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Now we'll be starting Module 3. 
Going back to our concept map, we have 

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finished the background module, the 
module on resistive circuits and now 

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we're starting the module on reactive 
circuits. 

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I'd like to stop and give you some 
background or some way of comparing 

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resistive circuits to reactive circuits 
to teach you or motivate you why we 

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want to study reactive circuits, and how 
it differs from resistive circuits. 

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To do that, I'm going to show a 
experimental result. 

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This is a display from an oscilloscope. 
Oscilloscope is a measuring device that 

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shows voltages. 
Along the horizontal axis is time. 

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So each division is a division in time. 
This particular one is milli-, in 

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milliseconds, 1 millisecond [UNKNOWN] per 
division. 

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The vertical axis is voltage. 
The green line shows the voltage source 

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to resistive network. 
And in this case, it's going up and down. 

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So here, it's at 1 volt, and then it goes 
down to minus 1 volt for 2.5 

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milliseconds. 
Then it goes up to plus 1 volt and then 

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back down to minus 1 volt. 
This is what's called the square wave. 

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The blue line is the voltage across one 
of the resistors in that circuit. 

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Notice how the voltage across the 
resistor is exactly in line and looks 

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exactly the same as the voltage source 
except for the fact that it's scaled. 

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And that's what it happens in a static 
circuit. 

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The voltage output exactly matches the 
form, the shape of the voltage input. 

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Now, a reactive circuit is different. 
In a reactor circuit, I've got, in this 

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particular case, a capacitor in there. 
The voltage source looks the same but the 

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response across any of the elements in 
that circuit looks different. 

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It doesn't exactly match the shape. 
It's got some. 

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Behavior to it. 
So this is equivalent, let's say in a 

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physical system, having an object, a 
physical object. 

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And it's got object, it's got inertia to 
it. 

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And we want to push that object up to a 
certain speed. 

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Well it takes you a while to get up to 
speed. 

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And that's equivalent to what's happening 
here. 

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It takes it a while to get up to the, the 
correct voltage. 

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It doesn't happen instantaneously. 
And that's the difference between a 

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resistive and a reactive circuit. 
And that basic behavior allows us to do a 

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lot of very interesting designs in our 
circuit, so it has a lot of interesting 

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behaviors. 
So, it's important for us to understand 

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those behaviors. 
And to understand those behaviors, we're 

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going to have to look at differential 
equations. 

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And we'll give you some very basic 
background in differential equations 

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during this module. 
Bringing back up the concept map. 

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We've done the background in resistive 
circuits, and we're pulling some of those 

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concepts forward into the reactive 
circuit module. 

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In particular, the definitions of current 
and voltage, voltage sources, and current 

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sources, and resistance were pulling 
forward from the background module. 

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From the resistive circuit module, We're 
going to be utilizing those methods to 

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obtain circuit equations. 
That includes Kirchhoff's Current Law, 

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Kirchhoff's Voltage Law, mesh, node, and 
Thevenin equivalence. 

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Everyone of those methods that we've 
already used are pertinent to the 

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reactive circuits. 
The particular concepts we're going 

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through in reactive circuits are 
capacitors and inductors, differential 

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equations and I'm not assuming that you 
know differential equations by now. 

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I'm only going to be using enough 
material to introduce to how to solve RC 

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circuits and RL circuits.