High School Physics Thermodynamics

Hi welcome to educator.com. Today we’re going to be talking about thermo dynamics.0000

At this point it should be really clear that the study of heat and temperature incredibly important to any kind of science.0005

We’ve talked about how it causes expansion, how it causes changes of phase, how there’s just all this stuff that happens based on the temperature of that substance is at.0011

So it’s going to affect chemistry, it’s going to affect biology; it’s going to affect physics. It really, really matters.0020

If you want to do electrical engineering that’s going to matter. Temperature is incredibly important.0025

It has a huge effect on how things operate and even through some clever arraignment we can start to take advantage of that.0030

We can create a thing an engine. You can use heat to do work. Through engines we’re able to cause heat to be beneficial to us in all sorts of ways.0035

In addition to the fact that we might want say a warm room, so it might be…we’ll be able to understand heat better from being able to get heat into a location or heat out of a location.0046

This study is called thermodynamics and it’s what we’ve been talking about for the past two lessons.0057

In general, thermodynamics is way to study heat and its relationship to energy and work.0061

Let’s take a look at some of the laws that make up thermo dynamics.0068

The first law of thermo dynamics is a slight expansion of conservation of energy. You’ve probably already take it for granted and probably figured this out on your own.0072

We’ve talked about it and we’ve almost certainly had this idea introduced to you for a long time. But we haven’t explicitly stated it before.0081

Let’s put it in words. The amount of heat energy we put into a system is equal to the change in that systems internal energy plus the amount of work this system does.0087

Symbolically we get that q; heat energy is equal to change in internal energy plus work.0097

Now why do we use the words internal energy instead of just temperature?0104

Well internal energy lets us talk about how much energy is in the system. Temperature is a specific measure of the average of the system.0108

If we’ve got a 50 kilogram block of steel versus a drop of water at the same temperature in both of those things it’s going to have different amounts of internal energy.0114

There’s even some other things to consider. We can instead be talking about the pressure and volume of the thing without directly relating to the temperature.0124

We can talk about internal energy as more than something that’s just a function of the temperature.0131

It’s the…how much motion is happening in internally, what’s happening, the vibration of the atoms that make up the substance that we’re talking about.0136

How can we talk about heat doing work? Let’s explore one possible way that heat could do simple work.0147

Heat does work in many ways. If we just have an open flame, it’s going to cause work to happen by making those molecules become more energetic and expand out and push out in all directions so it’s causing work by increasing their speed and moving them around.0154

That’s not a very easy way to think about of it though and its not certainly useful work. Let’s think in terms of an engine.0169

For a simplified example we’ll consider a piston, it’s a cylinder full of air where end can move but the enclosure is fully air tight.0175

What would happen if we heat the air at the bottom of it? If we heat the air down here we’re going to cause that air, remember if we had a single volume and we heated it, we caused increase in temperature, it would cause increase in pressure.0183

So in our piston, when we heat that air we’re going to cause those air molecules to start moving around more. Since they’re moving around more they’ve got more pressure.0197

Since they’ve got more pressure they’re now pushing on that piston plate more which means that they’re actually able to cancel out the force of gravity and able to push past it.0205

They’re going to push out as their volume increases, they’re going to wind up having less pressure and less temperature until more temperature gets in.0214

They’re going to be able sort of…as they become higher in temperature they’ll be able to push out the piston.0220

Pushing out the piston will have a connection to the amount of heat energy that would put in.0226

Some will go into raising the internal energy of the atoms, some of it will go into just actual doing work, actually doing work lifting the piston up.0230

We’ve got a way to be able to talk about how…we have a definite example of heat being able to both raise the internal energy but also at the same time cause work to be done external to just they system of the air and the heat.0240

We’re able to do something outside the system lifting that block.0254

The second law of thermo dynamics is similar to the first law. The second law is one that we’ve haven’t especially talked about it but it’s one that we’re definitely ready to understand.0259

One that is probably even certainly more obvious than the first one, at least the way it’s going to be stated here.0268

