Hi. It's Mr. Andersen and this is AP Physics essentials video 33. It is on the magnetic
field of a dipole. What is a magnetic dipole? Well it could be a magnet. It has a clear
north and it has a clear south. It could be the needle on a compass or it could even be
the earth as a magnetic dipole. And so to understand how it works we use an analog for
that, an electric analog for that. So imagine if we had a loop and that loop contains a
current and it is just moving around in a circle. We would have to hook it up to something.
But let's say the current is moving in that direction. We learned in the last video that
if you ever have current moving through a wire we are going to create a magnetic field.
And so let's use that right hand rule to figure out where the magnetic field is coming. So
I am going to grab that loop, just virtually, and then I am going to see that as I point
my thumb in the direction of the current then the magnetic field is going to come up right
through the middle. And so that would be our magnetic field. Let's move the current. So
let's move the loop a little bit. If I move my thumb now, it is pointing away from me.
Where is that magnetic field coming? It is coming up right through the middle. And let's
say I go all the way around to the back of that loop, I do not know if my hand could
turn that direction, but we still have the magnetic field coming right up through the
middle of that loop. So this would be our magnetic dipole. It is going to be affected
by magnetic fields just like a magnet. So a good analog for a magnetic dipole is current
moving around a wire. And so we are going to have a clear north and a clear south of
this magnetic dipole. But any kind of a magnet is going to be a magnetic dipole. So like
a compass needle, a floating compass needle is going to be a magnetic dipole. And so if
we put a compass needle next to a magnet, again that magnet is going to generate a magnetic
field, and so that compass needle, since it is a magnetic dipole is going to respond to
that field. It is going to move. It is going to experience a torque and move in relation
to that magnetic field. And we do not have to use a magnetic dipole. We could iron filings
and if we put them out inside this magnetic field they are both going to turn and respond
to that magnetic field. And our earth is also an magnetic dipole. And so other dipoles,
like compasses are clearly going to respond to that earth. And so the first thing I want
to make sure I mention is the difference between the dipoles we have talked about so far. So
this would be an electric dipole. We have a positive charge. Remember all the electric
field lines would come out of it. And a negative charge, they would all come into it. But if
we move them next to each other then we have this electric dipole. And you can see that
those field lines, electric field lines, are identical to those of the magnetic field lines.
In a magnetic dipole we have a north and a south. And so we have the magnetic fields
that are going to move around that. What is the difference between the two? Well I can
pull this positive off and the negative can stay there and I have two monopoles. But you
never have a magnetic monopole. It does not exist in science. If ever have north, then
you are also going to have south. And so to understand how this works we will lots of
times use this analog of current moving around a wire. And then we have a magnetic dipole.
That would be the m right here. And then this is going to be B which is going to be the
magnetic field. And so it is exactly the same as a compass. It has a clear north and it
has a clear south. And so an example of a magnetic dipole could be a compass needle.
And so let's say we have a compass needle and then we apply a magnetic field coming
from right to left. How is that magnetic dipole going to respond to that? Well it is going
to experience a torque, and it is going to move in that direction. Let's say we take
a series of compasses and put them on a table around a magnet, they are not just going to
keep pointing in the direction they were, they are going to follow these magnetic fields
of this magnetic dipole. It is kind of hard to visualize that. And so lots of times in
as science lab what we will do is put some paper over the top. And then we can sprinkle
iron filings on it and you can start to see those magnetic field lines. And you can see
how this magnetic dipole, the compass needle, is lining up along those field lines. And
the Earth remember is a magnetic dipole. It is filled with this hot fluid charged fluid.
And as it spins it is creating a dynamo. And that is really creating a magnetic field.
And so you can think of the earth as like a giant magnet. And the first thing you should
see, as you have this, is it seems like the north is in the wrong direction. But remember
it is north, on a compass needle that is pointed towards the north of our planet. And so that
is technically the south end of the magnet, even though we refer to it as going north.
And so if we were to take all of these compasses and put them on our planet and then instantly
turning on the magnetic field, what are they going to do? They are all going to line up
along those magnetic field lines. And so did you learn to describe the orientation of a
magnetic dipole inside a magnetic field? Again, you just follow the field lines.
And I hope that was helpful.