down once you look at it closely.

There are some parallels that can be drawn between Newtonian gravity and Lorentz force, like the geometric reasons for them having essentially the same mathematical representation, but that's a different kettle of fish.

5. Sep 18, 2014

### Staff: Mentor

Oh well, if you are just asking about orbits: every attractive force will give them, and typically (in particular, with a 1/r^2-law) they are stable.

Note that the atom needs quantum mechanics for a proper description. There is no electron zipping around like a billard ball.