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Faraday's Law Introduction

How a current can be induced in a loop of wire by a change in magnetic flux.

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  • female robot amelia style avatar for user Fernanda Palacios
    How do we know the direction of the induced current?
    (57 votes)
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  • piceratops ultimate style avatar for user andy xu
    Why is the current going in the clockwise direction? According to the right hand grab rule, if I grab the right side of the wire, my index fingers point out, which means my thumb points up! How confusing! Thanks for your answer.
    (13 votes)
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    • duskpin seedling style avatar for user Natasha.Kumar4
      So according to the right hand rule, the current would technically be running in a counterclockwise direction. However, if you think about what would happen if the current were to run in this direction, you will find that an increase in the current would further increase the magnetic field, which would in turn increase the current and create an unending increase in these variables. This is why the induced current due to flux always runs in an opposite direction than that of the right hand rule. Hope that makes sense. See the next video, "Lenz's Law" for further clarification.
      (12 votes)
  • female robot amelia style avatar for user Fernanda Palacios
    Alright so a change of magnetif flux induces current. But why??
    (11 votes)
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    • old spice man green style avatar for user Matt B
      Magnetic flux comes from a magnetic with positive and negative charge. Current is the flow of (negatively charged) electrons. A movement in the magnet, and thus change in magnetic flux, will force electrons to move in a coil and therefore induce a current
      (12 votes)
  • aqualine ultimate style avatar for user Norton
    At , when Sal said the bigger velocity arrows would be on the right side of the field, why is that? Wouldn't the bigger vector lines be on the left side since that is where the field is stronger (the field lines are more dense here)?
    (12 votes)
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  • starky tree style avatar for user Christina
    Why is a current induced when there is a change in magnetic flux? What is the mechanism exactly?
    (5 votes)
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    • male robot hal style avatar for user Andrew M
      Charges experience a force in the presence of a changing magnetic field.
      Asking why is like asking why do like charges repel and unlike charges attract. These are fundamental features of our universe and we don't have a deeper explanation of why those features are what they are.
      (3 votes)
  • blobby green style avatar for user Melissa Michelle
    How does electromagnetic strength (e.g. number of magnets) affect the induced current peak maximum?
    (3 votes)
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  • aqualine ultimate style avatar for user Minh Đức
    if i move the loop wire in a uniformly distributed magnetic field, would there be any induced current?
    (3 votes)
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    • purple pi purple style avatar for user APDahlen
      Hello Minh,

      No. As far as the loop is concerned there is no change in magnetic flux consequentially there will be no induced current.

      However, if you rotate the loop the magnetic field would change. This is the principle behind motors and generators...

      Regards,

      APD
      (1 vote)
  • marcimus pink style avatar for user seungheon choo
    At , does it always go clockwise?
    (3 votes)
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  • female robot ada style avatar for user Sanya
    So if a current is induced only when there is a change in the magnetic flux, will the current stop when I'm done changing it?
    For example, if I increase the area of a loop from 4 sq. cm to 16 sq. cm, will there only be a current in those few seconds where I am changing the area, and will it stop once I was set the loop at 16 sq. cm? Thanks!
    (2 votes)
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  • female robot amelia style avatar for user Fernanda Palacios
    For how long is the current induced?
    (0 votes)
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Video transcript

- [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.