Faraday's Law and Lenz's Law(HD)

brightstone has thousands of high quality videos covering all major subjects please check out more at www.lsnjlawhotline.org netic flocks you'll generate electro-motive force EMF you can think of EMF just like voltage it's not exactly the same thing but for all intents and purposes it kind of is all right so what's magnetic flux magnetic flux is magnetic field times the area alright so it consists of two different pieces how big is the magnetic field and how big is the area over which the magnetic field is acting all right so the way that it works is when I have a change in magnetic flux I generate an electro-motive force to run a current around now remember that when you've got a current you generate a magnetic field now the way that the law works is that systems do not like change so whenever the magnetic flux changes the current is induced in such a direction as to minimize that change alright the way that it looks mathematically is EMF EMF always looks like an electrified e equals minus change in magnetic flux divided by change in time this minus sign is Lenz's law Lenz said that the magnetic flux our change will what sorry that the electro-motive force will oppose the change in magnetic flux so he gave the minus sign this is berrin a so faraday Lance all right so let's do an example suppose that I've got this situation the magnetic field is directed out of the board and let's say that B decreases alright so it's out of the board but it's getting smaller now remember the way that this works systems don't like change it's not about trying to make the magnetic field zero it doesn't like change so it had all this magnetic flux already and now the flux is going away so the current is going to try to bring back the blocks and it's going to go it's going to generate a current in such a direction that the magnetic field generated from the current is out of the board so it's going to generate a current in that direction if the magnetic field goes down alright what if the magnetic field coming out of the board started to increase well now we got more magnetic flux but I don't want change so now the current is going to go in the other direction so that the magnetic field opposes that increase in arm in magnetic flux and so that's the way it goes it's just kind of all right what was the change and then let's try to mitigate that let's try to make it as small as possible all right now there's one very very very classy example of this and I'm going to show you a youtube video that illustrates what happens now as you watch this happen on the the demonstrator is going to drop a magnet down these two sides this is glass that's aluminum notice that the magnet running down the glass landed way before the aluminum dead so that's the idea this magnet was coming down we had a change in magnetic flux because the magnet was moving and then that generated a current in the aluminum that opposed that change it tried to slow the magnet down so that the change was not as big as it would have been otherwise now what's interesting about this is that aluminum is not a magnetic material you can take a magnet touch it to illuminate it doesn't it doesn't care it's not a magnetic material this was not a magnetic effect directly it was the current that generated there and even though aluminum is not a magnetic material it certainly is a conductor and it will definitely support a current so that's a very very very interesting example of how the Faraday Lenz law works another important example is the use of rail systems like the BART or like the Metro in DC the way that these things work is the car is going and it there it goes it goes it goes we get to the station and suddenly there's a magnetic field introduced it doesn't like the change so it tries to stop to make that change take place slower alright let's do an example so this is a numerical example I've got a 5 Tesla magnetic field and it's directed out of the page and it changes to zero Tesla in point one seconds and I want to know the EMF that's generated in a wire loop that has two square meters of area and I want to know the average current um if the resistance is 20 ohms all right so let's see how this goes first thing I need to do because the EMF is equal to minus the change in flux over the change in time I need to find out the change in flux well the area of the loop didn't change so it's not the area that change it's the magnetic field so the change in flux is going to be the change in magnetic field times the area well the magnetic field changed from five to zero so the change was negative five times the area which is two so we'll have negative ten and then it'll be Tesla square meters alright so that is my change in magnetic flux what about the change in time well the change in time is 0.1 seconds so the EMF will be equal to minus and I've got a negative 10 Tesla meters squared over 0.1 seconds and so that's going to give me a hundred and what do you think the unit is well we could work through what is a Tesla meters squared per second or we could say it's a EMF so it must be volt all right everything is in SI units so everything's at SI units very very simple all right now I want to know the average current well if I've got a EMF of a hundred volts resistance of 20 ohms current equals V over R so the current will be 5 amps now what direction will that current be in well if this is my wire and I've got magnetic field coming out of the board but it's going down right then I want to bring back the flux so this is going to be a counter clockwise current all right now another wonderful example of the use of the Faraday Lenz law is in the construction of something called a railgun what a railgun is is it's a construction like this we've got a wire that comes down I put a little resistor in there um wire that comes down like this and then we have here a movable metal bar and this is the rail all right and then we impose an extremely strong magnetic field on the whole configuration and then we want to fire it what do we do get rid of the magnetic field very quickly no magnetic field so now what does the system want to do what it wants to bring back the blocks look there was all that it was they'll bring it back so it's going to generate a current in this in the direction that will generate a magnetic field into the page so that's like that so this current is going to be going around like that after I turn off the magnet while I'm turning off the magnetic field now notice what happens I've got a current going down in a magnetic field directed into the page so now I've got a force out like that and this rail is going to accelerate very very very quickly and just be fired off the edge of it and that's a railgun now one interesting way that we can think about this effect instead of thinking about the current and the force on that we could change our viewpoint and we could say alright how can the system act to lower this change in flux how can it bring back the flux well one way that it could do it it doesn't have as much folks this thing's movable flux is magnetic field times area moving this can't change the magnetic field but it can certainly change the area so if I want the books to remain the same what am I going to do well I'm going to increase the area so that B times a won't go down as fast as it would have if I just had the same area and so that's a railgun and that's the Faraday Lenz law and by - I can't do this with you - laughing back there so if we had no that's not right three coplanar points so have you ever gotten up the airplane that should be yeah dang it like five hundred degrees in here what alright when you're in chemistry class they're gonna be doing a lot of work you're gonna go funny ever so as an example we can consider like you've got a chain hanging from - um - fix

Channel: Brightstorm

Views: 136,100

Rating: 4.9151292 out of 5

Keywords: Lenz's Law, faraday, magnetic flux, electromotive force, area, physics, brightstorm, study, help, sample problem, concept, law

Id: gm-U3v6K1Sk

Length: 10min 3sec (603 seconds)

Published: Wed Apr 23 2014