One way to put the second law is that without external work heat never flows from a cold object to a hot object.0275

If something's cold it isn’t going to colder to make something else even hotter, that doesn’t make sense.0283

It goes from high to low, is what we’re used to. Heat is in some way something that we think of as water.0289

If you’ve got something hot it pours out until it’s evenly spread out. Hot things go down to cold things but cold things don’t, of their own accord, go to hot things.0295

One possible way to fight this out is…to defeat this is by putting work into the system, that’s what a refrigerator does.0305

There’s a clever way to mechanically cause the heat in the box to be put into something else and have that dissipated somewhere else, but we have to put in external work.0313

Without external interference, heat always goes from hot objects to cold objects.0324

This probably seems shockingly obvious; you’d never drop an ice cube in water and expect the water to get hotter while the ice cube got colder. It’s completely intuitive to this at this point in our lives.0333

Hot things make the things around them hotter. They do not themselves get hotter from being around cold things. They make the cold things hotter and the cold things make hot things colder, simple as that.0344

But this idea has really important ramifications. One way to state this was what we just said, heat never flows too cold to hot.0357

Another way to state it is through the idea of entropy. This idea of entropy is really important.0364

We haven’t discussed entropy is yet but another way to put the second law of thermo dynamics is to say that entropy will either increase or remain the same for any process.0371

For any action for any reaction. Entropy is always going to stay the same or get larger.0380

Almost all the time, it’s going to get larger. Entropy never decreases.0385

What the heck is entropy? Entropy is a measure of disorder or chaos. Virtually any real life process will take something organized and cause it to become less organized.0390

As something becomes less organized, the organized something is the more entropy it has.0400

This idea of entropy can be put into a specific mathematical formula. We can talk about it mathematically. We’re not going to, it’s enough for our purposes right now to just explore it a little bit on the surface and get the idea that entropy means chaos, it means randomness, it means disorder.0405

A good example of this would be if you had a stick of dynamite and you lit it and it blew up.0421

A stick of dynamite starts off as this really tightly packed configuration of very complex molecules. We’ve got the very complex molecules that store all of that chemical energy inside of there.0425

We’ve got them tightly packed into this single orderly piece of dynamite that’s well wrapped up.0438

You light the fuse, so you apply a little bit of heat, the fuse runs in; it manages to apply some heat to the dynamite.0443

The dynamite takes that and has a chain reaction where way more heat is released. Way more heat is released; it manages to cause all of those complex molecules to transform into simpler molecules, which allows that heat to be released.0449

All of that heat and pressure causes the dynamite to blast out. We’ve got a release of heat which means our molecules are now vibrating around, they’re more random, there’s more motion going on.0462

They’re bouncing around, they’re more disordered. We’ve got all of those complex, complicated molecules that have been blow…not blow, been transformed into smaller, simpler forms of molecules.0471

Their transformation from the complex to the simple has released energy. Finally the process of exploding has caused the entire piece of dynamite to be blown over a large area randomly.0485

We’ve got all of this chaos, all of this disorder from lighting this stick of something that started off fairly ordered.0495

In general, order will tend to disorder. Over time ordered systems go to disordered system. Unless they’re perfectly isolated with no ongoing processes.0504

If nothing’s going on inside of them and they’re completely removed from everything else, they’ll be able to stay the same level of entropy.0512

But that’s not possible in the real world. We can’t perfectly insulate something from the world around it and we can’t keep everything from stopping happening inside of it.0519

Whatever we’ve what, whatever it is; it’s going to over time become more and more disordered. More and more entropic, more entropy and it’s going to have more randomness.0527

Over time order falls into disorder. A given system like a living being can increase its own order, but it causes this at a greater increase in disorder elsewhere.0537

While you might be able to lower the entropy in your body by eating a good breakfast and working out, by doing things like that your body is taking care of itself.0548

Its doing things, it’s making a very complex system. But over the course of doing that you have to eat that food, all those complex chemicals in the food are broken down into simpler chemicals, turned into waste products.0556

