So, one question to ask
ourselves is,
what is engineering?
How do we define,
what is engineering?
Well, the definition I like to
use is one put forth by Steve
Senturia, one of our professors
who is now retired.
He defined engineering to be
the purposeful use of science.
All right, so what is 6.002
about?
So, 6.002 is a first course in
engineering.
And I like to view 6.002 as the
gainful employment of Maxwell's
equations.
Many of you have seen Maxwell's
equations before.
Most of you should have.
And they are hard stuff.
6.002 is all about teaching you
how to simplify our lives,
make things simple.
So, if you can gainfully employ
Maxwell's equations,
gainfully employ the facts of
nature to build very interesting
systems.
So let me show you how the
transition is made.
So, there's a world around us,
nature, so we made some
observations in nature.
We make measurements,
and we can write down large
tables of measurements.
So, for example,
we can take objects and measure
the voltage across them,
and look at the resulting
current through the elements.
So, we may end up getting a
bunch of values such as
[CHALKBOARD].
So, we start out life with
making measurements on what
exists.
And we build a bunch of tables.
Now, we could directly take
these tables,
and based on observations of
these tables,
we could go ahead and build
very interesting engineering
systems that help us out in
day-to-day lives.
But that's incredibly hard.
Imagine having to resort to a
set of tables to do any kind of
useful work.
So what we do as engineers,
we first layer a level of
abstraction.
We look at all the data,
and somehow layer abstraction
such that we can simplify or
much more succinctly put in a
simple equation or a simple
statement what these numbers are
telling us.
OK, so for example,
our physics laws,
so laws of physics for example
are simply abstractions,
the laws of abstractions.
So, these sets of numbers can
be codified by Ohm's law,
for example,
V is equal to RI,
the voltage current,
relates to the resistance of
the object.
So, V is equal to RI is a law
that succinctly describes a set
of experiments,
and replaces a large number of
tables with a very simple
statement.
You could call this the law,
or you could call it an
abstraction.
OK so you see laws of physics,
call them abstractions of
physics if you like.
Similarly, there are Maxwell's
equations and so on and so
forth.
So, this is what is.
This is what's out there.
OK, and a law as an abstraction
describe the properties of
nature, as we see it,
in some succinct form.
Now, if you want to go and
build useful things,
we could take these
abstractions,
take Maxwell's equations,
and go and build things.
But it's hard.
It's really,
really hard.
And what you learn in,
at MIT is this place is all
about simplifying things.
Take complicated things,
build layers of abstraction,
and simplify things so that we
can build useful systems.
Even in 6.002 we start life by
making a huge leap from
Maxwell's equations to a couple
of very, very simple laws.
OK, I'm going to show you that
leap that we will make today.
So, the first abstraction that
we layer is called the lump
circuit abstraction.
OK, in the lump circuit
abstraction, what we do is we
make a set of simplifications
that allows us to view a set of
objects as discrete or lumped
elements.
So, we may, I will define
voltage sources.
We'll define resistors.
We'll define capacitors,
and so on.
OK, and I'm going to make the
jump, and show you how we make
the jump in a few minutes.
So, on that sort of
abstraction, we then layer yet
another abstract layer.
And let me call that the
amplifier abstraction.
OK, remember,
here we are absolutely down and
dirty.
We are setting the probes,
measuring objects,
and building huge tables.
We abstracted things into
simple laws, and life got a
little better.
OK, I'm going to show you can
abstract things further out and
build discrete objects,
and, you could build even more
interesting components called
amplifiers and begin playing
around with amplifiers.
OK, so when you are using
amplifiers, you don't really
have to worry about the details
of Maxwell's equations.
OK, I'll give you some very
simple abstract rules of
behavior for an amplifier,
and you can go build very
interesting systems without
really, really knowing how
Maxwell's equations applies to
that because you will be working
at this abstract layer.
However, since you're
engineers, and you are good at
building such systems,
it's very important for you to
understand how we make this leap
from the laws of physics into
some of our very primitive
engineering abstractions.
