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Current time:0:00Total duration:5:46

Video transcript

so right over here have depicted a square loop of a conductor let's say it's a wire and it's stationary and it's sitting in a magnetic field and I've drawn a few vectors that represent the magnetic field and you can see at least on the surface that is defined or this contoured by the wire that the magnetic field looks constant so if we just had this scenario nothing too special going on but it becomes interesting if I were to actually change the flux going through the surface so both of these pictures right over here they actually show the same scenario where we have increased the flux we have increased the flux at these points on the surface defined by the wire at every point the magnetic field has now got gotten stronger so we have increased the flux so let me write that the flux we use a Greek letter Phi to usually denote flux the flux of the magnetic field the flux of the magnetic field has gone up and we know from Faraday's law that when you have a change in your flux then that's going to be that's going to induce a current in the loop and so an interesting question is what direction is that current actually going to go in we have two options the current could go the current let me find a nice color for the current the current could go in the the current could go in the clock in the clockwise direction or it could go in the or it could go in the counter clockwise direction so which of these two do you think the current will actually go in well let's think a little bit about it we know that a current flowing through a wire actually on its own will induce a magnetic field above and beyond the magnetic field that's already there so let's think about the type of magnetic field that this orange current would would actually induce so if it's going in the counterclockwise direction remember we use the right hand rule I can take my right hand point my thumb in the direction of the current so let me see if I can do that so my thumb in the direction of the current and then my hands my hands are going to loop or as you say my fingers are going to loop in the direction in the direction of the magnetic field so when I do that with my right hand so my right hand looks something like that thumb in the direction of the current right over here we see that it shows a magnetic field being induced that would that would wrap around like that or if we were to show it if we were to sample it points right on the surface the magnetic field that would be induced would look something like this and I'm just doing it at some sample points so notice it would be additive it would be additive whoops it would be it would be it would be additive to the existing magnetic field in fact it would increase the flux even more in fact at these points these vectors would increase even more these vectors let me see if I can draw that these vectors would increase even more well what would that do if the flux increases even more than the current is going to increase the current is going to increase even more which is going to make the flux increase even more and which makes the current increase anymore and you would have this never-ending cycle where the current keeps increasing and the flux keeps on increasing and you would have this energy that's appearing out of nowhere which would violate the law of conservation of energy and so that's a pretty good argument for why you would not that's a pretty good argument for why we would not expect the counter the counter clockwise scenario we would not want the current that's an induced to induce a magnetic field that goes in the same direction as our increase in flux so just by deductive reasoning we know that this is going to be the scenario and let's see what think about what happens here we can do the right-hand rule again we take our right hand point our thumb in the direction of the current point our thumb in the direction of the current and we see if we do that if we do that with our right hand well now this would this would induce a magnetic field that would decrease the flux so it would it would produce a magnetic field the current right over here would produce a magnetic field that's going downward the current in that direction would also when you take your when you take your right hand and you were to put it along here once again if you're going in the if you are going in the counter clockwise or sorry if you're going in the clockwise direction over here that to when you do your right hand rule right over here your fingers would coil around that way and so once again when you look at the surface it would produce it would induce a magnetic field that is going in that direction and so it would have the induced magnetic field from the induced current will go against the change in flux and this makes sense because we won't go into this never-ending positive feedback loop where the current keeps getting stronger and stronger and the flux keeps increasing and increasing and increasing and this idea that the orientation of the current that is induced will produce a magnetic field that counteracts the the the change in flux this is Lenz's law lenses lenses law and once again if you if that wasn't true then you would have a violation of the of the conservation of energy so in general whenever someone if someone says ok well this is the change in flux is happening in a certain direction well to think about which way the current would flow you just have to say well what type of a magnetic field would each direction of the current produce and that magnetic field should go in a direction that goes against your change in flux so if the flux is increasing the magnetic field that's induced by the induced current should make the flux decrease if the flux was decreasing then the induced magnetic field by the induced current should make the flux decrease less or should make the flux should be additive to the flux