The air we breathe in it becomes less complex to some extent turning oxygen into carbon dioxide is an interesting question the way it’s working.0567

We’re able to take all of these things around, we produce heat, causing more motion, more randomness. The whole process of being a complex thing, being able to stay complex and not break down yourself means that you have to cause complexness in other things to break down.0575

You have to take complexity from elsewhere and break it down to keep up your own level of complexity.0592

While you’re able to have order in your own body, it causes an even greater increase in disorder elsewhere. The entropy also goes up; it isn’t something that’s conserved.0598

Entropy is just constantly marching forward. In fact we can look at entropy; entropy is a one way arrow. We can think of it as the direction that times flows.0607

Entropy is always flowing in the direction of time moving forward. Things get more chaotic with time.0614

With that idea of things getting more chaotic with time, I’d just like to point out that the poets got it right. They might not have stated it scientifically, but poets have stated this intuitively for centuries.0622

For example, so dawn goes down to day, nothing gold can stay’. Nothing complex, nothing perfect can stay. Nothing beautiful is what’s its expressed in this poem, can stay for a long time.0632

That idea of break down is right there. Robert Frost. Another one, William Butler Yates, ‘things fall apart, the center can’t fold, mere anarchy is loosed upon the world.’0644

This is a part of larger poem, which actually has some different ideas going on, but that definitely applies to the notice of entropy, the fact that things just don’t last.0654

If this sounds interesting to you, there is a lot more poetry out there. I recommend it, go check it out. There’s some really cool stuff, more than just physics.0662

If you already like poetry, I think it’s really interesting to see how much there is a connection between the things that we study and the arts that are out there.0670

There’s a lot of connection between the science and the arts and it’s really interesting.0676

Back to the science. Engines in general, an engine takes in heat and it turns some of that into useful work and puts out the rest as exhaust.0680

An example of a car, it takes in gasoline, a complex molecule. It burns that to make the wheels spin through the use of the engine and some heat comes out of the engine and the engine compartment through the exhaust pipe.0689

Understand that that’s the general way of a car working? So we can come up with a really simple diagram, we can diagram this as a hot source that goes into some engine and then energy comes out of that engine in the form of work.0700

Then the rest of the energy goes into a cold sink. We’ve got some large amount of heat going into our engine and then out of that we’ve got work being turned out of that heat.0711

At the same time, some of it manages to get just lost to that cold sink. The best kind of engine would be something that gets lots of lots of work for very, very little energy.0721

The best kind of car would be where you put in a teaspoon of gasoline and then after driving your car you’ve got a liter of gasoline.0733

You put in a tiny amount of gasoline and it actually causes you to have more. But that doesn’t make sense because that would be a destruction of the conversation of energy.0740

We can’t get more work out than the energy we put in. At best, the best kind of engine from our point of view then, knowing that the conservation of energy is defiantly something we can’t beat.0748

The best engine would be one that’s able to convert all the heat energy we put into it into useful work. Every drop of heat energy gets turned into work.0759

That would be the best kind of thing. To describe this, to be able to measure the amount of heat that gets turned into work, we need to talk about efficiency.0767

How efficient something is. The amount of energy that has to go in for the amount of energy we get out.0774

We can compare the heat in to the work out. The efficiency is the work that we get out, w divided by the amount of heat we put in, the energy we put in.0780

The energy you get out divided by the energy you put in is the efficiency. The efficiency can be anywhere between 0 to 1 but no more.0789

We can also easily turn this into a percent by multiplying by 100. It would be great if there was an engine out there that did have a 100% efficiency that would be awesome.0796

It turns out that that’s not possible. A Carnot engine is a theoretical engine that can be proven to have the maximum possible efficiency.0806

It’s not possible to get better than a Carnot engine. Sadly a Carnot engine doesn’t even allow for 100% energy conversion.0814

The most that a Carnot engine will allows for is this: the upper limit on efficiency is connected to how hot your source is, how hot your heat source is, and how cold that exhaust is.0821