So, once we make the amplified
abstraction in 6.002,
by the way, 6.002 starts here.
We start from the laws of
physics and then proceed all the
way out.
So, once we talk about
amplifiers we will take two
pads.
On the amplifier,
you will build the next
abstraction called the digital
abstraction.
OK, and with the digital
abstraction, we will build new
elements such as inverters and
combinational gates,
OK?
So, notice we are building
bigger, and bigger things,
which have more and more
complicated behavior inside
them, but which are very simple
to describe, right?
So, following the digital
abstraction, we will superimpose
the combinational logic
abstraction on top of that,
and define functional blocks
that look like this:
some inputs,
some function,
some outputs.
The next abstraction on top of
that will be the clock digital
abstraction, where we will have
some notion of time introduced
into the system.
There will be a clock,
and this will be some function.
And there will be a clock that
introduces time into the sort of
logic values that functions
operate upon.
Following that,
the next level of abstraction
that we build is called
instruction set abstraction.
OK, now you begin to see things
that consumers get to look at.
Can someone give me an example
of, or name an instruction set,
or instruction set abstraction?
Bingo.
So, x86 is one set of
abstractions.
And in fact,
in many universities,
education could well start just
by saying, OK,
here's an abstraction.
These are the x86 instructions,
OK?
Some MIT gurus have designed
this awesome little
microprocessor,
OK?
So you just worry about,
you take this abstraction layer
here, the assembly instructions,
and you go and build systems on
top of that.
OK, so this is an abstraction
layer called the x86 layer.
There are other abstraction
layers.
In 6.004, you will learn about,
I believe, the alpha or the
beta, OK, and various other
abstractions at this point.
So, 6.002 kind of goes until
here.
6.002 takes me from the world
of physics all the way to the
world of interesting analog and
digital systems.
OK, 004, the course on
computation structures,
will show you how to build
computers all the way from
simple digital objects all the
way to big systems.
Following that,
you learn about language
abstractions,
Java, C, and other languages,
and that's in 6.002.
And there are several other
courses that will cover that.
Following this,
you learn about software system
abstractions,
and software systems,
you will learn about operating
systems.
Any example of an operating
system abstraction that people
know out there?
What's that?
Linux.
What else?
I'm just wondering how long
I'll have to go before I hear
what I want to hear.
[LAUGHTER] OK,
so we have a bunch of software
systems.
So, if we have a bunch of
software systems,
these are nothing but
abstractions.
Linux simply implies a set of
system calls that the programs
must adhere to.
Windows is another set of
system calls.
That's it.
And see how much money they
made out of it?
OK, it's all about abstraction
layers, that all start from
nature.
All right?
Build abstraction upon
abstraction upon abstraction
upon abstraction,
and someone out here are lots
of dollars.
OK, so based on these
abstractions,
we can then build useful things
for human beings.
We can build very useful
things, video games,
so we can send space shuttles
up, and a whole bunch of other
systems.
But it's based on these
abstraction layers.
What's unique about education
at MIT?
What's unique about 6.002 and
EECS?
Is to my knowledge,
there are not many other places
in the world where you will get
an education in everything going
all the way from nature to how
to build very complicated analog
and digital systems.
OK, we will show you layer upon
layer upon layer upon layer,
peel away the onion until you
are down to raw nature,
OK, through Maxwell's
equations.
So, 6.002, 004,
this is 033,
OK, 6.170, and so on.
OK, the whole EECS is about
building abstraction layers,
one on top of the other.
So that's one path.
There's the analog path.
The analog path would take an
amplifier, and build an
abstraction layer called the
op-amp.
See how similar they all look?
You know the amplifier,
the inverter of the digital
world, and the operational
amplifier in the analog world,
just different ways of looking
at the same devices.
So, to build an analog system,
to build an operational
amplifier, and then,
here we go end up building a
whole bunch of different
interesting analog system
components.
OK, and these components might
look like oscillators.
They might look like filters.
