# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Mr Andrew on 16/10/2007 17:13:41

Title: magnetic fields...why?
Post by: Mr Andrew on 16/10/2007 17:13:41
Charge is a property of matter which tells other matter with charge how to move.  So why does moving charge generate a circulating magnetic field?  I understand the derivations but they were based off of physical observations.  I am not questioning those observations, I am only wondering why things happen as they do.  Does anyone have a good explanation?
Title: magnetic fields...why?
Post by: lyner on 16/10/2007 22:21:50
A magnetic field doesn't 'circulate', as such.
The concept of a 'field' is an odd one. You don't have to believe it's there at all. It's just a way of describing what happens (not explaining it, necessarily).
People draw lines of force as if they're really there - whether it's magnetic, electric or gravitational field lines.  But the lines just show how an object  /charge/ pole will move if you let it go. It shows the  shortest 'downhill' route, based on the change in potential energy.
It isn't necessary to use magnetic fields to describe the interaction between moving charges. There is a cool relativistic explanation, based on the difference in spacing of positive and negative charges in a wire, for instance, as a current flows.
In this explanation, the minuscule relativistic effects on electrons which are moving at a few mm per second can be seen to be enough to produce significant forces between the wires (because there are so many of them). We normally think of relativity as coming into play only at very high speeds. Here, it operates at very low speeds.
But magnetic lines of force are just what you want when you get to real, practical situations - so we use them.
If anyone thinks they 'understand' a piece of Physics then they have missed something. (To mis-quote someone much more erudite than me!)
Not much of an explanation, I'm afraid, but you're unlikely to get one in  a few words. Text books won't help you, either, if that's what you want. They go through the mechanics of what and how, rather than why.