The best is going to be when there’s a really, really wide thing. The absolute best would be if your cold exhaust was absolute zero but that’s not possible.0832

You can’t have something in real life that is still at absolute zero. Once again you can’t perfectly insulate something.0839

If you were to have heat flowing into it, it wouldn’t be absolute zero for long. The upper limit is going to be 1 minus the ratio of the cold to the hot.0845

1 minus temperature of the cold divided by the temperature of the hot.0856

Notice temperature cold and temperature hot have to be measured in kelvin otherwise this formula won’t work because once again, if temperature cold was below zero Celsius, we’d have a negative number.0860

Suddenly we’d be able to have an efficiency that was greater one doesn’t make any sense.0869

We have to be talking in kelvin because we have to always be above that zero. We have to be working with kelvin.0873

But this is kind of disappointing. It would be great if there could be a perfect efficient engine but we can prove that it’s not because the best kind has got to be a Carnot engine and a Carnot engine has a maximum efficiency.0878

This efficiency isn’t what you’ll get in real life because the Carnot engine is forgetting the annoying things that come with real life, like friction and heat through other sources, the fact that you can’t have perfect insulation, those sorts of things.0889

It’s like talking about sort of a perfect theoretical thing and we can’t even create a perfect theatrical thing in real life.0903

So you’ll never see something in real life that is better than this. It’s always got to be less than that.0909

Ready for some examples. First example is a nice easy one to knock out of the park.0914

If a system has 100 joules of heat energy put into it and the system does 47 joules of work on its environment, how much will the internal energy have to be raised?0918

Well the first law of thermo dynamics is the heat in is equal to the change in the internal energy plus the work out.0926

Our heat energy was 100 joules, the work out was 47 joules. So we toss those numbers in and we get that 53 joules is the amount that the internal energy has gone up by.0939

Depending on the specific heat of what we’re dealing with we’ll get different amounts.0958

If we had a really low specific heat we’d get a higher temperature raise. If we had a really high specific heat we’d get a lower temperature raise.0962

We do know that the change in the internal energy is going to be 53 joules, whatever temperature that winds up connecting to.0967

Second example, what would be the efficiency of the system from the previous question?0974

Remember we had heat of 100 joules put in, we had a work of 47 joules and we had to change in internal energy of 53 joules.0978

First thing to notice, this is actually a red herring. We don’t care what the change in internal energy is. All we care is what’s the connection between the work out and the heat in.0985

Because that’s how we figure out efficiency. Efficiency is equal to the work divided by the heat in. Work out divided by heat in.0992

47 over 100, we get 0.47 is our efficiency; we would could also talk about as 47%.1002

We’ve got a 47% efficient engine, which is actually really, really, really good.1012

We’ll talk about why that’s so great. You’ll see precisely why that’s so great when we get to the fourth example but 47 is actually a really great efficiency to get.1016

Third example. We’re going to do this one without any math, but we will talk about it.1026

If you’ve got a bunch of coins scattered on the floor and you pick them all up and you stack them into one neat ordered column. You’ve created more order, right?1030

You’ve got this disordered bunch of coins sitting on the floor and you manage to bring it into one tightly bunched thing were they’re all together, they all have a unified temperature because they’re now touching.1038

Haven’t you made there less entropy? Haven’t you lowered the entropy in those coins? Yes, you have lowered the entropy in the coins, but you’ve introduced entropy to the rest of the world.1047

The rest of the universe will now have a total of more entropy. If we look at the entropy over the whole thing, we’re going to get more entropy.1058

Where is this coming from? How are we not violating the second law of thermo dynamics with the fact that we’re stacking these coins?1065

It’s because we can be sure in the fact that we’re introducing more entropy. How are we introducing more entropy?1070

Sure, the stack has less entropy, but there's other things out there. As you move around, one of the primary things, is you move around, you’re going to be generating heat.1075

Heat is generated by your motion. As you walk around the room picking up those coins, bending over. You’re going to introduce heat.1088

