And so what makes magnetic fields? Well
the answer is moving charges. So we saw
that if we have an infinitely long wire
carrying a current, then the magnetic field
a distance R away from that would have a
strength given by (inserts equation). But the directions
are more complicated. You have to use the right hand rule. So you put your right
hand thumb in the direction of the
current, and then your fingers point in
the direction that the magnetic field is
going to go. So in this case it's going
to go into the page, which we usually
denote by a circle with a cross in it
which looks like the back of an arrow.
The arrow is going into the page so we
see the fletching at the back of the
arrow. And a common example of something
that is definitely not an infinitely
long straight wire that we've looked at
is the solenoid, with a magnetic field
due to each piece of current in each
piece of wire here. So we've got all
these little bits of wire that we have
to take into account separately because
they're all pointing in different
directions. But if we add up the magnetic
field due to all those pieces of wire
then we end up with a surprisingly
constant magnetic field through the
middle of this solenoid. And the field is
otherwise very similar to a bar magnet.
Now a bar magnet's magnetic field is
also made by moving charges, though in
this case it's the microscopic quantum
mechanical spin of the individual
particles inside it. And if we were to
isolate just a single moving charge, and
it was moving at a velocity V, and I wanted
to know who the magnetic field at a
position that was a distance R away, then
again would have to do a right-hand rule.
And this moving charge here looks
exactly like a current and so we use
essentially the same right-hand rule for
a positive charge: we put our right thumb
along the velocity and then our fingers
curl down and so we can see that the
magnetic field in that case would be
going again into the page. Now
calculating the strength of that
magnetic field or the strength of magnetic
fields of our magnets or the strength of
magnetic fields and solenoids can be
done of course, it does require a little
bit more complicated calculation so
we're not going to worry about that now.
So long as we can figure out what
direction we expect the magnetic field
to be going, we're pretty happy and we
know that we have a nice long wire we
can figure out how strong the magnetic
field is away from that.
