Faraday’s Law Introduction | Physics | Khan Academy

Faraday’s Law Introduction | Physics | Khan Academy

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– [Instructor] In other
videos, we talk about how a current flowing through a wire can induce a magnetic field. Now what we’re going to
talk about in this video is how we can go the other way. How a change in a magnetic field can affect or induce a
current in a loop of wire. We’re gonna be going from a change, change in, and actually we’re
gonna focus on magnetic flux. Magnetic. Magnetic flux through a loop. Through a loop. Now we’re going to see how this actually induces a current in that loop. So, induces. Induces a current. Induced current. This right over here, all right, this is my attempted
drawing a magnetic field and these are the magnetic field lines. They don’t look like lines because they’re all
popping out of the screen. They’re moving towards
you, towards the viewer so you could view these
as the tips of the arrow. Now there’s different ways
of showing magnetic fields. You could show magnetic
field lines like this, you could also use vectors. And when you’re using field lines, it’s the density of the field lines tell you how strong the magnetic field is. If you got to the right here, they are less dense and
so the magnetic field is less strong on the
right hand side over here than they are on the left hand side, or at least that’s what
I’m trying to depict. Now if we did it with vectors we would have bigger arrows
pointing out over here than we would over here. But now let’s do a loop of, let’s do a loop of wire. Let me draw a loop of wire here. Let’s say I have a loop of wire that is, this is my loop of wire. My loop of wire. If I just throw that loop of wire and if it’s just stationary,
it’s in this magnetic field and the magnetic field isn’t changing, I do have some flux going through the, I guess you can say the
surface defined by this wire. And if you’re unfamiliar
with the term magnetic flux, I encourage you to watch
the video on magnetic flux. But if I just have this wire stationary in the magnetic field, nothing is going to happen. But I will be able to induce a current if I change the magnetic flux going through this surface in some way. So for example, right
now the magnetic field is pointing out of the screen. If I were to make it even stronger in the direction pointing
out of the screen. So, I guess one way to think about it is if I were to, if the change
in the magnetic field, the change in the flux
were to get even stronger in the outward direction. So I don’t know, a good way to, just gonna get even stronger
in the outward direction. I’ll draw a big arrow there. I guess you could say that these things became even denser
in this outward direction. It is actually going to induce a current. And the current that it
will induce is going to go, is going to go in that direction and let me draw it a little bit clearer. The current is going to
go in that direction. It’s going to go in a clockwise
direction around this. And that is because that
change in the magnetic flux that induces a voltage, an electromotive force that
causes this current to flow. That causes the current to flow and there’s other ways to
change the magnetic flux. If I were to lessen the magnetic field in the outward direction or another way, if the change in the
magnetic field were inwards, then the current would
go the other direction. But the key here is when I change the flux through this surface defined by this wire, it’s going to induce a current. The current isn’t going to
be there if the thing is, if the magnetic field is stationary and I’m not changing this loop in any way. But as soon as I change
the flux in some way, I am going to induce a current. I could also instead
of changing the field, I could actually move my coil. I could move it that way. And if I were to move it that way, the flux going through this surface or I guess coming out of
this surface will increase. Because if I move this to the left, the magnetic field is denser, I guess it’s stronger
so there’ll be more flux through this area here. So, if you move it that way, you also would have a current like this. Now if you moved it the other way, if it was on the left hand
side and you moved it that way, it would also induce a current but now since the flux is lessening in the outward direction, the current would go in the other way. Now there’s other ways to change the flux. You could actually change
the area of this actual loop if somehow it was made out of some maybe stretchy, stretchy wire somehow. If you increased its, if you would increase. Let me draw it this way. If you were somehow able to stretch it. Stretch it so it contains, so the actual area increases. If you were able to stretch it out so that the actual area increases which would cause the flux in the out of the screen direction
to increase even more, that also would induce, that also would induce the current. And so this whole idea of
a change in magnetic flux inducing a current, this is the essence and we’ll go deeper into
it in future videos, this is the essence of Faraday’s Law. Faraday’s, Faraday’s Law. And we’ll quantify this
more in future videos but it’s just the notion
that if I have a loop of wire and I have a changing magnetic flux through the loop of wire, that is going to induce
a current in that wire.

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