OK, they look like power
supplies, a whole bunch of very
interesting abstract components,
which pulled together can then
give you the next set of
systems.
And these systems might be
toasters, or say for example
other analog systems like the
various control systems for
various power plants and so on
and so forth,
and ultimately,
fun and dollars.
OK, so 6.002 is about going
from physics all the way to this
point.
We will build interesting
analog systems,
and take you up to interesting
digital system components,
from which 004 will take you
all the way to building computer
architectures.
So that, in a nutshell,
kind of gives you a feel for
the space of EECS.
OK, this chart here is almost a
vignette of what EECS at MIT is
all about.
And this is the world according
to Agarwal, because he's
teaching 002.
OK, so this is 6.002,
and the rest of EECS is
somewhere out there.
OK, so I'm going to do now is
throughout this course;
I want you to think about which
part in this vignette we are in.
So, right now,
I'm going to start here and
take you here.
OK, and as you get closer and
closer, things get simpler,
and simpler,
and simpler.
Still, the final abstractions
are pedal, brake,
steering wheel.
I mean, that's the abstraction
to play a game,
right, four or five very simple
interfaces, and that's all you
need to know.
And everybody in the world can
play stuff.
So remember,
this stuff is complicated.
This stuff is very,
very simple.
OK, and the more we build
abstractions and come to this
side, things get simpler and
simpler.
So, a large part of what I'll
cover today is make the biggest
simplification.
The biggest simplification we
will make his go from Maxwell's
equation to some very,
very simple algebraic rules.
OK, I did Maxwell's equations
myself.
And I tell you,
they were very interesting
stuff but complicated.
I can't imagine building
efficient systems using
Maxwell's equations.
So, let's take an example,
OK?
So, let's say I have a battery.
Just switch to page three of
your course notes.
And let's say I connect that to
a bulb.
OK, and this is a wire.
And, the battery supplies some
voltage, V, and I ask you a
simple question.
What is the current through the
bulb?
OK, so here is something that I
can build using objects.
I can pick a round from stores
and so on.
And I can collect them up in
this way, and ask the question,
what is the current,
I?
Now, if all you've done is
learn about Maxwell's equations,
you can roll up your sleeves
and say, ah-ha!
The first step is to write down
all of Maxwell's equations,
and you can say,
del cross E is minus del and go
on, and on, and on,
OK, and write out all of
Maxwell's equations and say,
now how do I get from there to
here?
OK, it's very good.
You can do it.
OK, you can do it,
but it's very complicated.
OK, so instead,
what you're going to do is take
the easy way.
So, what I want to remind you
is that this course is actually
very easy.
OK remember,
we're going to be building
abstraction upon abstraction to
make your lives easier.
If you think your lives are
getting more complicated,
then you are not using
intuition enough.
OK, just remember the big I
word.
It's all about making things
simple.
OK, so let me give you an
analogy.
So, suppose you have an object.
OK, and I apply a force to the
object.
It's an analogy,
OK to get some insight into how
to do this.
So, I say here's an object.
I apply a force,
and I ask you the question.
What is the acceleration of the
object when I apply a force,
F?
So, how would you do it?
OK, and eighth,
or ninth, or tenth grader can
do this.
OK, they would ask me,
what's the mass of the object?
OK, I ask you what is the
acceleration?
You would turn around and ask
me, what is the mass of the
object?
I tell you, the mass of the
object is M.
And then you say,
oh sure, A is F divided by M,
done.
It's as simple as that.
OK, I could have gone into all
kinds of differential equations
and so on to figure that out,
but you asked me for the mass.
And you gave me the answer,
A is F divided by M.
So, you ignored a bunch of
things.
You ignored the shape of the
object.
You ignored its color.
You ignored its temperature.
OK, and you ignored the soft or
hard or whatever.
OK, you ignored a whole bunch
of things.
You were focused on one thing.
OK, you're focused on its mass.
And, it turns out that the
process really was developed
from a set of simplifications.
That is called,
does anybody remember this?
Point mass simplification.
