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Welcome back to linear circuits. 
Today, we're going to begin our 

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discussion of transformers. 
This lesson is essentially just going to 

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present transformers as a device and 
describe the physics on how they work. 

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This concept of transformers is fairly 
extensive one and so to help make it a 

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little bit easier to learn. 
It's been broken up into several smaller 

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pieces. 
So that it is nicely compartmentalized. 

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In the previous lesson we talked about 
maximum power transfer in AC systems. 

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Sometimes to get maximum power 
transferred, transformers are actually 

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used for the impedance matching that was 
discussed. 

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But we're going to talk about 
transformers generally and look at that 

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number of different places that they, 
they turn off when they're used. 

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The objectives for this lesson are to 
identify physical transformers and their 

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circuit representations and then to 
describe the physical function of 

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transformers. 
This is representative of a transformer. 

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Here the grey is indicitive of a ferrous 
metal core. 

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So ferrous meaning that it's got some 
iron in it. 

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And the reason behind it being ferrous is 
ferrous materials hold magnetic fields 

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very well. 
Then we have two coiled wires represented 

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in these somewhat orangish-colored coils 
here. 

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So, this side, as we put a current 
through it Is going to generate the 

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magnetic field according to Ampere's Law, 
which we've already discussed when we 

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talked about conductors. 
When the magnetic field is established in 

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this coil, because of this ferris core, 
is going to then have an impact on the 

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other coil. 
Since now there is a magnetic filed that 

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is moving. 
Through this second coil, the second 

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inductor. 
So when we put them together in this 

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configuration, we call it a transformer. 
It doesn't necessarily have to be quite 

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like this. 
The only thing that makes it a 

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transformer is this idea of mutual 
induction, that current going through one 

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coil generates a magnetic field that has 
an impact or an effect on the other coil. 

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When we do circuit diagrams they will be 
drawn somewhat like this where we 

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basically have 2 inductors that are 
placed side by side and the dots here are 

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representative of reference directions. 
The reference directions that are present 

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in transformers we with regard to the 
directions of these coils and. 

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So it's possible that you could coil this 
the opposite direction. 

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In which case, you would move one dot 
down to the other side. 

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This lets you know whether this magnetic 
field is basically working in the same 

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direction or the opposite direction, that 
the magnetic field is coming from the 

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other coil. 
In these systems, we're typically going 

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to connect one side to a source and the 
other side to a load. 

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So to help distinguish the two sides of 
the transformer, we'll refer to one side 

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as the primary winding or the primary And 
now there's the secondary winding. 

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And typically, the primary winding 
relates to the winding that is connected 

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to your power source, and the secondary 
winding is connected to some sort of 

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load. 
It's entirely impossible to flip the 

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direction of the transformer. 
And then you could then change, calling 

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the other the primary, and then the 
previous one the secondary. 

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It really doesn't particularly matter. 
It's just to keep things clear as to 

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which side you're referring to. 
As far as the relationship of magnetic 

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field and current, first of all, we know 
about Ampere's Law. 

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As current flows through the coil, it 
generates a magnetic field. 

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The other thing that makes these devices 
operate is Faraday's Law of Induction. 

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Faraday's Law of Induction states that a 
change in magnetic flux leads to a 

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voltage. 
We're not going to get in the finer 

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details of Faraday's Law of Induction but 
to be able to talk a little bit about it, 

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we need to have some idea of what 
magnetic flux is. 

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So if we take some sort of closed loop, 
and we make a surface that connects all 

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the sides of this loop, and then we count 
the amount of b field, or the magnetic 

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field that's going through this surface, 
and then divide it by the area. 

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That gives us the magnetic flux, which is 
represented by the, by capital phi. 

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This is somewhat similar to the way that 
we calculated currents. 

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We calculate the amount of charge that 
was moving through some surface, in time. 

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So it's a similar kind of an idea. 
Faraday's law of induction states that 

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changing Magnetic flux leads to voltages, 
which means that if the b field here is 

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constant, there's no changing magnetic 
flux, so zero voltage, which is why 

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inductors behave like wires if you let 
the currents flow through them stay 

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constant in time. 
So the implication of this is that 

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transformers are AC devices. 
In order for them to work the way that 

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we've described, you need to have an 
alternating current. 

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If the current is DC or if it's constant 
in time, it doesn't quite have the same 

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impact as it would if the current were 
alternating. 

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There's two primary models for analyzing 
transformers in a linear sense. 

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The first is the linear transformer 
model, where it uses impedances for the 

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analysis. 
Impedances for the two coils as well as 

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impedance for the neutral induction. 
This is primarily used in communications 

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applications. 
The ideal transformer model is primarily 

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used for power transfer applications. 
It requires a few different assumptions 

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to be made. 
That are never quite, actually true. 

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But they generally give us a good idea of 
how the transformer is going to operate 

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in a certain circumstance. 
In per ideal transform models, we're 

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simply going to make use of the voltages 
and the number of coils turns for the 

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analysis. 
To summarize, we just introduced 

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transformers as a circuit device, 
described their physical behavior and 

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introduced the 2 analysis modes. 
In the next lesson we'll start by talking 

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about the first of these, the linear 
transformer model. 

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To see how we can use it for analysis, 
until then.