100:00:00 --> 00:00:01

200:00:01 --> 00:00:02The following content isprovided under a Creative

300:00:02 --> 00:00:03Commons license.

400:00:03 --> 00:00:06Your support will help MITOpenCourseWare continue to

500:00:06 --> 00:00:10offer high-quality educationalresources for free.

600:00:10 --> 00:00:13To make a donation or viewadditional materials from

700:00:13 --> 00:00:15hundreds of MIT courses,visit MIT OpenCourseWare

800:00:15 --> 00:00:17at ocw.mit.edu.

900:00:17 --> 00:00:19PROFESSOR: Thermodynamics,all right, let's start.

1000:00:19 --> 00:00:31Thermodynamics is the scienceof the flow of heat.

1100:00:31 --> 00:00:42So, thermo is heat, anddynamics is the motion of heat.

1200:00:42 --> 00:00:46Thermodynamics was developedlargely beginning in the

1300:00:46 --> 00:00:521800's, at the time of theIndustrial Revolution.

1400:00:52 --> 00:00:54So, taming of steel.

1500:00:54 --> 00:01:02The beginning of generatingpower by burning fossil fuels.

1600:01:02 --> 00:01:05The beginning of the problemswith CO2 and [NOISE OBSCURES]

1700:01:05 --> 00:01:06global warming.

1800:01:06 --> 00:01:10In fact, it's interesting tonote that the first calculation

1900:01:10 --> 00:01:18on the impact of CO2 on climatewas done in the late

2000:01:18 --> 00:01:211800's by Arrhenius.

2100:01:21 --> 00:01:26Beginning of a generation ofpower moving heat from fossil

2200:01:26 --> 00:01:30fuels to generating energy,locomotives, etcetera.

2300:01:30 --> 00:01:34So, he calculated what wouldhappen to this burning of

2400:01:34 --> 00:01:38fossil fuels, and he decided inhis calculation, he basically

2500:01:38 --> 00:01:42got the calculation right, bythe way, but he came out that

2600:01:42 --> 00:01:45in 2,000 years from the timethat he did the calculations,

2700:01:45 --> 00:01:48humans would be in trouble.

2800:01:48 --> 00:01:51Well, since his calculation,we've had an exponential growth

2900:01:51 --> 00:01:54in the amount of CO2, and ifyou go through the calculations

3000:01:54 --> 00:01:57of -- people have done thesecalculations throughout times

3100:01:57 --> 00:02:01since Arrhenius, the time thatwe're in trouble, 2,000 years

3200:02:01 --> 00:02:06and the calculation, hasgone like this, and so now

3300:02:06 --> 00:02:08we're really in trouble.

3400:02:08 --> 00:02:11That's for a different lecture.

3500:02:11 --> 00:02:13So, anyway, thermodynamicsdates from the same period

3600:02:13 --> 00:02:19as getting fossil fuelsout of the ground.

3700:02:19 --> 00:02:20It's universal.

3800:02:20 --> 00:02:24It turns out everything aroundus moves energy around

3900:02:24 --> 00:02:24in one way or the other.

4000:02:24 --> 00:02:26If you're a biologicalsystem, you're burning

4100:02:26 --> 00:02:28calories, burning ATP.

4200:02:28 --> 00:02:31You're creating heat.

4300:02:31 --> 00:02:34If you're awarm-blooded animal.

4400:02:34 --> 00:02:37You need energy to move yourarms around and move around --

4500:02:37 --> 00:02:41mechanical systems, obviously,cars, boats, etcetera.

4600:02:41 --> 00:02:44And even in astrophysics, whenyou talk about stars, black

4700:02:44 --> 00:02:46holes, etcetera, you'removing energy around.

4800:02:46 --> 00:02:49You're moving heat aroundwhen you're changing matter

4900:02:49 --> 00:02:51through thermodynamics.

5000:02:51 --> 00:02:54And the cause of somethermodynamics have even been

5100:02:54 --> 00:02:59applied to economics, systemsout of equilibrium, like big

5200:02:59 --> 00:03:01companies like Enron, youknow, completely out of

5300:03:01 --> 00:03:03equilibrium, crash and burn.

5400:03:03 --> 00:03:11You can apply non-equilibriumthermodynamics to economics.

5500:03:11 --> 00:03:15It was developed before peopleknew about atoms and molecules.

5600:03:15 --> 00:03:18So it's a science that'sbased on macroscopic

5700:03:18 --> 00:03:21properties of matter.

5800:03:21 --> 00:03:25Since then, since we know aboutatoms and molecules now, we

5900:03:25 --> 00:03:29can rationalize the conceptsof thermodynmamics using

6000:03:29 --> 00:03:34microscopic properties, and ifyou are going to take 5.62,

6100:03:34 --> 00:03:35that's what you'd learn about.