OK, so, in physics,
you've done this before.
OK, you've simplified your
lives by viewing objects as
having a mass at a point,
and force is acting at that
point.
OK, M is that property of the
object that is of interest to
you.
This process is called,
in physics, point mass
discretization.
OK, now using an analogy,
and I'm going to show you a
similar simple process to do the
problem with the light bulb.
OK, so take my light bulb
again,
And I focus on the filament of
the light bulb.
OK, all I care about is the
current flowing through the
light bulb.
OK, I don't care about whether
the filament is twisted,
whether it's hot.
I don't care about its shape.
I don't care about its color.
All I care about is the
current.
OK, so to do that,
what we can do here at a very
high level is since we just need
the current and don't care about
a bunch of other properties,
we will simply replace the bulb
with a discrete object called a
resistor.
So the discrete object is a
resistor, much like the point
mass simplification that we did
earlier that replaced the bulb
filament with a object called a
resistor, a discrete object
called a resistor.
Or a lump object called
resister, and put a value next
to it just like the mass for the
object, a resistance value,
R.
OK, now what I can do is in the
same manner, replace the battery
with an object called a battery
object, and connect that here,
the voltage,
V, applied to it.
V falls across the resistor,
and I get my I simply from
Ohm's law as we divide by R.
So, notice here,
to replace this complicated
bulb, this really twisty,
weird old thing with this
discreet thing called a
resistor, and its only property
of interest was its resistance
value, R, direct analogy to what
we did there.
So, since R represents the only
property of interest,
we can simply ignore all the
other things.
So, notice here,
we've done things the simple
way.
And remember,
in EE, in the electrical
engineering, we do things the
simple way.
OK, we could go the hard route
and do Maxwell's equations,
and get PhD's in physics,
and so on.
But out here,
we are looking to do useful,
interesting systems in the
simplest way that we can.
OK, we do things a simple way.
All right, so we just did this,
and boom, I found out what the
current was.
Now, I cheated a little bit.
I've cheated a little bit.
R is a lumped abstraction for
the bulb.
So, you look at this resistor
here.
That is simply a placeholder.
It's a stand-in for this
complicated thing called a bulb.
It's a discreet object.
It's a lumped object,
and represents the bulb.
Now, so most of 6.002 will take
off from here,
OK, and that's it.
To very simple stuff,
like V is equal to IR,
it's a simple high school
algebra to take off in that
direction.
But before we go there,
it's important to understand,
why was it that we were able to
make the simplification?
OK, we did something else.
Something's going on under the
covers here.
On the one hand,
I say let's use Maxwell's,
and then I jump out and say,
hey, we can just use this
simple thing.
I did something that allowed me
to go from here to here.
And you need to understand why
I did that and how I did that.
Understand it once,
and then you won't have to need
that information again.
You just need to understand it.
So, let's take a closer look at
the bulb filament,
and look at what we really did.
So, here's my filament,
A, and let's say that the
surface area here,
I label that SA,
and the one down here SB,
my voltage, V,
applied there,
and this is what I call my
black box that I've replaced
with a resistor.
Notice that,
in order for this to work,
V and I need to be defined.
So I needs to be defined,
and V needs to be defined.
OK, if I give you a random
object, and I don't tell you
anything else about the object,
it's not clear I can do that.
OK, if it's a much more general
situation, I have to write down
Maxwell's equations,
and this is what I would write
down.
Write down J dot dS as a
function of the coordinate here
integrated over the area minus,
OK, I would have to start from
there from one of Maxwell's
equations.
All right, notice that this
becomes IA, and this becomes IB
in our simplification.
But, if I don't tell you
anything else,
you have to start from here.
You will have some varying
current here by point.
You might have some other
current coming out here because
I may have some charge buildup
happening inside.
If charge is building up inside
the filament;
then I would have to put del q
by del t out here,
right, the current in minus the
current out must equal charge
buildup.
Whoa, where is this and where
is that?
So this is reality.
This is really,
really what I have to do.
But how did I get there?
