The term “thermodynamics” is an interesting one. “Thermo” comes from the Greek word thermé and means “heat.” “Dynamics” refers to motion and/or equilibrium, and I didn’t look up what it means in Greek because I don’t really care all that much. Anyway, when you put it all together, what it means is that “thermodynamics” refers to the movement of energy from one place to another. Which is really what I should have said in the first place.
Now that we’ve gotten that out of the way, let’s learn some of the basic ideas behind the magic that we call thermodynamics. Better yet, let’s get ready to learn that many of these basic ideas involve the redefinition of words you’ve used your entire life. Isn’t science great?
Energy is defined as the ability of something to do work or produce heat. Basically, energy measures the ability of something to do stuff.
Energy can take two forms. Kinetic energy and potential energy.
Kinetic energy is the energy that’s in the form of moving stuff. If you have a baseball flying at your head, it has kinetic energy because the eventual ability of the baseball to do stuff is derived from the fact that it’s moving. In gases, the kinetic energy is due to the motions of the gas molecules, and the vibrations of the atoms in molecules are also a form of kinetic energy.
- Temperature is related to kinetic energy. Specifically, the temperature is a measure of how much the particles in something are moving around. If you remember back to the kinetic molecular theory, you’ll recall that the kinetic energy of the particles of a gas is proportional to its temperature in Kelvin. That’s where this comes in: The faster molecules move around, the more temperature they have, and more kinetic energy, too.
Temperature and energy are not the same thing. Temperature measures the movement of the particles, which kinetic energy (the ability of something to do work or produce heat) is related to. They’re similar concepts, but I wouldn’t mix them up on a test if I were you.
Potential energy is stored energy. You can’t really see that it’s there, but it is. For example, if there’s a painter standing on a ladder next to your house, you probably don’t want to walk under the ladder. Not only does it bring bad luck, but you also run the risk of having a paint bucket (or painter) fall on you. On the ladder, the bucket has no kinetic energy, but it does have gravitational potential energy that can be turned into kinetic energy if it falls off the ladder. In a chemistry class, potential energy is stored in the form of chemical bonds.
These two concepts lead to an idea you’ve probably seen before: The first law of thermodynamics. Also called the “law of conservation of energy”, what it means is that the total amount of energy in the universe will always stay the same. You can convert kinetic energy to potential energy (throw a ball onto the roof) or convert potential energy to kinetic energy (fire a bullet out of a gun) – either way, the energy will neither be created nor destroyed.¹
How energy moves around: Work and heat
If you want to move energy around, you’ve got two options: Heat and work. Let’s take a look at both.
Work is the movement of energy from one place to another via mechanical means. For example, if you make a machine that lifts bowling balls into the air, this machine is performing work on the ball because the ball has more potential energy. Likewise, if you were to smash a priceless Ming vase with the suspended bowling ball, you’d be performing work on it.
Heat (q) is energy that’s moving from one place to another through molecular motion. For example, let’s say you fill a cooking pot full of ice water. If you then turn on the stove, energy from the burning gas will be transferred into the cooking pot. As it moves, this energy is known as heat. A general property of energy is that if one area has a lot of energy and another does not, heat will transfer energy from one to the other until both areas have uniform energy.
It’s important to note that heat and temperature are not the same thing. Heat is energy as it moves from one place to another, while the temperature of an object is the amount of movement that the molecules of an object have. As a result, if you want to raise the temperature of something, you must heat it (i.e. add energy to it in the form of moving molecules).
When you say that “it’s hot outside”, what you’re really saying is that the temperature outside is high. However, if you take an ice-cold Fanta outside with you, the heat (i.e. transferring thermal energy) will cause its temperature to rise.
System, surroundings, and universe
So, where does all this heat and work and stuff come from, and where does it go? To find out, we need to talk about a few more terms:
- A thermodynamic system is the point of reference from which you keep track of everything else. For example, if you’re trying to figure out how much energy it takes to cook a squirrel, you’d probably think of the squirrel as the system that you’re interested in.
- Thermodynamic surroundings are whatever interact with the system. In the example of the squirrel, we’d say that the inside of the oven is surrounding the system of the squirrel because that’s the thing adding energy to the squirrel.
- The universe is everything, everywhere. However, from a thermodynamic standpoint, we usually like to limit this to just the system and surroundings. After all, Proxima Centauri is part of the universe, but it probably doesn’t play a big role in cooking a squirrel.
Some finishing words:
There’s a lot more about thermodynamics that needs to be covered, but I can assure you that it will be a lot easier if you understand these terms before you learn the rest of it. Seriously, it’s a good idea.
- Though a cornerstone of modern science, it turns out that the law of conservation of energy isn’t exactly right. It turns out that when you perform a nuclear reaction, some of the mass is converted to energy using Einstein’s equation E = mc². Fortunately for us, this doesn’t negate the first law – it just caused us to rename it the “law of conservation of mass-energy”, because for nuclear processes the amount of mass + energy before the process will equal the amount of mass + energy afterwards.
- Nerd, painter: Image courtesy of stockimages at FreeDigitalPhotos.net
- Businessmen: Image courtesy of Ambro at FreeDigitalPhotos.net
- Fanta: By Lexlex (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)%5D, via Wikimedia Commons
- Alien squirrels: Image courtesy of Simon Howden at FreeDigitalPhotos.net