If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:03

Video transcript

what I want to do in this video is give ourselves an introduction or an intuition for the term flux in general and then think about how it applies to the idea of magnetic magnetic flux so first of all when people are just talking about flux and this is the easiest way that I know how to conceptualize it they're talking about how much of something is flowing through a surface in a given amount of time so if you imagine that this is this is you know I'm just defining a volume of air right over here and let's say the air is denser near the near the bottom of this volume of air so there's more air down here then there is up here which is generally true air density goes down as as the as you increase altitude so there's very low density up here this is in between I don't have to draw all of the air particles but you get the sense a lot of air a lot of air down here and the air is moving and so let's say that the air let's say the air is so this we do the velocity vectors in a different color so these are some of the velocity vectors of the air let's say the air on this side is moving is moving and it's a a medium velocity so right over there but as we move more in that direction the air the air is moving faster so they have a they have larger larger velocity vectors like that we see that in general all of the air is moving in that general direction that's the way I'm drawing it and I can draw the velocity vectors over here the air is less dense but the trend and the velocity vector when I go from the left to the right is roughly the same so that's the flow now I'm just sampling some velocity vectors there so let's talk about flux and to think about flux really of any form you have to think about a surface so let's imagine you were to put some type of a net let's say you were to put a net right over right over here right over here and if we were to think about the flux we would say well how much air is traveling through that net in a certain amount of time we could say how many Maalik fuels are travelling and say each second and so this would have some flux associated with relative the air but what if we were to take that same net and we're assuming is some type of a theoretical net that actually does not impede the air the airflow but it allows us to it helps us visualize a surface what if we were to move that net a little bit to the right where the density is the same but the particles are just moving faster well now our flux would increase so this is larger flux larger larger flux why is our flux increase because well the density is the same but in any given amount of time I'm going to have more things going through that surface now what if I were to put take that same net and move it up to this high altitude right over here that high altitude well the velocity is of the molecules are the same and they're going in the same direction but there's just fewer of them so you're going to have less fewer molecules traveling through the surface in a given amount of time so this is going to have smaller flux and this is all relative to my first one smaller smaller flux now what if you were to take the same net and instead of the air going perpendicular to the surface being normal to the surface what if you were to take the net and and reorient it so that the air is going in the same direction of the surface so what if you were to take the same net and you were to make it like this so that it's the same net and it doesn't look exactly the same but it's the same net and you're to make it like this well now how much air is traveling through that net in a given amount of time well now very little to zero air is going to be traveling through that net in a given amount of time the air is going along the surface not through the surface so this one we could say this one let's just say that all the air is going exactly in the same direction as the surface so nothing is going through the surface we would say that this one has zero zero flux now let's say this this net this theoretical net that actually does not impede the air flow let's say we can stretch it or contract it so if we stretch that net in the same let's say we were to do it let's say we were to stretch it up like this so it becomes a bigger net so it becomes a bigger net like this well now this thing is going to have larger flux because there's just more area there's more to flow through because now if you said for this surface you're going to have a larger flux because there's just going to be more air is going to go through that in a given unit of time so as you can see when we think about how much of something just flux in the traditional sense how much of something goes through a surface in a given unit of time it depends on the in in this case the density of the substance it depends on its velocity both the magnitude of the velocity and the direction of the velocity we see if we orient the surface or if we oriented the velocity so it's not going normal to the surface perpendicular to the surface well then we could have our flux go down and you could have things in between you could have a net let's say we took that same original net but the direction of the air is neither normal nor exactly in the same direction of the surface well this flux is going to be in between is going to be in between that original flux and this one right over here there is going to be air flowing through there is going to be air flowing through the actual surface but it's not going exactly normal to the surface and as we will explore later when we get a little bit more mathy into it we actually care about the component of the vector of the air that is exactly normal when we have eventually calculate flux but this flux is going to be someplace in between this one and the zero flux because the area isn't going exactly perpendicular well that's just the general the more basic or the for me the more intuitive notion of flux but what do we mean by magnetic by magnetic flux well-liked regular flux we're still dealing with how how things are kind of you could say going through a surface but instead of thinking about air particles or water molecules or things like that we're going to be thinking about a magnetic field so let me draw a surface so I have a I have a little bar magnet this is an this is the north side this is the south side we see our field lines and then I've drawn a couple of the magnetic field vector in white there and let's say that we have a surface we have a surface like this and so for this surface when we think about the flux we want to care about how much we're not actually things are actually actually moving when we think about magnetic flux we aren't actually thinking about actual physical things moving through this surface the way we did when we thought about I guess you could say traditional or more or flow-based flux but it's a similar idea we care we care about the component of the magnetic field and the density of the magnetic field that is normal to this surface so let's say that this has a given flux so let's call that and the notation is Phi and I'll say that's the flux of the magnetic field and once again it's based on it's based on the strength of the magnetic field and especially the component of the magnetic field vectors that are perpendicular that are perpendicular to this actual surface so this would have one magnetic flux but if I were to take the same surface and make it parallel make it parallel to the two the magnetic field vectors instead of being normal or now the magnetic field vectors are parallel to init's that are being normal so now our flux is zero so this would have zero are pretty close to zero so approximately zero magnetic flux magnetic flux flux now if I were to take this surface and instead of orienting this way I just moved it further away so instead of putting here I were to take it all the way out here where the magnetic field where the magnetic field is weaker where the magnetic field is weaker so we have a weaker magnetic field out here the flux would also decrease so it has a lot of the same properties as this more physical flux that we were talking about before but instead of talking about say the velocity of the air molecules we're talking of illicit e vector of the air molecules and how those relate to the surface here we're talking about the the magnetic field vectors and how that those relate to the surface but the the for the analogies is still the same if they are perpendicular you have larger flux if they are if they are going in the same direction you might have zero or very little you might have very little flux if you have a weak magnetic field out here the flux is going to be lower than if you have a strong that if you have a strong magnetic field that's analogous to when you had high density and high velocity that was a lot of flux versus low density or low velocity that is lower flux and also we could increase the total surface so if we stretch that surface or we had a larger surface right over here the flux through this surface is going to be larger than the flux through this surface let's say this surface is analogous to a surface like this right over here let's say that the magnetic field is symmetric on both sides so the flux through this surface and this surface would be the same so if you were to stretch it out to have a to be a larger surface well now you just have more you have more of the magnetic field that is now normal to the surface so hopefully this gives you at least a beginning ideas of the notion of magnetic flux and how it relates to as you can think of more physical flux