Re: What causes clockwise or counterclockwise magnetic fields?

Dear Henry,

There are left-hand rules as well as right-hand rules, but I don’t think 
that is exactly what you are asking.

First, let me talk a bit about electric and magnetic fields. These fields 
do indeed propagate across space – that’s how light from the Sun reaches 
the Earth, for example. Now, if you look at that ray of light coming to us 
from the sun, it consists of perpendicular electric and magnetic fields, 
and both fields are perpendicular to the direction of propagation of the 
wave. To visualize this, draw a cross on a blank piece of paper. This 
gives you two perpendicular directions (N-S and E-W, for example). Now 
stand a pencil on its point where the two lines meet. You have created a 
third perpendicular direction. If the pencil is the direction of 
propagation (the way the ray is pointing), the two perpendicular lines on 
the paper represent the electric and magnetic fields. 

So, which one is the electric and which the magnetic field? This is one 
place a right-hand rule comes in. On the paper, pick one direction to be 
the positive direction for each line. Point your thumb along the pencil 
and your fingers along the positive direction of one of the lines. If your 
palm is facing the other positive direction, your fingers are along the 
electric field; if not it’s the magnetic field.

Note that when an electromagnetic field propagates, it doesn’t rotate. The 
electric and magnetic fields stay in the same direction all the time; 
their magnitude oscillates sinusoidally. A wave on a rope has a similar 
motion.

There are a number of other right-hand rules that are useful when dealing 
with magnetic fields (magnetic fields are inherently three-dimensional in 
their generation and propagation). If you put your thumb along the 
direction of the current in a wire, your fingers curl around the wire in a 
way that shows you how the magnetic field is (it makes circles around the 
wire). A charged particle moving in a magnetic field experiences a force 
perpendicular to the field and to its direction of motion. A right-hand 
rule is used to tell you which direction the force is. Of course, if the 
particle has a negative charge, the force would be in the other direction –
 or you could just use a left-hand law.

If you look at these examples, there are a lot of arbitrary choices 
implied in them. Which charge do we choose to be positive? Does the 
magnetic field point out of north poles or into them? Which way does the 
current flow in a wire? (The choice that was made, before the electron was 
discovered, is actually opposite to the direction the electrons move).

Still, once you define all these choices, you are left with the 
fundamental question you are asking: Why, if you point your right thumb 
along a wire, do your fingers point in the direction of the field? Why 
doesn’t it go the other way?

So, here is where you may not like my answer, because we don’t know. We 
observe that it is so; we construct our models and theories so that they 
correspond to the world as we see it. Therefore the right-hand rules are 
built into our theory of electromagnetism; we put them there because if we 
used a left-hand rule instead (where we now use a right-hand one) we’d be 
getting everything backwards all the time. But why the field is one way 
rather than the other – we don’t have an explanation for that. 

You can view lectures on electromagnetism here; start with #11:
 http://ocw.mit.edu/OcwWeb/Phys
ics/8-02Electricity-and-MagnetismSpring2002/VideoLectures/index.htm


Here is the Website for the chapter on electromagnetic waves from a 
college textbook:
 http://cwx.prenhall.com/bookbind/pubbooks/walker2/chapter25/deluxe.html