Hi. It’s Mr. Andersen and this AP Physics
essentials video 71. It is on electromagnetic
induction which is the ability of a magnetic
field to create current inside a conductor.
Now scientists like Hans Christian Orsted
have already shown that if you have current
in a wire like this it creates a magnetic
field. And that magnetic field could affect
this compass needle which is really a small
little magnet. And so current can produce
magnetic fields. But what scientists like
Michael Faraday wondered is could you take
a magnetic field and produce current from
it? In other words was the opposite true?
And this is the apparatus that he used to
study that. And so we have basically have
two parts. We have a ring of iron but on the
left side we have an electromagnet. So you
are going to connect this to a batter and
that is going to produce magnetic fields over
on the left side. And so the hope is on the
right side, that could produce current inside
this wire. And then we would be able to measure
that using a galvanometer. And so he set it
up like this and then he closed the switch.
And so what what happens to the current in
the wire on the right side as I close that
switch. And you can see that we generate a
little bit of current. But then it goes away.
And this is puzzling. So he opened up the
switch and watched what happened. We have
a little bit more current but it is in the
opposite direction. So let me kind of close
that switch and we have a little bit of current.
But it goes away. And then we open the switch
and we have a little bit of current but its
going in the other direction. And so that
is electromagnetic induction. But what he
wondered is why is it only occurring right
when I close the switch? So to understand
that you really have to understand the specifics
of electromagnetic induction. And to understand
that you have to understand what magnetic
flux is. Magnetic flux is how a conducting
material or any material for that matter is
affected by a magnetic field. And so what
would be something similar? It would be how
you or any material on our planet is affected
by light from the sun. And so it is going
to be the amount of light from the sun. But
it is also going to be the angle at which
that light hits. And so magnetic flux is going
to be the product of the strength of the magnetic
field, how much or how large that magnetic
field is and then we are going to multiply
that times the surface area perpendicular
to that magnetic field. And so imagine that
this right down here is a wire loop. And so
we have a little bit of a wire. And then we
have a magnetic field. And so if we ever have
change in that magnetic flux, then we are
going to have electromagnetic induction occurring.
And so what happened right when he closed
that circuit? Well the magnetic field before
he closed the circuit was zero. But then he
added this magnetic field, so was there change
in magnetic flux? Yes. And so was there electromagnetic
induction? Yes. So it created current. What
would happen if we were to increase the magnetic
fields? So let’s double the magnetic field.
Well for a moment as we are doubling the electromagnetic
field, are we getting a change in the magnetic
flux? Yes. Are we getting induction? Yes.
And therefore we are going to have current
in that wire. And so by varying the amount
of that magnetic field we can get induction,
or we can get current in that wire. Now what
is another way we could go at that? Again
we could look at the surface area that we
are impacting. And so if you think of this
as a wire, all of these magnetic field lines
are perpendicular to this wire. And so we
are going to have a large magnetic flux. But
watch what happens when I turn it at an angle.
And a good way to do this is simply count
the lines of the magnetic field that it is
hitting. You can see there is a reduction.
And so the number of lines is different but
also these are not hitting it straight on.
It is not perpendicular so we would have to
use a little bit of trigonometry to figure
out what component of that magnetic field
is actually perpendicular to the surface area.
But did it change between those two rotations?
Yes. And so was there electromagnetic induction?
Yes. And so there would be current as well.
And so let’s say we turn it like this. The
magnetic flux is going to be zero because
none of these magnetic field lines are going
to be perpendicular to the surface. But it
changed between those two points. And so we
are going to have induction and we are going
to have current. So that seems a little non
intuitive, but it has real world applications.
And so the electricity that you are using
right now and the microphone that I am using
right now as well, use this idea of electromagnetic
induction. And a great way to look at that
would just be looking through a generator.
And so how does a generator work? Well in
a magnetic field what we can do is we can
take these wires and we can start to rotate
them. And as we rotate them we are getting
huge changes in magnetic flux and so we are
going to have huge changes in the current
itself. And so if we look at the equation,
magnetic flux or phi sub B is going to be
a product of the magnetic field, how big that
magnetic field is, times the cross-sectional
area perpendicular to that magnetic field.
And so if I take this wire, right here and
I compare it to this wire right here, which
one is going to have a larger magnetic flux?
It is going to be the one on the left. And
the reason why is we are going to have more
of those magnetic field lines go through it.
What is a good way that we could increase
magnetic flux is we could just wrap that wire
a bunch. And so each of those wires, we are
going to have the magnetic flux of that individual
wire. And so by changing the magnetic field
or by changing the size of that area we can
change magnetic flux. But also remember we
could angle it. And so if I angle it like
this, we are going to have fewer of those
magnetic field lines go through it and so
we are going to get a change in the magnetic
flux. And here we would actually have no magnetic
flux. But it is not a measure of magnetic
flux that is important in producing current.
It is are we getting changes over time. And
so this is a PHET simulation that gets at
that. And so we have magnet over on the left
side. And then on the right side we simply
have a wire hooked up to a galvanometer or
it is a voltmeter. It is going to measure
the amount of voltage. And you can see that
there is no current right now. But as I start
to change that position of the magnet, I am
starting to get current. And you can see,
you can even see the electrons moving in the
wire. And so if I increase the number of wires
I can increase how much I am deflecting that
needle. How much current I am actually moving
inside the wire itself. Why is that? Now the
magnetic field lines, you can see the magnetic
field lines are changing. And as they change
we are getting change in magnetic flux. And
so we are getting current inside the wire.
Now another way to look at this would be through
a generator. And so in this set up what I
have is a magnet down here on a wheel so that
I can spin that magnet. I have my wire again.
And I am trying to induce current inside that
wire. And then I am going to measure the amount
that I am going to change. And so watch what
happens now when I start to turn on the water
inside that faucet. It is changing the position
of the magnet. As is it changes the position
of the magnet it changes the magnetic field.
You can see the magnetic field lines are changing.
And so I am starting to get a little bit of
current. What happens if I increase the speed
of that water? I am getting more current.
You could even have a light bulb connected
to it. And so now I am starting to get usable
energy. Watch what happens as I increase the
number of wires. I even have more energy coming
out. Now if I increase the water again I even
have more energy being produced. And so we
are taking that energy of the flow of the
water and actually making electromagnetic
energy out of it. And so this is how the generators
in a dam would work. We have the water flowing
down through the dam. It is spinning these
magnets and it is creating that electricity.
And so did you learn to construct an explanation
for how a simple electromagnetic device, like
a generator, works? I hope so. And I hope
that was helpful.