6200:03:35 --> 00:03:38You'd learn about statisticalmechanics, and how the

6300:03:38 --> 00:03:41atomistic conceptsrationalize thermodynamics.

6400:03:41 --> 00:03:46It doesn't prove it, but ithelps to getting more intuition

6500:03:46 --> 00:03:53about the consequencesof thermodynamics.

6600:03:53 --> 00:03:55So it applies to macroscopicsystems that are in

6700:03:55 --> 00:03:59equilibrium, and how to go fromone equilibrium state to

6800:03:59 --> 00:04:03another equilibrium state, andit's entirely empirical

6900:04:03 --> 00:04:05in its foundation.

7000:04:05 --> 00:04:08People have done experimentsthrough the ages, and they've

7100:04:08 --> 00:04:11accumulated the knowledge fromthese experiments, and they've

7200:04:11 --> 00:04:16synthesized these experimentsinto a few basic empirical

7300:04:16 --> 00:04:19rules, empirical laws, whichare the laws of thermodynamics.

7400:04:19 --> 00:04:25And then they've taken theselaws and added a structure of

7500:04:25 --> 00:04:30math upon it, to build thisedifice, which is a very solid

7600:04:30 --> 00:04:36edifice of thermodynamics as ascience of equilibrium systems.

7700:04:36 --> 00:04:43So these empirical observationsthen are summarized

7800:04:43 --> 00:04:46into four laws.

7900:04:46 --> 00:04:51So, these laws are,they're really depillars.

8000:04:51 --> 00:04:55They're not proven, butthey're not wrong.

8100:04:55 --> 00:04:59They're very unlikelyto be wrong.

8200:04:59 --> 00:05:01Let's just go through theselaws, OK, very quickly.

8300:05:01 --> 00:05:07There's a zeroth law The zerothlaw every one of these laws

8400:05:07 --> 00:05:09basically defines the quantityin thermodynamics and then

8500:05:09 --> 00:05:11defines the concept.

8600:05:11 --> 00:05:13The zeroth lawdefines temperature.

8700:05:13 --> 00:05:19That's a fairly common-senseidea, but it's important to

8800:05:19 --> 00:05:23define it, and I call thatthe common-sense law.

8900:05:23 --> 00:05:30So this is thecommon-sense law.

9000:05:30 --> 00:05:37The first law ends up definingenergy, which we're going to

9100:05:37 --> 00:05:44call u, and the concept ofenergy conservation, energy

9200:05:44 --> 00:05:48can't be lost or gained.

9300:05:48 --> 00:05:51And I'm going to call thisthe you can break even law;

9400:05:51 --> 00:05:55you can break even law.

9500:05:55 --> 00:06:00You don't lose energy,you can't gain energy.

9600:06:00 --> 00:06:02You break even.

9700:06:02 --> 00:06:09The second law is going todefine entropy, and is going to

9800:06:09 --> 00:06:11tell us about the directionof time, something that

9900:06:11 --> 00:06:15conceptually we, clearly,understand, but is going to put

10000:06:15 --> 00:06:20a mathematical foundationon which way does time go.

10100:06:20 --> 00:06:24Clearly, if I take a chalk likethis one here, and I throw it

10200:06:24 --> 00:06:27on the ground, and it breaksin little pieces, if I run

10300:06:27 --> 00:06:30the movie backwards, thatdoesn't make sense, right?

10400:06:30 --> 00:06:33We have a concept of time goingforward in a particular way.

10500:06:33 --> 00:06:39How does entropy play intothat concept of time?

10600:06:39 --> 00:06:48And I'm going to call thisthe you can break even at

10700:06:48 --> 00:06:49zero degrees Kelvin law.

10800:06:49 --> 00:06:53You can only do it atzero degrees Kelvin.

10900:06:53 --> 00:07:06The third law is going to givea numerical value to the

11000:07:06 --> 00:07:13entropy, and the third law isgoing to be the depressing one,

11100:07:13 --> 00:07:23and it's going to say, youcan't get to zero degrees.

11200:07:23 --> 00:07:26These laws areuniversally valid.

11300:07:26 --> 00:07:29They cannot be circumvented.

11400:07:29 --> 00:07:33Certainly people have tried todo that, and every year there's

11500:07:33 --> 00:07:38a newspaper story, Wall StreetJournal, or New York Times

11600:07:38 --> 00:07:42about somebody that hasinvented the device that

11700:07:42 --> 00:07:46somehow goes around the secondlaw and makes more energy than

11800:07:46 --> 00:07:50it creates, and this is goingto be -- well, first of all,

11900:07:50 --> 00:07:53for the investors this is goingto make them very, very rich,

12000:07:53 --> 00:07:57and for the rest of us, it'sgoing to be wonderful.

