Hi. It’s Mr. Andersen and the AP Physics essentials video 56. It is on the electromagnetic
force. Remember there are four fundamental forces in the universe. We have gravity, electromagnetism
and then finally the strong and the weak nuclear force. Now in the last video I showed you
how gravity is this attractive force between all matter. Operates along all size scales
but it dominates at the very large. Well electromagnetism is the same way. It operates along all size
scales but it is going to dominate at our scale, at human scale. And so we really do
live in an electromagnetic world. And so to look at electromagnetic forces we could look
at charges. So if I have a positive and a negative electric charge, what we find is
that there is going to be an attractive force pulling them together. Now how is electromagnetism
different than gravity? If those charges are the same, let’s make them both positive,
now there is going to be a repulsive force between the two. Now another component of
electromagnetism is magnetic forces. If we have two magnetic dipoles there is going to
be an attraction between the north and the south. The larger the magnet, the greater
that force is going to be. Now magnets remember are different than charges in that they do
not exist as a discrete north or south. They are always going to exist as a dipole, but
there are connections between electric charges and magnets. If we move a magnet we can generate
charge. And if we move a charge we can generate a magnetic field. And so they both are essentially
the same thing. And they operate along all scales, however they are going to dominate
at the human scale. To show you how an electromagnetism is really one force we could look at charges.
So if I run charges through this wire, in other words if I run current through this
wire and I wrap it around a metal which is not normally magnetic it will generate a magnetic
field and we have created something called an electromagnet. And so a moving charge can
generate a magnetic field. We could take this electromagnet and then spin it in a large
generator, so just by spinning a magnet, as water moves through this dam, we are going
to start to generate current. And so these are the same force. They are going to operate
along all scales and they are billions and billions of times stronger as far as a force
goes than gravity. A way to look at how we could quantify those charges is Coulomb’s
Law. And so if we have two positive charges there is going to be repulsion. If we have
opposite charges there is going to be an attractive force. We can use Coulomb’s constant to
figure out how great that force is going to be. But remember the bigger the charge the
greater that either attraction or repulsion is going to be. And the same applies to a
magnet. Now these electromagnetic forces remember operate along all scales but they are going
to dominate at the human scale. And so if we take this human right here, and let’s
put a helmet on them and strap it on, and then we are going to have an industrial magnet
come down, we could attract that human and we could pull him up a little bit. Now where
are the electromagnetic forces in this diagram? It is going to be in the cells of the human.
It is holding all of us together. It is also holding this hat together. And it is holding
this magnet together. And in fact as we run electricity through it, it is generating magnetic
force which is holding this person off the ground. So it really is dominating at the
human scale. Since this is an electromagnet, I could turn it off and the person is going
to fall down. Now why do they not just fall down through the earth, because gravity is
pulling them towards the earth? It is because electromagnetic forces right down here are
way stronger than the forces of gravity. And so at the human scale electromagnetism, these
charges, are what dominate our planet. So did you learn to connect the size of electromagnetism
to the size of the charge or the size of the magnetic dipole and their movement?
I hope so. And I hope this was helpful.