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Imagine that here, in front of me, is a box.
But this is no ordinary box.
It’s indestructible, could be any size,
and anything could be happening inside it.
Maybe it’s making steel from iron.
Or maybe it’s burning coal to generate electricity,
or creating compounds to be used in medicines.
No matter what’s going on inside this
magical box, it’s our job as engineers to make
it the best most efficient box it can be.
And to do that, we’ll need to rely on an
incredibly basic rule. A law, if you will.
It’s called the Law of Conservation, and
it says that matter and energy can neither
be created nor destroyed.
And we can use this law to figure out both,
where our box isn’t operating as efficiently
as it can be, and how to fix it.
[Theme Music]
Whatever it’s doing, our box is what
engineers call a system, and what’s happening
inside it is a process.
Everything beyond the box is known as the
surroundings.
Let’s say you have an entire steel mill
inside this particular box, so the process
is converting iron to steel.
Now, iron is expensive.
Running steel mills is expensive.
So if there’s something inefficient about
the way your box works, you’re going to want
to know about it – and fix it.
Ideally, whatever you feed into the system
at the beginning, is what you get at the end.
Input equals output.
If your box is perfectly efficient,
the law of conservation tells you that the amount
of steel you get at the end, should weigh exactly as
much as the ingredients you put in.
So if you have 100 kilograms of iron and other
ingredients going into your box every hour, you’d
also have 100 kilograms of steel coming out.
You know you’re not going to get 105 kilograms
of steel, because you can’t create 5 kilograms of
matter from nowhere.
And you can’t have 95 kilograms at the end,
either, because everything you put into this perfect
version of the box is being turned into steel,
and 5 kilograms of matter just
can’t disappear.
If you’re running this system continuously,
with ingredients always flowing in and an equal
amount of steel always coming out,
you have what’s called steady-state.
When a system is steady-state, that means the
variables at the input and output remain constant,
despite what’s happening inside the system.
It’s like water flowing through a full tank: if the
system is steady-state, water comes out at the
same rate it flows in, and the tank stays full.
Of course, we’re just talking about what
would happen in an ideal situation – if the
box is perfectly efficient at steel-making.
The thing is, the real world doesn’t usually
work that way.
Engineers deal with systems that are way
more complex, and they’re not going to be
perfectly efficient.
But you can use the law of conservation to
get a system as close to perfect as possible
– in other words, to get as much output
from your input, as you can.
Before we go back to our steel mill, though,
let’s bake a cake.
The pan will be our system, the oven will
be our surroundings, and the cake batter will
be our raw materials, or input.
The cake itself will be our delicious output.
Like most processes, we’re using this one
to make something we want – the cake.
But, again like other processes, it’s probably
gonna produce some byproducts.
When you burn coal to get electricity, for
example, you also get ash, sulfur oxides,
and nitrogen oxides.
Steelmaking creates dust from zinc and
other metals, which is considered abrasive,
hazardous waste.
And don’t even get me started on chemical
waste from manufacturing pharmaceuticals.
Our cake will probably have some byproducts, too,
like if we burn some of the batter on the pan, creating
some charred bits that we wouldn’t want to eat.
Look, not everyone’s a master chef.
There are lots of other ways to end up with
leftovers, too.
Sometimes you can’t get all of your raw materials to
react or turn into what you want – like if there’s some
batter in the pan that doesn’t get cooked.
Some of the final product can also get stuck
in the system.
Maybe some of the batter spilled onto the
oven racks.
Any of these examples could be waste since
you couldn’t simply separate them back into their
initial parts to use them as input next time.
There’s also the possibility our raw materials
were contaminated, which could easily end
up in our final product.
When we cracked open the eggs, for example, some
small pieces of shell might have fallen into the batter,
which means we’ll probably still find them
when we try a slice of finished cake.
You know, on second thought, maybe I’ll
pass.
So, clearly our cake-baking process has a
lot of room for improvement.
Engineers define problems like these in terms
of conversion and yield.
Conversion describes how much of our initial
input was used in the process.
If our system has a conversion rate of less than
100%, it means you’ll have some leftover or waste.
Now that our cake is done, let’s go back
to the steel mill in a box.