12100:07:57 --> 00:07:59And they go through thesearguments, and they find

12200:07:59 --> 00:08:02venture money to fund thecompany, and they get

12300:08:02 --> 00:08:06very famous people toendorse them, etcetera.

12400:08:06 --> 00:08:10But you guys know, because youhave MIT degrees, and you've,

12500:08:10 --> 00:08:14later, and you've taken 5.60,that can't be the case, and

12600:08:14 --> 00:08:16you're not going to getfooled into investing money

12700:08:16 --> 00:08:18into these companies.

12800:08:18 --> 00:08:22But it's amazing, that everyyear you find somebody coming

12900:08:22 --> 00:08:26up with a way of going aroundthe second law and somehow

13000:08:26 --> 00:08:36convincing people who are verysmart that this will work.

13100:08:36 --> 00:08:39So, thermo is also a bigtease, as you can see

13200:08:39 --> 00:08:43from my descriptionsof these laws here.

13300:08:43 --> 00:08:46It makes you believe,initially, in the feasibility

13400:08:46 --> 00:08:49of perfect efficiency.

13500:08:49 --> 00:08:54The first law is very upbeat.

13600:08:54 --> 00:08:58It talks about theconservation of energy.

13700:08:58 --> 00:09:01Energy is conservedin all of its forms.

13800:09:01 --> 00:09:04You can take heat energy andconvert it to work energy and

13900:09:04 --> 00:09:08vice versa, and it doesn't sayanything about that you have

14000:09:08 --> 00:09:11to waste heat if you're goingto transform heat into work.

14100:09:11 --> 00:09:13It just says it's energy.

14200:09:13 --> 00:09:16It's all the same thing, right?

14300:09:16 --> 00:09:19So, you could break even if youwere very clever about it,

14400:09:19 --> 00:09:22and that's pretty neat.

14500:09:22 --> 00:09:24So, in a sense, it says, youknow, if you wanted to build a

14600:09:24 --> 00:09:29boat that took energy out ofthe warmth of the air, to sail

14700:09:29 --> 00:09:33around the world,you can do that.

14800:09:33 --> 00:09:37And then the second lawcomes in and says well,

14900:09:37 --> 00:09:40that's not quite right.

15000:09:40 --> 00:09:42The second law says, yes,energy is pretty much the same

15100:09:42 --> 00:09:46in all this form, but if youwant to convert one form of

15200:09:46 --> 00:09:51energy into another, if youwant to convert work, heat into

15300:09:51 --> 00:09:55work, with 100% efficiency,you've got to go down to zero

15400:09:55 --> 00:09:57degrees Kelvin, to absolutezero if you want to do that.

15500:09:57 --> 00:10:01Otherwise you're going to wastesome of that heat somewhere

15600:10:01 --> 00:10:04along the way, someof that energy.

15700:10:04 --> 00:10:06All right, so you can't getperfect efficiency, but at

15800:10:06 --> 00:10:09least if you were able to goto zero degrees Kelvin,

15900:10:09 --> 00:10:11then you'd be all set.

16000:10:11 --> 00:10:14You just got to find a goodrefrigerator on your boat,

16100:10:14 --> 00:10:16and then you can stillgo around the world.

16200:10:16 --> 00:10:18And then the third lawcomes in, and that's the

16300:10:18 --> 00:10:19depressing part here.

16400:10:19 --> 00:10:21It says, well, it's true.

16500:10:21 --> 00:10:23If you could get to zerodegrees Kelvin, you'd get

16600:10:23 --> 00:10:27perfect efficiency, but youcan't get to zero degrees

16700:10:27 --> 00:10:31Kelvin, you can't.

16800:10:31 --> 00:10:34Even if you have an infiniteamount of resources,

16900:10:34 --> 00:10:42you can't get there.

17000:10:42 --> 00:10:48Any questions so far?

17100:10:48 --> 00:10:52So thermodynamics, based onthese four laws now, requires

17200:10:52 --> 00:10:56an edifice, and it's a verymature science, and it

17300:10:56 --> 00:10:58requires that we definethings carefully.

17400:10:58 --> 00:11:01So we're going to spend alittle bit of time making sure

17500:11:01 --> 00:11:06we define our concepts and ourwords, and what you'll find

17600:11:06 --> 00:11:13that when you do problem sets,especially at the beginning,

17700:11:13 --> 00:11:18understanding the words and theconditions of the problem sets

17800:11:18 --> 00:11:23is most of the way intosolving the problem.

17900:11:23 --> 00:11:33So we're going to talkabout things like systems.

18000:11:33 --> 00:11:37The system, it's that partof the universe that

18100:11:37 --> 00:11:38we're studying.