How did I get there?
The key answer is,
as engineers,
when in doubt we simplify.
Remember, we are engineers.
Our goal in life is to build
interesting systems.
OK and some are motivated by
money.
OK, so our goal is to build
interesting systems and do good
to humanity.
So, as long as we can build a
good light bulb,
we are happy.
So what we can do is we can
say, look, all I care about is
building interesting systems.
So I can say,
hey, this stuff is too hard.
Let's make the assumption that
all the systems that we will
consider will have this thing be
zero.
OK, in other words,
if I take a complete object,
if I take an element like a
resistor or a capacitor,
the box around the entire
element, OK, and I want to just
deal with those systems in which
this thing is zero.
You can come and beat me up and
say, but why?
Why not?
Why am I doing this?
And I am saying the world is
arbitrary.
I'm an engineer;
I want to build good systems.
By making this simplification,
I eliminate this squiggle
thing, and so on.
I don't want to deal with it.
I want to make my life simple.
So this is gone to zero
because, why?
Because I have said that in the
future I will only deal with
those elements for which this is
true.
I'm going to discipline myself.
I'm going to discipline myself
to only deal with those systems.
OK, Maxwell is turning around
and, you know,
mad at me and all that stuff,
but tough.
So this, what I've said about
making a simplification here,
and this is one of the
simplifications I'm making.
And I give a name to the
simplification.
And that's called the lumped
matter discipline.
OK, so I'm saying I will only
deal with elements for which if
I put a black box around it,
this is going to be true.
And if this is going to be
true, then notice,
there is no charge buildup.
Current in must equal current
out.
Ah-ha!
So this becomes IA.
This becomes IB.
Yes.
OK, I can now deal with IA's
and IB's.
And IB and IA are equal because
this is zero.
Notice that there is a whole
bunch of depth here in the jump
from here to here.
As MIT graduates,
you really, really need to
understand why it is that we
made that jump,
and then go and use that,
and do cool things.
All right, this allows us to
define I.
We have a unique I associated
with an element for the current
through the element.
We still have to worry about B,
and I won't go through that in
detail.
The course notes have some
discussion of that and so does
the textbook.
So V, AB is defined when del
phi B, the rate of change of
magnetic flux is zero.
So, if I take the element and I
take any region outside the
element, this must be true.
And you say,
why should that be true?
That's not true in general.
Absolutely.
It's not true in general.
But I, because I choose to,
I going to deal with only those
elements.
I will discipline myself.
But these are only those
elements for which this is true,
and this is true.
I'm going to limit my world.
I'm going to create a play
field for myself.
You want to play;
follow my rules.
OK, and that's called the
lumped matter discipline.
So once you say that I'm going
to adhere to the lump matter
discipline, and this is true
inside your elements.
This is true outside the
elements.
You can define VA and VB,
and good things happen to you.
OK, let me show you a few
examples of lumped elements.
But remember,
a large part of what we're
doing is based on these two
assumptions.
And to just go through the
background on that,
I would encourage you to go to
chapter 1 of your course notes
and read through just as how
this came about,
that comes about.
So, by doing that by adhering
to a lumped matter discipline,
we can now lump objects.
We could lump a bulb into a
resistor.
OK, so to be clear,
a certain number of lumped
objects, and now,
the universe is going to be
comprised into lumped objects.
OK, so before this,
when he went home,
we talked about eggs,
and omelets,
and light bulbs,
and switches,
but once you come to MIT,
and after you've taken 6.002,
you begin talking about lumped
elements, you know,
resistors, voltage sources,
capacitors, little inky-dinky
objects that follow the lumped
matter discipline.
OK, they stick to very simple
rules, and the math that you
have to do to analyze them is
incredibly simple.
What could be simpler than V is
equal to IR?
So, let me give you an example
of interesting lumped elements,
and then show you a couple of
really nasty lumped elements.
OK.
OK, so what you see out here,
so we characterize lumped
elements by the VI
characteristics.
OK, you apply voltage,
measure the current.