You’re going to cause heat to get put into it. The motion of the air molecules is going to become more frenetic, they’re going to be bouncing around, moving around more.1099

They’re going to become more random, more chaotic, less ordered, disordered. You’ve caused disorder in the air molecules by your raw motion, walking around the room and also by the heat generated off your body.1106

As you’ve done this, sugars in your body have been broken down. So you’ve got these complex chemicals that are supplying you with the energy.1116

So you’re breaking down these complex chemicals to be able to have less complex chemicals so you can have energy. You start with a complex chemical, you break it down into something simpler, some energy is released and that’s how your body is doing its thing.1125

It’s able to eat food and break it down into things that are simpler and so you get energy out of it. But in breaking it down you’re taking a complex thing and turning it into simpler.1136

In that case, you’re also once again releasing them. Just as you go around living, parts of your body, they start to degrade. You’ve got your skin cells breaking off, turning into dust.1145

Your skin cells are decaying along with varies other cells in your body. And your body is replenishing them, it’s causing more order, but this is going to all turn into waste products.1156

You’ve got these complex systems that are by nature just breaking down over time. Well you can go around doing something where you’re creating more order in the system.1163

On the whole when we look at the large picture more entropy is introduced just by the fact that you’re moving around doing anything.1172

The only way we could keep the entropy as low as possible would be to just let the coins sit and not have any effect on them and leave the door closed to it.1178

At least in that case no more entropy would be introduced. It would stay at its already disordered state but it wouldn’t become more disordered.1185

But as we go in and start to bustle around and do things we can cause some order to show up in one spot but on the whole more disorder will be introduced than order.1192

The entropy always goes up. Entropy wins. In the long run entropy is the winner.1200

Ready to do the final example. Iron melts a little bit after 1,500 degrees Celsius. As you start to go a little bit past that iron will start to break down and melt into a liquid.1211

If we’ve got a car engine made of iron, the absolute hottest we could run that engine would 1,500 degrees Celsius right?1221

We don’t…that would probably be…well okay first of all in real life, that would be magical because the oil that’s used to lubricate the engine to make sure it runs smoothly would be burned off, completely gone by that.1228

The gaskets involved, charred. The engine would just completely stop working way before 1,500 degrees Celsius. But we can defiantly see the…but it would be hard to figure out what the precise top level would be.1238

We can defiantly see one good place to say that the absolute top value is when you’re engine turns into a molten slag of iron, right?1249

When it turns just too liquid iron it’s no good anymore. We can say that the top operating heat would be 1,500 degrees Celsius, absolute top level for an engine.1257

That’s still higher than we could possibly get in real life but the absolute highest temperature we could operate an engine at is 1,500 degrees Celsius.1268

If that car engine is running on a 20 degree Celsius day, what’s the maximum efficiency we could get out of it?1275

Remember, the maximum efficiency is equal to…by the way, this guy right here he’s called epsilon, once again yet another of our friendly Greek letters, he’s epsilon.1280

Didn’t say that earlier but all of our efficiency has been using the letter epsilon. Our maximum efficiency is 1 minus the temperature of the cold sink divided by the temperature of the hot input.1293

If we managed to run our engine at its absolute highest temperature which is 1,500 degrees Celsius and sort of a ridiculously high temperature, higher than we could ever achieve in real life.1307

The absolute top we could pull off is 1,500 degrees Celsius and that’s going to be the absolute best we could have because the bigger the denominator, the better the efficiency we can get.1316

If we can get this number equal to zero we’ve got perfect efficiency. We want to make a giant denominator and tiny cold. Well what’s the cold sink going to be?1325

The cold sink, the best we could do would be the temperature around us in the atmosphere. We’re not able to drive around with a bucket of ice attached to us.1334

Although that’s an interesting idea, but in real life we’re going to have to deal with a bunch of other things.1343

We’ve got the fact that part of the sink is going to be engine compartment around us, we’ve got the engine compartment and that’s defiantly not going to be fully room temperature.1347

