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  • 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 siblingsthe first and second lawshad already been established.

  • But it was considered so fundamental to thermodynamics that it was promoted to be more than firstso, “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 phaselike a solid to a gasor changes in chemical structure.

  • All of these types of energykinetic, potential, and internalshow 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 wordadiabatos”, meaningnot 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.

Whether you're putting food into your body or fuel into your car, you're always trying to get something out of raw materials.

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熱力學第一和零點定律。工程學速成班#9 (The First & Zeroth Laws of Thermodynamics: Crash Course Engineering #9)

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    林宜悉 發佈於 2021 年 01 月 14 日
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