OK, so what I can do is I can
plot I here, and V here,
and see what it looks like.
OK, I can characterize elements
by their VI relationship.
And there are a bunch of
elements that I can create based
on the VI relationship.
So let me show you a few
examples.
So for the resistor,
since V is directly
proportional to I,
and R is a constant,
I get a straight line.
That's the I axis,
the V axis, and this is the
resistor.
What I actually have is a
variable resistor,
so I'm going to change the
resistance value,
R, and the curve will also
change slope.
OK, I changed the value of R
because it's a variable
resistor, and the changes slope
because my R is different.
OK, next, let me go to a fixed
resistor, and this guy here on
the screen to your left is a
fixed resistor.
And you see that its IV
characteristic is a line of a
given slope, 1 by R,
and that's it.
I can't change it.
Number three,
I have another lumped element
called a Zener diode that you
will see in the fourth week of
this class, and the
characteristics for the Zener
diode look like this:
IV.
If my voltage goes across the
Zener diode goes up slightly,
the current shoots up.
But if the voltage becomes
negative I don't have any
current flowing into it until
the voltage passes on the
threshold, at which point my
current begins to build up.
OK, so I can increase the
voltage a little bit,
and it can show that the
current starts building up
again.
So that's another interesting
lumped element called a Zener
diode.
Let's switch to the next one
called a diode.
So a diode looks like this:
IV.
As the voltage across the diode
becomes positive,
around .6 volts,
or thereabout,
the current begins to shoot up.
But when the voltage is below
that threshold of .6,
then my current is almost zero.
It's another lumped element
called a diode.
And you will begin using these
elements in your 002 lives to
build interesting systems.
The next example is a
thermistor.
A thermistor is a resistor
whose resistance varies with
temperature.
OK, so this is a very expensive
little hairdryer,
and what I'm going to do is
blow some hot air at my
resistor, and you're going to
see that its value is going to
change depending on how much I
heat it.
So as it cools down,
let me cool it down,
so you can see it's coming
down.
I can zap it again.
I could do this all day.
This is so much fun.
OK, so that's another
interesting lumped element.
As the temperature rises,
its resistance changes.
The next thing is called a
photo resistor.
It's a resistor.
It used to be a resistor;
Lorenzo?
Oh OK, that's fine.
So this is a photo resistor.
And notice that it almost
behaves like an open circuit.
But what I'm going to do is
shine some light on it.
When I shine light on it,
it begins to conduct and
becomes a resistor of some
value.
There you go.
OK, so that's a photo resistor.
So now I'm going to show you a
battery.
Notice we did talk about
batteries before.
I'll show you a battery.
So before you show a battery,
just thinking your own minds,
what should the IV
characteristic of a battery look
like?
IV.
A battery supplies a constant
voltage.
You know your little cell,
the AA battery,
1.5 volts?
So, think of what the IV
characteristic of a battery
should look like for three
seconds before it shows you.
This is the one I showed,
Lorenzo?.
It's a straight line.
This is a good battery.
It's a straight,
vertical line,
but says that the voltage is
1.5 volts, or thereabouts.
No matter what current it
supplies as an ideal voltage
source, it has a fixed voltage,
V, and no matter what the
current going through is.
Now, I'll show you a dud,
a bad battery,
and this is what the bad
battery looks like.
So, many of you have had your
car batteries die on you.
When you go to the store,
they check your batteries.
They use exactly this
principle, that dead batteries
have resistance.
By the way, you see slopes
here.
You're thinking of resistance.
OK, they can use this property
to figure out that your battery
is dead.
So that's a dead battery.
And finally,
let me show you a bulb.
We started with a bulb,
and so I need to end,
OK, we started with a bulb,
so I need to end with a bulb.
And what you will see is that a
bulb simply behaves like a
resistor.
Its IV curve is going to look
like this.
OK, notice this is my bulb.
And guess what,
it behaves like a resistor.
It's a very interesting kind of
resistor, so I won't go into
details for now.
But notice its IV
characteristic behaves like a
resistor.
