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- [Voiceover] We're already looked at

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Faraday's law in some detail.

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It showed us that if we have some

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loop of conductor and we have a change

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in magnetic flux over
time through the surface

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defined by that loop, it's going to induce

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an EMF through that loop which will cause

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a current to start to
circulate through that loop

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and that current, of
course, will be dependent

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on the actual resistance of the conductor.

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And there's many ways
that we've already seen

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of having a change in magnetic flux.

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One, you can have a change
in the magnetic field.

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It could be a change in its magnitude

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and, or it's orientation.

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You could have a change in the shape

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of the actual loop of conductor.

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If its area increases or decreases,

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that will change the flux.

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Remember, the flux is just your,

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the component of the magnetic field

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that is perpendicular to the surface.

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If you took the average of that times

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the area of the surface.

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And then the other way that we're going

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to study in this video is
inducing an electromotive force

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by changing it, not the shape of the loop

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and not the magnetic field but by changing

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the orientation of the loop.

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And in particular, we're
going to have the loop rotate.

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So let's think about this.
So I have this loop here.

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It's connected to this axle
and I'm going to rotate it

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in a, I'm going to rotate
it in a clockwise direction

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through this constant magnetic field.

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You can see it's constant.

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All of the magnetic field vectors

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I've just sampled'em at
different points in the field.

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They're all pointing straight
up and I've drawn them

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so that they all have the same magnitude.

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Now we'd appreciate is as we rotate this,

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the angle between the magnetic field

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and the surface is changing
and right from this point,

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as we rotated this clockwise direction,

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the component of the magnetic field

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that is perpendicular to the
surface is going to increase.

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Now what am I talking about?

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Well, let's look at it
from this point of view.

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Let's look at it from the point of view

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of the actual loop of wire,
so from this point of view

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the magnetic field is at some angle.

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I can draw that angle here
so, you know, whatever angle,

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it's a little hard to see,
whatever angle this is,

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we could say that is that angle here

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and as we rotate the
entire loop, it's attached

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to some type of an axle here,
in a clockwise direction.

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What is going to happen to this angle?

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Well, after let's say delta
T, let's say we're just

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rotating it a constant
rate, we're going to have

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the magnetic, so after
we've rotated a little bit,

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the magnetic field of vector is going

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to look something like that.

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So it's the component
that is perpendicular

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to the surface was what
we care about for flux,

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it's going to go from being
like this, it's gonna go

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from being like that to going to being,

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to being like this, to being like this.

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So at least for this
point of the rotation,

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for this point of the rotation till we get

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to the flat, until we
get to the flat point,

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until we have our loop
being completely flat,

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the component of our magnetic field

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that is perpendicular is going to increase

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which is going to have,
so we're going to have

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an increase in flux over that time.

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So if we have an increase
in flux over that time

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as we rotate up, at least
until we get to the flat point,

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what is going to happen?

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Well, we're going to induce a current

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and then we just have to think about what

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is the orientation of the current?

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So we want to have a current that

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will induce a magnetic field that will

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go against the change
in flux so the current

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should induce a magnetic field that is,

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if our flux, at least for
that part of the rotation

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is increasing in the upwards direction,

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if we're from the point
of view of this loop,

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then we need to create a magnetic field

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that is acting against that.

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So magnetic field that
is acting against that

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or we need to, we're
ready to induce a current

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that induces a magnetic field that acts

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against that change in flux.

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So, how do I, what type of current

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would induce a magnetic field like that?

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So I'll just use the
right-hand rule, my fingers

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would go in the direction of these,

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of the magnetic field, so my fingers

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and I have to use my right hand,

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so my fingers are gonna
go in that direction,

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so my right hand, my thumb,

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my thumb would go in this direction.

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So that is going to be the direction

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of the current that is induced.

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So the whole point of me showing you this,

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is that there's multiple ways to have

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a change in magnetic flux and there's

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multiple ways to induce a current.

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And this one is particularly interesting

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because it lets you or we
can start to think about,

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wow, I could turn, I could
turn a mechanical rotation

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into an induced current and
this is, this basic principle

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although they wouldn't use
such a simple loop like this,

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is exactly how electric generators work.

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They're actually, in some ways,

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the reverse of an electric motor.

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An electric motor has a current

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that causes something to rotate.

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Here, we're having something rotate

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causing a current to
form and that's actually

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what we have happening when you look

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at something like windmills or

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when you look at hydroelectric generators.

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Right over here, this windmill, the wind

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is going to cause these
blades to turn around

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and then inside, inside
of this little place

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right over here, you're going
to have a more fancy version

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of this rotating which is
going to induce a current.

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The magnetic field isn't
going to be exactly like this.

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It's going to be a more
sophisticated mechanism

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but it's the same underlying principle.

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It's just Faraday's law at work.

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Same thing with hydroelectric generator,

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you're using the potential energy

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of the flowing water to turn an axle

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and then that helps us
generate electricity

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by the exact same principle.