ECE3300 Lecture 2-5 Magnetic Fields

Uploaded by cfurse on 24.08.2009

Now, let's talk about the magnetic field. You've seen
magnetic fields since you were a kid. This is your basic bar magnet.
Your north, south magnet has a set of field lines that are very dense
right here in the center and transverse out in circles away from this
bar magnet. Even though these lines are shown going straight up, they
come around and come back in on this side so all of our magnetic field
lines like this one and so on, all of our magnetic field lines are loops.
They are all closed lines and that is typical of magnetic fields.
Magnetic field lines are always closed lines.
Now, the magnetic field we're going to represent a couple
of different ways like we did with the electric field. The magnetic
field has two variables. This is the magnetic flux, H is the magnetic
field, and the constant that's related to them is the permeability or
the magnetic permeability. The magnetic permeability is given in
henrys per meter (H/m). If it's in free space, the value of magnetic
permeability is four pi times ten to the minus seventh and relative
permeability is given -- we'll be looking at relative permittivity in a
minute. Mu is equal to the relative permittivity times the permittivity
of free space.
Now, there are two things that cause magnetic fields or
magnetic flux lines. One of them is permanent magnets like you see
here. Typically, these are materials that are ferromagnetic, they
most often have iron in them and they create large magnetic fields
just because of their nature. In another case, the magnetic field is
caused by current so if we have a current, I, like this, it will produce a
magnetic field going around it like this. It's a closed line.
What you do is you put your thumb in the direction of the
current so this is your thumb and you wrap the fingers of your right
hand. So these are your fingers of the right hand. Don't use the left
hand. I did on my first electromagnetics exam and I got a very poor
grade so don't use your left hand. Use your right. The fingers of your
right hand will show you that the direction of the magnetic field. So
the two things that produce magnetic fields are static permanent
magnets and dynamic or static currents.
Now, I'd like to also talk about the relationship between
Mu, Mu R, and Mu knot. Mu knot remember is four pi times ten to the
minus seventh henrys per meter (H/m) and here's some typical values
of Mu R. If you have materials like gold, mercury, silver, their Mu R
value, their relative permeability is very similar to that of air. Those
are called diamagnetic materials. They are virtually exactly like air.
Paramagnetic materials are air, aluminum, tungsten, titanium, platinum,
and they have values that are also very similar to air. The
ferromagnetic materials, cobalt, nickel, iron and so on, refined iron,
have values that are much, much higher than air.
So what happens in that case? If I have a magnetic flux
where Mu is very, very large that means that the magnetic field in
something such as iron is going to be much greater than the magnetic
field in pure air so the magnetic field becomes stronger because of
my thorough magnetic material.
Now, let's talk about what you are going to see in this
course. We're going to be doing three things. We're going to be doing
things that are static, so electro statics or magneto statics, and
we're going to be doing things that are dynamic where you have
electro dynamics, and you could say you have magneto dynamics but
the two of these always go together.