Okay, now we need to talk about real voltage and current sources.

And that's going to bring us to the point where we can then start talking about

impedance matching. So now, up to now, we've been talking

about only ideal voltage sources. So, an ideal voltage source.

The definition of that is that, it has a fixed voltage no matter how much current

is drawn from the source. And we just represented that with, with

this sort of symbol. Now, a real voltage source can only

provide a limited amount of current. There is a you know, if you take, for

example, a 1 1 half volt battery, it's 1 1 half volts.

but if I, completely short-circuit that battery, it cannot provide an infinite

amount of current. A very large current, but it's limited.

And the way to represent that is to add a small internal resistance inside the

battery. There actually is a small amount of

resistance inside the battery. And that limits the maximum amount of

current available from that source. So then, if we take this model of a real

voltage source here, and then I connect a short circuit across it.

I'm going to have some current flowing through this short.

And the amount of current that flows through that short is going to be limited

by the internal resistance of the battery.

So, let's say this is a 1 and 1 half volt battery.

And it has an internal resistance of 0.1 ohms, for example.

Then, the total amount of current flowing through this short will be limited at 15

amps. So, that's a lot of current, but it's not

infinite. Now the, in a practical sense then we're

getting closer to the idea of impedance matching.

So, in a practical circuit. I want to let's say I'm trying to build a

heater circuit. And I want to get the maximum of power

transfer from the battery into this load resistor to make the maximum amount of

heat. And now, in practice, the load resistance

and the internal resistance form a voltage divider.

So remember, this is inside the battery and this is the external load.

So, the voltage developed across RL, the load, is just going to be familiar

voltage divider equation. So, it's the total voltage times this

factor, which is the ratio RL over the total series resistance RL plus R

internal. Now, we're ready to talk about the

subject of impedance matching. So, this is the same circuit that we just

looked at, you have a volt, voltage source, a real voltage source with some

internal impedance. And we attached some lower resistor and

there's a current flowing around this circuit.

So the question is, what is the value of the load resistance rl that will give us

maximum power transfer from this source to the load?