Whether you’re putting food into your body
or fuel into your car, you’re always trying to get
something out of raw materials.
You’re trying to convert energy.
But if you want to understand how this works,
we need to talk about thermodynamics, and the
laws behind it.
Only then can we truly harness the power of
energy as engineers.
[Theme Music]
Energy is constantly being converted all around
you.
When you take a bite of an apple, you
take in the fruit’s energy and convert it into
something that your body can use.
Maybe you’ll use it to help power the marathon
you’re training for.
Maybe it’ll go to power your normal bodily
functions.
Or you might store the energy to use later,
as fat.
But energy conversions don’t just happen
on a personal scale.
They’re also at the core of many engineering
designs, like with hydroelectric dams.
In a hydroelectric dam, water turns a turbine, which then
turns a metal shaft in an electric generator, converting
the movement of the water into electricity.
These conversions are important, because
energy doesn’t just come out of nowhere.
It needs to come from some other type of energy.
So, to better understand how energy can be
converted, you need to understand thermodynamics.
Thermodynamics is the branch of physics and
engineering that focuses on converting energy,
often in the form of heat and work.
It describes how thermal energy is converted to
and from other forms of energy and also to work.
And thermodynamics is one of the main focuses
of mechanical engineering.
Because thermo, as it’s often called, is
critical to engines.
Engineers need to know how much heat or work
they’ll get out of an engine if they put energy into it.
We’ll talk a lot more about engines in the
next episode.
Even when we’re not focused on heating or cooling
something, like with heat pumps and refrigerators,
we still don’t want our machines overheating.
After all, engineering is not just about getting more of
what we want, but also controlling what we don’t want.
It’s not just mechanical engineers that
deal with thermodynamics.
It also plays a big role in chemical engineering.
When chemical reactions form new compounds,
they often create energy.
And often that energy is thermal energy.
Now, to understand how all this works, we
should start at the bottom: the zeroth law
of thermodynamics!
Yes, that’s really what it’s called!
We only came to understand the zeroth law after its
more famous siblings – the first and second laws –
had already been established.
But it was considered so fundamental to
thermodynamics that it was promoted to be
more than first – so, “zeroth”!
Now, this law focuses on temperature and defines
thermal equilibrium.
In general, an equilibrium is where certain
properties, like pressure, volume, or temperature,
remain the same across the system.
So, if two or more things are in thermal equilibrium,
then they’re all at the same temperature.
The zeroth law says that when two objects are
individually in thermal equilibrium with a third object,
then they are also in equilibrium with each other.
This is important because when a body is left in a
medium at a different temperature, energy will be
transferred until a thermal equilibrium is established.
That’s why, if you leave a cold soda out
in the sun, it will warm up and reach the
same temperature as the air outside.
The basic ideas behind why this happens lie
within the next law, the first law of thermodynamics.
The first law of thermodynamics applies the law
of conservation that we learned a few episodes
ago to thermodynamics.
It basically defines heat as a form of energy,
which means it can neither be created nor
destroyed.
So we can’t create or destroy energy, but
we can convert it from one form to another.
This might seem pretty simple, but it’s
a powerful idea.
It allows us to better understand a system, how
we can get energy from it, or how we can stop the
conversion of energy when we want to.
Now, no matter what system you’re looking at,
there are two areas of energy that we need to
concern ourselves with:
the energy contained within the system, and the
energy that can move between boundaries.
Let’s start with the energy inside a system.
We can break it down into three main parts.
The first is kinetic energy. This is the type
of energy that’s involved with movement.
The most common form is translational kinetic
energy, which is when something moves from
one location to another.
There’s also rotational kinetic energy, when
something spins or rotates, and vibrational kinetic
energy, when something shakes or vibrates.
Think about it in terms of throwing a baseball.
As it flies through the air, the ball will
have kinetic energy.
The kinetic energy would be translational
as it moves from your hand to your friend’s
mitt, and rotational as it spins in the air.
The second type of energy inside a system
is potential energy.
This is energy that can come from where something
is, even if it’s not moving.
We can basically think of it as stored energy.
Potential energy often has to do with how
high something is.
The higher it is, the more potential energy
we can have.
This is often called gravitational potential
energy.
Like, if you’re climbing a ladder, you’ll have more
and more potential energy with every step you take.
But potential energy can also come from an
object's horizontal position.
Think about a bow and arrow.
Using elasticity, we can transfer potential
energy to an arrow as we draw it back in a bow.
As we fire the arrow, the potential energy
will be transformed into kinetic energy.