OK, so those are some pretty
standard lumped elements.
You deal with a lot more sets
of lumped elements,
switches, MOSFETs,
capacitors, inductors,
a bunch of other fun stuff.
But before we do that,
what I wanted to tell you,
don't go berserk on this
abstraction binge.
Too much of anything is bad for
you.
So what I'm going to show you
is, abstractions or models are
only valid provided you work
within a set of constraints.
Notice, we have already had
this tacit handshake which said
that we follow the discipline.
Even after we follow the
discipline, there are ranges to
how well physical elements can
behave like ideal lumped
elements.
OK, for example,
what we will do is show you the
resistor.
And it's going to look like a
resistor.
And I'm going to keep
increasing the voltage around
it.
OK, what's going to happen at
some point?
I just keep doing that.
If it's an ideal element,
if you're a theorist,
you say, oh yeah,
the curve will keep extending
until I reach infinity.
But this is a practical
resistor, so people out here can
cover your eyes or something.
OK, so you're abstraction can't
predict that.
All it says is the current is
an amp.
It can't predict the heat,
light, or the smell.
In the laboratory,
even, you get the smell.
You know what somebody has just
done.
So that's one example of the
lumped abstraction breaking
down.
So, if I really believe that my
own BS, anything is a lumped
element.
So here's a pickle.
A pickle is a lumped element.
I can choose it as a lumped
resistor.
But this is a very interesting
lumped resistor.
Don't try this at home.
This is a standard pickle into
which you are pumping 110 V AC.
I promise you,
this is a standard pickle.
So, it has a fixed resistance,
but your lumped abstraction
cannot predict the nice light
and sound effect.
OK, so the last two or three
minutes what I want to do,
so remember,
don't get carried away by
abstractions.
There are limits.
OK, you can't predict
everything.
OK, that's the smell of a
pickle.
OK, so let me give you a
preview of some upcoming
attractions, and show you one
more quick simplification in the
last few minutes.
So what we can do,
once we build these lumped
elements, we can connect them in
circuits.
OK, so I can build a circuit,
of the sort.
So here's a voltage source with
a bunch of resistors.
I can connect them with wires
and build a circuit of the sort.
One interesting question we can
ask ourselves is,
under the lumped matter
discipline, what can we say
about the voltages?
OK, if I go around the loop,
provided my world adheres to
the lumped matter discipline,
what can I say about the
voltages around this loop?
Ah-ha, Maxwell again,
right?
So, I can write Maxwell's
appropriate equation to solve
that.
OK, voltages have something to
do with E and your integral of E
dot dl and all of that stuff,
right?
So this is the appropriate
Maxwell's equations to use.
And I want to find out what
happens here.
Now remember,
under LMD, I made the
assumption.
OK, my world,
my playground,
has del phi B by del t being
zero.
The rate of change of flux is
zero.
So, under these circumstances,
I can write this.
I can break up this line
integral into three parts across
the voltage source and across
the two resistors and write that
down.
OK, and then when I can do,
is now that the right-hand side
is zero, I can simply take this.
And I know that E dot dl across
this element is simply VCA.
This is VAB,
and this is VBC equals zero.
OK, so when I make the
assumption that del phi B by del
t is zero, and I go around this
loop, apply Maxwell's equations,
what do I find?
I find that the sum of the
voltages, VCA plus VAB plus VBC,
is zero.
That's fantastic.
So now, I could say hasta la
vista to this baby here.
And I can focus on this guy and
say, Maxwell's equations,
this thing with squiggles and
dels and all that stuff,
can be simplified to the sum of
the voltages across a set of
elements in a loop in a circuit
is zero.
OK, and this is called
Kirchhoff's first first law,
KVL.
OK, similarly,
in recitation section,
you'll see the application of
Kirchhoff's current law,
which comes from this be equal
to zero, and all the currents
coming into a node being zero.
So, KVL and KCl directly come
out of the lumped matter
discipline.
And you can use those to solve
circuits like this.
