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In today’s episode we’ll explore thermodynamics and some of the ways it shows up in our daily lives. We’ll learn the zeroth law of thermodynamics, what it means to reach a thermal equilibrium, and define the first law of thermodynamics. We’ll also explore how stationary, adiabatic, and isochoric processes can make our lives as engineers a little easier.

Note: Different branches of engineering sometimes define the first law of thermodynamics differently, depending on how work is defined. Essentially, work released from a system might be defined as a positive value or a negative value, and thus the first law can be defined as either Q-W or Q+W. Both are acceptable forms, depending on how the system is defined! We chose to focus on only one definition here to limit the confusion.

Crash Course Engineering is produced in association with PBS Digital Studios:


Çengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. 8th ed., McGraw-Hill Education.


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CC Kids:
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.