We’re going to at best be able to pull off that 20 degrees Celsius cold sink. Remember we can’t use Celsius though when we’re dealing with this because it’d be possible to drop Celsius into the negatives and then this whole formula would get screwed up.1357

We have to be working in kelvin since kelvin is the SI unit. If we want to make this into kelvin, we take each of these and we add 273.15. The cold is going to be 293.15 kelvin.1368

The hot in kelvin is going to be 1773.15 kelvin. The maximum efficiency, we plug both of those in and our maximum efficiency is 1 minus the cold, 293.15 kelvin divided by the hot, 1773.15 kelvin.1382

That comes out to 0.835 which is equal to 83.5%. Keep in mind 83.5% is ridiculously high because there is no way we could get a real life engine to operate at those kinds of temperatures.1406

If we’d tried to get a real life engine to operate at those kinds of temperatures, kapoot, there’s no way we’re going to actually be able to pull it off without completely destroying, just ruining our cars engine.1423

Real life we can’t get that kind of temperatures. That kind of temperature in the engine is just ridiculous. We’re not dealing with the best temperature we could have.1433

It’d clearly be better the way this works for us to be driving around in the winter, a colder temperature than 20 degrees Celsius, is something we could defiantly pull off.1442

But we’re not going to do way better, we can’t drop more than…if we were to manage to drop to -80 Celsius, that’d be colder than any day has ever been on Earth.1449

That’d be only dropping by 100, the efficiency just not going to bump up that much more.1457

83.5, when we’re dealing with this magical super engine is the best we can do.1461

To me at least, 83.5 that doesn’t sound that great. That’s getting pretty close to a B- if this were a test.1468

Think about this, efficiency is actually really, really hard to come by in real life. 83.5% is the best this magical engine can do. It’s able to withstand these crazy internal temperatures.1474

If that’s the best we can do for our engine that’s magical and remember this not even including things like the friction, the other real life forces that are going to occur.1486

This is this theoretical maximum for an already magical engine. If we were to introduce just a little bit of real life, those numbers are start to plummet.1494

In real life for an actual car engine, the theoretical amount that could put out. The theoretical for a real car engine operating at real temperatures, so theoretical car engine is able to pull off an efficiency of around 55% or so.1503

That’s pretty great but an actual car engine once you have to start factoring in all the friction resistances, all the pressure, the various turbulence that’s starts to happen, that sort of randomness that can’t be controlled for.1520

Actual car engines manage to pull of around 25% efficiency rate. Efficiency is really, really hard to come by. Most of the energy that we wind up breaking down from our chemical bonds, actually just goes to heat.1531

We aren’t able to get most of our heat to turn into work. Which is a real shame, because if we were able to manage to just double this, we’d be able to do great things with the amount of energy we have.1547

One of the best things we could do to get more energy is to be able to increase our efficiency, but there’s this hard limit to the best efficiency we can get.1556

That’s based on our hot temperature and our cold temperature. Unless we’re able to get those really far in difference, more importantly get a really nice cold sink temperature, you just can’t get that great in efficiency because of our Carnot maximum efficiency.1565

In real life efficiency is really hard to come by, so an actual car is able to pull off around 25%. Most of the gasoline you wind up burning in your car actually just goes to heating up the air around you, kind of disappointing.1581

But certainly really interesting. That’s the nature of thermo dynamics, a lot of stuff, all the order eventually falls into disorder. It’s not exactly the happiest of endings but that’s how things are going to go with time.1595

Entropy is the winner in the long run. We can at least have long period of order in thought and intelligence and great complex beings.1607

In the really, really long end, as the universe runs to its end, things will eventually tend to disorder and just have compete fall apart as the universe goes into just hot heat death mode as it breaks apart.1615

That’s a long, long, long time from now. For our purposes we can basically forget about that, we’re going to have the chance to live long complex interesting lives in a nice cool universe that hasn’t seen the long painful end of entropy yet.1628

Things are good for a long time to come so don’t worry about it.1643

Alright, hope that was interesting, hope you learned a lot and we’ll see you at educator.com later.1645