# I Ampere's law?

Tags:
1. Jul 21, 2016

### Cozma Alex

I know the ampere's law but I want an explanation on why it exist in that way, consider a wire where flows current, i know that when an observer is in relative motion to the current there is a magnetic field but why in this geometric configuration and not in another? What about the electric field outside? I guess is zero unless we don't move at the same velocity on the current so that our frame of reference is solidal with it, then the electromagnetic field is just electric field, is that TRUE? I mean, if we see current flowing all the field is magnetic, if we don't see current all the field is gonna be electric due to Lorentz contraction law and all that stuff right?
Why the magnetic field outside a wire obeys to the right hand rule?

2. Jul 21, 2016

### Staff: Mentor

Can you focus your question down to one most important key question? You have so many scattered questions that you will get scattered responses.

3. Jul 21, 2016

### Cozma Alex

Why does the magnetic field outside the wire carrying current obey to the right hand rule, why does it formes circumferences around the wire?

4. Jul 21, 2016

### Staff: Mentor

That is just a convention. We could have chosen a left handed convention and all of the physics would have worked out the same.

5. Jul 21, 2016

### Staff: Mentor

We don't observe the magnetic field directly. Instead we observe the forces between current-carrying wires and moving charges. Our rules for the direction of the magnetic field produced by a current (or moving charge), and the force exerted by a magnetic field on a current (or moving charge) were chosen so as to accommodate our observations of the forces between currents and moving charges moving at various angles with respect to each other.

As Dale noted, we could use a "left-hand rule" for the magnetic field if we also used a matching "left-hand rule" for the magnetic force exerted by that field. Mathematically, this would be equivalent to putting minus signs in the two equations, which would cancel out when we combine them.

6. Jul 21, 2016

### jim hardy

Back up a step - why does charge in motion cause a magnetic field at all ?

7. Jul 22, 2016

### Cozma Alex

P
Is a relativistic effect

8. Jul 22, 2016

### jim hardy

an observed trait of the universe

What if it'd been hexagonal instead of circular ? The formulas would be a little different, that's all..

9. Jul 22, 2016

### Cozma Alex

So... it's like that because the universe is made this way

10. Jul 22, 2016

### sophiecentaur

That would imply a very arbitrary case of symmetry. If it had to be a hexagon pattern, would the sides of hexagons for different wires all be parallel from wire to wire? It would imply 'preferred' directions in space which would be very unsatisfactory for Mathematicians, if nothing else (lol). The circle is the least arbitrary pattern in this case. Geometry is a pretty powerful argument here, as it is when you justify the Inverse Square Law.

11. Jul 22, 2016

### jim hardy

I think so.

Place iron filings around a wire and observe the pattern.

Then figure out what math describes it.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/biosav.html

Scientific method:
Observe something.
Then figure out what math explains the observations.
Then see if the math ever fails, and when it does adjust it.

Isn't that how relativity came about ?

I think if i understood what magnetism really is i'd have a nice lab at Princeton.

old jim

12. Jul 22, 2016

### jim hardy

hmmmm would preferred directions infer that dreaded "absolute frame of reference" ?