Welcome back to linear circuits. 
Today, we're going to begin our 
discussion of transformers. 
This lesson is essentially just going to 
present transformers as a device and 
describe the physics on how they work. 
This concept of transformers is fairly 
extensive one and so to help make it a 
little bit easier to learn. 
It's been broken up into several smaller 
pieces. 
So that it is nicely compartmentalized. 
In the previous lesson we talked about 
maximum power transfer in AC systems. 
Sometimes to get maximum power 
transferred, transformers are actually 
used for the impedance matching that was 
discussed. 
But we're going to talk about 
transformers generally and look at that 
number of different places that they, 
they turn off when they're used. 
The objectives for this lesson are to 
identify physical transformers and their 
circuit representations and then to 
describe the physical function of 
transformers. 
This is representative of a transformer. 
Here the grey is indicitive of a ferrous 
metal core. 
So ferrous meaning that it's got some 
iron in it. 
And the reason behind it being ferrous is 
ferrous materials hold magnetic fields 
very well. 
Then we have two coiled wires represented 
in these somewhat orangish-colored coils 
here. 
So, this side, as we put a current 
through it Is going to generate the 
magnetic field according to Ampere's Law, 
which we've already discussed when we 
talked about conductors. 
When the magnetic field is established in 
this coil, because of this ferris core, 
is going to then have an impact on the 
other coil. 
Since now there is a magnetic filed that 
is moving. 
Through this second coil, the second 
inductor. 
So when we put them together in this 
configuration, we call it a transformer. 
It doesn't necessarily have to be quite 
like this. 
The only thing that makes it a 
transformer is this idea of mutual 
induction, that current going through one 
coil generates a magnetic field that has 
an impact or an effect on the other coil. 
When we do circuit diagrams they will be 
drawn somewhat like this where we 
basically have 2 inductors that are 
placed side by side and the dots here are 
representative of reference directions. 
The reference directions that are present 
in transformers we with regard to the 
directions of these coils and. 
So it's possible that you could coil this 
the opposite direction. 
In which case, you would move one dot 
down to the other side. 
This lets you know whether this magnetic 
field is basically working in the same 
direction or the opposite direction, that 
the magnetic field is coming from the 
other coil. 
In these systems, we're typically going 
to connect one side to a source and the 
other side to a load. 
So to help distinguish the two sides of 
the transformer, we'll refer to one side 
as the primary winding or the primary And 
now there's the secondary winding. 
And typically, the primary winding 
relates to the winding that is connected 
to your power source, and the secondary 
winding is connected to some sort of 
load. 
It's entirely impossible to flip the 
direction of the transformer. 
And then you could then change, calling 
the other the primary, and then the 
previous one the secondary. 
It really doesn't particularly matter. 
It's just to keep things clear as to 
which side you're referring to. 
As far as the relationship of magnetic 
field and current, first of all, we know 
about Ampere's Law. 
As current flows through the coil, it 
generates a magnetic field. 
The other thing that makes these devices 
operate is Faraday's Law of Induction. 
Faraday's Law of Induction states that a 
change in magnetic flux leads to a 
voltage. 
We're not going to get in the finer 
details of Faraday's Law of Induction but 
to be able to talk a little bit about it, 
we need to have some idea of what 
magnetic flux is. 
So if we take some sort of closed loop, 
and we make a surface that connects all 
the sides of this loop, and then we count 
the amount of b field, or the magnetic 
field that's going through this surface, 
and then divide it by the area. 
That gives us the magnetic flux, which is 
represented by the, by capital phi. 
This is somewhat similar to the way that 
we calculated currents. 
We calculate the amount of charge that 
was moving through some surface, in time. 
So it's a similar kind of an idea. 
Faraday's law of induction states that 
changing Magnetic flux leads to voltages, 
which means that if the b field here is 
constant, there's no changing magnetic 
flux, so zero voltage, which is why 
inductors behave like wires if you let 
the currents flow through them stay 
constant in time. 
So the implication of this is that 
transformers are AC devices. 
In order for them to work the way that 
we've described, you need to have an 
alternating current. 
If the current is DC or if it's constant 
in time, it doesn't quite have the same 
impact as it would if the current were 
alternating. 
There's two primary models for analyzing 
transformers in a linear sense. 
The first is the linear transformer 
model, where it uses impedances for the 
analysis. 
Impedances for the two coils as well as 
impedance for the neutral induction. 
This is primarily used in communications 
applications. 
The ideal transformer model is primarily 
used for power transfer applications. 
It requires a few different assumptions 
to be made. 
That are never quite, actually true. 
But they generally give us a good idea of 
how the transformer is going to operate 
in a certain circumstance. 
In per ideal transform models, we're 
simply going to make use of the voltages 
and the number of coils turns for the 
analysis. 
To summarize, we just introduced 
transformers as a circuit device, 
described their physical behavior and 
introduced the 2 analysis modes. 
In the next lesson we'll start by talking 
about the first of these, the linear 
transformer model. 
To see how we can use it for analysis, 
until then.