Electromagnetic Induction

So let's talk about Electromagnetic Induction now this is actually one of the most important discoveries of the 19th century that has to do with electricity and magnetism it actually follows from the Kirchhoff, Faraday - Lenz law but the ideas that we kind of brought out of this, led to the modern power grid and power transmission so let's see how this works.

Electromagnetic Induction is associated with the property that if you change one current you can induce changes in another current so let's see how this works alright we got a current in along wire that's like that so going to the right and it's going to increase so let's see what that increase in current means as far as the Faraday - Lenz law is concerned. Alright well this current since it's going this direction is associated with magnetic field that's going into the page inside this second circuit loop. Alright this current is increasing that means that the value of this magnetic field is increasing that means that the magnetic flux through this loop is increasing into the board. Now Faraday - Lenz systems don't like change it wants to remain the same so the increase of the magnetic field into the board generates a current in this loop such that the magnetic field generated by this current points out of the board to try to cancel that increase in magnetic flux.

Alright, so how does the current have to go to generate a magnetic field that's coming out of the board? Well right hand rule, magnetic field comes out fingers show the direction of the induced current so when we've got this long wire like that we increase the current we generate a current down here. Now the beautiful beautiful thing about this property of electromagnetic induction is that it allows us to transmit current with no physical contact at all. There is no physical contact between the top wire and the bottom current loop but despite that fact, we use the magnetic field in compliance with the Faraday - Lenz law to transmit current down into the other loop.

Now one important thing about this that made Edison very unhappy was that it requires changing current you cannot do this with just a standard dc current because what would you have to do? Now you just have to increase the current and just keep on increasing it I mean that's not sustainable so what people do instead is they use something called alternating current so here is the idea when this current is increasing to the right the generated current will be counter clockwise, when this current is decreasing, the generated current down here will be clockwise so if we look over at a graph and this is an alternating current notice it looks like a cosine function because that's what they look like cosine functions, what we need is we need to write down when this current is decreasing this current should be negative when this current is increasing this current should be positive so let's see; decrease so it's negative so it's going to go down like that here it's increasing at the maximum rate so that means that it's going to be maximum current in the second loop right here when this current's zero and then keeps on decreasing so this is still negative until here where it's going to start to increase and so now this current generated in the secondary loop will be counter clockwise again so positive so we'll go positive maximum right here and then down still positive until here where this current is now decreasing and then it will be negative like that so you can see that we input an alternating current and our output current will also be alternating so this is what allows us to transmit alternating current or ac, well really I should just say ac lot's of time people say ac current or ac, ac stands for alternating current so we're kind of done with that so the idea is that we could transmit alternating current using electromagnetic induction and we can't do so for dc and that's what led in 1893 to a pretty much all hands on board changeover to ac power in this country and that's electromagnetic induction.