Faraday's Law of Magnetic Induction or WHY IS THERE ROCK AND ROLL? | Doc Physics

so if you have kids it's inevitable they're gonna come up to you and say something like daddy why is there rock and roll and you're gonna say to them rock and roll comes from Faraday's effect of electromagnetic induction and it's like this it's like if you have a hunk of metal and this is really just gonna help us transfer flux from one place to another and oh we should label this it's not just any metal but let's say that it's a ferrous metal it's got to be magnetizable but we probably don't want to be very magnetizable it's just gonna be a little bit magnetizable I'm going to put a switch in the circuit here and have the circuit go through battery and then I'm gonna have the circuit go through a resistor just to show the fact that every circuit has a resistor that's fine and then I'm gonna have it go in front and then back around and in front and then back around and in front and then back around and in front and then back around and then it will be leaving to that switch right there you get the idea here it is looping around that piece of metal and then I got to have this other one and this other one is gonna be doing the same thing let's go with a different color why the heck not right so there's another coil here it goes in front and then back around in front and back around in front and back around let's not bicker about number of times around or anything like that yet and all I'm gonna put over here is a voltmeter probably gonna be a really sensitive voltmeter probably a galvanometer but in this case we got the same number of coils n is equal to 4 here and what I want you to believe is that if I create a magnetic field inside of this coil by flipping that switch close the switch the battery makes a current go this direction which makes a magnetic field well let's figure it out it's going that way children so it makes a magnetic field pointing this direction inside the coil but inside this coil inside the soul there's metal and the metal then magnetizes which tends to magnetize a little bit more the metal a little bit more the metal a little bit more the metal so in general this whole piece of metal will become an electromagnet during the time that the current is flowing so if I do close the switch it will magnetize the piece of metal and this will be the North Pole and that will be the South Pole let's see if we can label that sucker right there North Pole and then the beautiful thing is here's the most beautiful thing since the magnetic field inside of this coil changed the coil induces a voltage and that voltage can be read off by this voltmeter right here that is Faraday's law of electromagnetic induction and michael faraday is one of the he's one of the we'll go Spanish flava he's one of the physicists who sought symmetry because he knew that a current could make a magnetic field he was wondering if a magnetic field could make a current and he found that most definitely it could there will be a steady current through this voltmeter if it's well we want to put a resistor there and let's put a resistor in here there will be a steady current through that resistor if we have wait a second is that right no there will be a temporary current through that resistor if we create a steady current through this resistor whoa interesting so there's something more complicated going on here we need to go a little bit further let's define magnetic flux first Oh we'll probably see that what am I talking about Les Paul and his electric guitar that he invented were I think at the very basis of the ability to make rock and roll and Les Paul sorta made that awesome and Jimi Hendrix brought it to fruition so what I want to do next is define flux this is magnetic flux looks like like electric flux doesn't it mm-hmm so this is the Greek letter Phi with capital and so Phi's will add to flux because it's flowing something or other but flux is just filled dotted in to Eretz the field that goes through a particularly so it's just going to be B times a times cosine of the angle between but the angle between oh I need to get my loop you need to remember that a loops Direction is outward for the loop that loop right there has a well it has a direction the surface itself has a direction that is out from it how do you tell out from in very carefully you probably are not going to be able to tell unless you know which direction the current is going so we're going to define direction in a little bit but we're saying that this is the definition of flux and it has a dot product which is also called the scalar product because it does not give a vector notice that flux is not a vector it's simply a quantity and it has units I would say that the units of flux well the units of flux have to be unit of magnetic field so that's Tesla and then the units of area is meter squared Tesla per meter squared is called a Weber or W B that's kind of cool for Webster all right but here's my point magnetic flux as defined is not particularly interesting but I need to give you Faraday's law of magnetic induction and Faraday's law of magnetic induction says that an induced voltage an induced voltage is caused by a change in magnetic flux if magnetic flux changes then you're going to get an effort on the part of the coil that's experiencing the flux changing like here's the idea the idea is if I've got no magnetic field going through here and then I turn on a magnetic field through here then the the loops not going to like that and it's going to get a voltage that will try to oppose that and I'll go into a little bit more detail about this minus sign right here but the minus sign says nature hates change let's just define that little very carefully here all right the next that this is actually my favorite part of this entire law but the next part says that if you've got more loops than you get a greater induced voltage I think that sort of makes sense a single loop doesn't have a lot of induced voltage but if you get a heck of a solenoid then you have a great induced voltage and we also want to say that this is change in flux over change in time and let's quit screwing around I mean you could go all formal on this you could say negative n times flux final minus flux initial divided by T final minus T initial we're using knots in this year sorry all right you could do that but let's be honest and just say that we're actually talking about the derivative it will actually be negative n times D Phi DT as flux changes a voltage is induced and a lot of books like to put this as the EMF but you know that I hate that I don't like that it's just a silly notation thing an induced voltage occurs and the faster flux changes the greater the induced voltage is that's awesome

Channel: Doc Schuster

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Id: UAjCrl-SPcY

Length: 8min 18sec (498 seconds)

Published: Mon Jan 28 2013