But the third type of energy that we’ll
find in a system is a bit different.
It’s called internal energy.
Internal energy is the energy associated with
the seemingly random movement of molecules.
It’s similar to kinetic or potential energy,
but on a much smaller, microscopic scale.
Take a glass of water for example.
As it just sits there on a table, the water
doesn’t seem to be moving.
But on a microscopic level, the water is 
teeming with molecules that are traveling
around at super high speeds.
While this type of energy might not seem as
important, it can have major effects on a system.
That’s because changes in internal energy can result
in changes in temperature, changes in phase – like a
solid to a gas – or changes in chemical structure.
All of these types of energy – kinetic,
potential, and internal – show us what can
exist within a system.
But these types of energy can’t cross the
boundary from their system to the surroundings.
But we’ve already talked about the main
types of energy that can cross boundaries.
One is heat, which we know to be the flow of
thermal energy, and another other is work, which
is essentially any type of energy other than heat.
So knowing all of these different types of energy
involved with a system can help us understand the
first law of thermodynamics.
Let’s start with a closed system, where
no fluid is moving in or out.
A good example would be a piston enclosed
in its cylinder.
The first law of thermodynamics states that
the change in internal energy, kinetic energy,
and potential energy of a system
is equal to the heat added to the system,
minus the work done by the system.
This equation may look pretty complicated,
but there are a few different scenarios that
can help clear it up.
One is a stationary system.
If you look at the left side of the equation, you’ll
see that the changes in kinetic and potential
energies will be 0 for a system that isn’t moving.
Another special case is an adiabatic process.
An adiabatic process is when there is no heat
transfer.
It’s rooted in the Greek word “adiabatos”,
meaning “not to be passed”.
This can happen if there are no differing temperatures,
or if something is so well insulated that only a negligible
amount of heat can pass through the boundary.
Think of it like how a good thermos bottle
can keep your hot chocolate warm.
Now you can also simplify this equation if
you have an isochoric process.
When a process is isochoric, the volume of
the system remains constant.
This often means that there won’t be any
work, leaving us with only heat on the right
side of the equation.
Any of these special cases help give you a
much simpler equation to work with, but this
all has to do with a closed system.
Oftentimes you’ll find yourself dealing
with more complex, open systems.
Unlike closed systems, open systems have a
flow going in and out.
A good example would be if your basement flooded
and you wanted to pump the water out of it.
With a system like this, you’ll need to
introduce a different energy measurement:
enthalpy.
Enthalpy includes internal energy, but also
adds in the energy required to give a system
its volume and pressure.
For an open system, you’ll also want to
refine what you mean by work.
Here you’ll want to focus on shaft work,
which is basically any type of mechanical energy
other than what’s necessary for flow.
Going back to our equation, you’ll want to replace
your internal energy with enthalpy and change your more
general work to focus specifically on shaft work.
This will let you apply the law to open systems
as well.
So let’s use a flooded basement as our open
system.
First off, we should establish that we’ll be treating
the basement as our system and the outside, where
we want the water to go, as our surroundings.
When we run the pump, it will take in electricity
and convert it to shaft work, which turns the pump.
That energy will then be used to get the water
moving, which will change some of its potential
energy to kinetic energy.
Hydroelectric dams are open systems too.
If you think of the dam as a system and its
environment as its surroundings,
then you see that there’s flow coming in, in the form
of water, and flow coming out in the form of electricity.
It’s a little more complex than just draining
a basement,
and it’ll take a lot longer to learn everything
that’s involved with generating electricity, but
the laws behind it are exactly the same.
So you see, you can’t always find the exact
answers to problems quickly.
But through science and engineering, you’ll
have the tools and knowledge to solve them
the best you can.
So today we learned about thermodynamics and
how it shows up in our lives.
We started by learning the zeroth law of
thermodynamics and what it means to reach
|a thermal equilibrium.
Then we talked about the different types of
energies involved with a system and defined
the first law of thermodynamics.
We also found out that stationary, adiabatic,
and isochoric processes can make our lives
as engineers a little easier.
I’ll see you next time, when we’ll learn
about entropy and move on to the second law
of thermodynamics.
Crash Course Engineering is produced in association
with PBS Digital Studios.
You can head over to their channel to check
out a playlist of their amazing shows, like
Brain Craft, Global Weirding with Katharine
Hayhoe, and Hot Mess.
Crash Course is a Complexly production and this
episode was filmed in the Doctor Cheryl C. Kinney
Studio with the help of these wonderful people.
And our amazing graphics team is Thought Cafe.