If you had 100 kilograms of iron and other
ingredients going in, and a 60% conversion rate,
that means you’d have 40 kilograms
of iron left over at the end of the process.
That’s a lot of wasted iron!
Yield, meanwhile, describes how much of the final
product you were able to get from your initial input.
So if your box has a 30% yield, 100 kilograms
of raw materials would get you 30 kilograms 
of steel, and 70 kilograms of waste.
The most basic way to think about a system
is in terms of “balance”.
Engineers measure the values that go in and the
values that come out, and if there’s a difference,
they’ll try to figure out what’s causing it.
But conversion and yield only cover the beginning
and end of the process.
Engineering, like life, is usually not that
simple.
If you have a system that’s not in steady-state,
then you probably don’t have the same amount
of mass coming in and out.
Real-life processes are messy, and stuff gets
stuck along the way.
That’s accumulation, and as engineers, we can
use it to keep track of the differences between
what’s coming in and what’s going out.
The basic idea is pretty simple: if you subtract your
output from your input, you get your accumulation.
If you’re measuring all this in terms of mass,
that simple equation is probably all you need.
Mass goes in, and whatever mass doesn’t
come out at the end, is stuck inside the system.
But when there are chemical reactions happening
inside the system – which there often are –
it can be more useful to think about accumulation
in terms of molecules.
During a process, your raw materials might go
through a chemical reaction that generates some
molecules that don’t end up in your product.
Or a reaction might consume some molecules
that are hanging around inside the system and
weren’t part of the input.
To keep track of where all your molecules are, and
which of them are accumulating inside the system, you
need two more terms: generation and consumption.
To calculate accumulation, you’d take your
input and subtract the output like usual.
But then you’d also add the molecules being
generated within the system by chemical reactions,
and subtract any molecules being consumed.
Whatever’s left over is what’s accumulated
inside the system.
This equation is essential to engineering.
Even in its simplest form, engineers can
use this equation to figure out how to improve
a system.
So let’s see how we can improve ours!
The steel mill in our box has some raw materials
flowing in and steel flowing out.
As engineers, our goal is to make as much
steel as possible at the end of the process.
Let’s say we begin with 100 kilograms of
raw materials with a 70% conversion rate,
which means we’re only making 70% of what’s
possible, or 70 kilograms of steel.
Not bad, but not great.
We’d be losing out on 30 kilograms of raw
materials, and probably a good bit of money!
This is where we’ll need to problem-solve
as an engineer and figure out how to make
the process better.
How about a recycling system?
We can introduce a separator at the end of
our system to sort out any leftovers.
Then we can run that stream of unused raw
materials back into the beginning of our process.
We already have 70 kilograms of steel and
30 kilograms of iron and other ingredients
that weren’t used.
Let’s send those 30 kilograms back.
Now, instead of 100 kilograms being fed into
the system, we have 100 kilograms plus the
leftovers from the previous cycle.
So, even though we still have a 70% conversion
rate, we’re still getting more steel from the same
amount of raw material, than we were before.
But what if our raw materials aren’t perfectly
clean?
What if we have contaminants?
Sometimes contaminants can lead to big problems,
literally ruining machines and halting production.
It can be very serious and costly.
For example, soda bottling plants have had
to shut down production because of possible
chlorine contamination.
With our steelmaking example, let’s assume
the contaminants are relatively harmless and
won’t react with anything.
We’ll keep using our recycling system from
before, but now we’ll also need a ‘purge system’
to filter out the contaminants.
If we don’t get rid of them, we’ll be passing
them back while adding new contaminants
during the next cycle.
As the contaminants build up, we’d run into more
and more problems with our machines and tools.
Remember, just because we’re following the
Law of Conservation, or any other principles
of engineering for that matter,
it doesn’t mean everything is working out
the way we want it to.
As engineers, we’ll encounter limitations
like these and many others.
But with clever designs, we can work around
them.
Engineering is all about testing our limits
and pushing them as far as we can until we
reach something truly extraordinary.
So today we learned all about the law of conservation,
beginning with simple, steady-state systems.
We then introduced the terms conversion and
yield, showing how they apply to a system.
We ended with accumulation and covered how
generation and consumption can affect how
much accumulation there is in a system.
I’ll see you next time, when we’ll learn
all about reversibility and irreversibility
and how they affect engineering.
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