Now we'll be starting Module 3. 
Going back to our concept map, we have 
finished the background module, the 
module on resistive circuits and now 
we're starting the module on reactive 
circuits. 
I'd like to stop and give you some 
background or some way of comparing 
resistive circuits to reactive circuits 
to teach you or motivate you why we 
want to study reactive circuits, and how 
it differs from resistive circuits. 
To do that, I'm going to show a 
experimental result. 
This is a display from an oscilloscope. 
Oscilloscope is a measuring device that 
shows voltages. 
Along the horizontal axis is time. 
So each division is a division in time. 
This particular one is milli-, in 
milliseconds, 1 millisecond [UNKNOWN] per 
division. 
The vertical axis is voltage. 
The green line shows the voltage source 
to resistive network. 
And in this case, it's going up and down. 
So here, it's at 1 volt, and then it goes 
down to minus 1 volt for 2.5 
milliseconds. 
Then it goes up to plus 1 volt and then 
back down to minus 1 volt. 
This is what's called the square wave. 
The blue line is the voltage across one 
of the resistors in that circuit. 
Notice how the voltage across the 
resistor is exactly in line and looks 
exactly the same as the voltage source 
except for the fact that it's scaled. 
And that's what it happens in a static 
circuit. 
The voltage output exactly matches the 
form, the shape of the voltage input. 
Now, a reactive circuit is different. 
In a reactor circuit, I've got, in this 
particular case, a capacitor in there. 
The voltage source looks the same but the 
response across any of the elements in 
that circuit looks different. 
It doesn't exactly match the shape. 
It's got some. 
Behavior to it. 
So this is equivalent, let's say in a 
physical system, having an object, a 
physical object. 
And it's got object, it's got inertia to 
it. 
And we want to push that object up to a 
certain speed. 
Well it takes you a while to get up to 
speed. 
And that's equivalent to what's happening 
here. 
It takes it a while to get up to the, the 
correct voltage. 
It doesn't happen instantaneously. 
And that's the difference between a 
resistive and a reactive circuit. 
And that basic behavior allows us to do a 
lot of very interesting designs in our 
circuit, so it has a lot of interesting 
behaviors. 
So, it's important for us to understand 
those behaviors. 
And to understand those behaviors, we're 
going to have to look at differential 
equations. 
And we'll give you some very basic 
background in differential equations 
during this module. 
Bringing back up the concept map. 
We've done the background in resistive 
circuits, and we're pulling some of those 
concepts forward into the reactive 
circuit module. 
In particular, the definitions of current 
and voltage, voltage sources, and current 
sources, and resistance were pulling 
forward from the background module. 
From the resistive circuit module, We're 
going to be utilizing those methods to 
obtain circuit equations. 
That includes Kirchhoff's Current Law, 
Kirchhoff's Voltage Law, mesh, node, and 
Thevenin equivalence. 
Everyone of those methods that we've 
already used are pertinent to the 
reactive circuits. 
The particular concepts we're going 
through in reactive circuits are 
capacitors and inductors, differential 
equations and I'm not assuming that you 
know differential equations by now. 
I'm only going to be using enough 
material to introduce to how to solve RC 
circuits and RL circuits.