18200:11:38 --> 00:11:41These are going to be fairlycommon-sense definitions, but

18300:11:41 --> 00:11:44they're important, and when youget to a problem set, really

18400:11:44 --> 00:11:50nailing down what the systemis, not more, nor less, in

18500:11:50 --> 00:11:53terms of the amount of stuff,that's part of the system, it's

18600:11:53 --> 00:11:55going to be often very crucial.

18700:11:55 --> 00:11:57So you've got the system.

18800:11:57 --> 00:11:59For instance, itcould be a person.

18900:11:59 --> 00:11:59I am the system.

19000:11:59 --> 00:12:00I could be a system.

19100:12:00 --> 00:12:04It could be a hotcoffee in a thermos.

19200:12:04 --> 00:12:07So the coffee and the milk andwhatever else you like in your

19300:12:07 --> 00:12:08coffee would be the system.

19400:12:08 --> 00:12:12It could be a glass ofwater with ice in it.

19500:12:12 --> 00:12:13That's a fine system.

19600:12:13 --> 00:12:15Volume of air in apart of a room.

19700:12:15 --> 00:12:18Take four liters on thiscorner of the room.

19800:12:18 --> 00:12:21That's my system.

19900:12:21 --> 00:12:28Then, after you define whatyour system is, whatever is

20000:12:28 --> 00:12:32left over of the universeis the surroundings.

20100:12:32 --> 00:12:35So, if I'm the system,then everything else

20200:12:35 --> 00:12:35is the surroundings.

20300:12:35 --> 00:12:37You are my surroundings.

20400:12:37 --> 00:12:39Saturn is my surroundings.

20500:12:39 --> 00:12:41As far as you can go inthe universe, that's part

20600:12:41 --> 00:12:43of the surroundings.

20700:12:43 --> 00:12:44And then between the systemand the surroundings

20800:12:44 --> 00:12:47is the boundary.

20900:12:47 --> 00:12:56And the boundary is a surfacethat's real, like the outsides

21000:12:56 --> 00:13:01of my skin, or the inner wallof the thermos that has the

21100:13:01 --> 00:13:06coffee in it, or it couldbe an imaginary boundary.

21200:13:06 --> 00:13:09For instance, I can imaginethat there is a boundary that

21300:13:09 --> 00:13:12surrounds the four liters ofair that's sitting in

21400:13:12 --> 00:13:12the corner there.

21500:13:12 --> 00:13:15It doesn't have to be a realcontainer to contain it.

21600:13:15 --> 00:13:21It's just an imaginaryboundary there.

21700:13:21 --> 00:13:24And where you place thatboundary becomes important.

21800:13:24 --> 00:13:28So, for instance, for thethermos with the coffee in it,

21900:13:28 --> 00:13:31if you place the boundary inthe inside wall of the glass or

22000:13:31 --> 00:13:33the outside wall of the glassand the inside of the thermos,

22100:13:33 --> 00:13:35that makes a difference;different heat

22200:13:35 --> 00:13:36capacity, etcetera.

22300:13:36 --> 00:13:39So this becomes wheredefining the system and the

22400:13:39 --> 00:13:42boundaries, and everythingbecomes important.

22500:13:42 --> 00:13:43You've got to place theboundary at exactly the right

22600:13:43 --> 00:13:46place, otherwise you've got abit too much in your system

22700:13:46 --> 00:13:50or a bit too little.

22800:13:50 --> 00:13:51More definitions.

22900:13:51 --> 00:13:59The system can be an opensystem, or it can be a closed

23000:13:59 --> 00:14:06system, or it can be isolated.

23100:14:06 --> 00:14:09The definitions arealso important here.

23200:14:09 --> 00:14:16An open system, as the namedescribes, allows mass and

23300:14:16 --> 00:14:21energy to freely flowthrough the boundary.

23400:14:21 --> 00:14:37Mass and energy flowthrough boundary.

23500:14:37 --> 00:14:43Mass and energy -- I'man open system, right?

23600:14:43 --> 00:14:45Water vapor goesthrough my skin.

23700:14:45 --> 00:14:52I'm hot, compared to the airof the room, or cold if

23800:14:52 --> 00:14:54I'm somewhere that's warm.

23900:14:54 --> 00:14:56So energy can goback and forth.

24000:14:56 --> 00:15:02The thermos, with the lid ontop, is not an open system.

24100:15:02 --> 00:15:04Hopefully, your coffee isgoing to stay warm or

24200:15:04 --> 00:15:05hot in the thermos.

24300:15:05 --> 00:15:07It's not going to get out.

24400:15:07 --> 00:15:09So the thermos isnot an open system.

24500:15:09 --> 00:15:13In fact, the thermos isan isolated system.

24600:15:13 --> 00:15:17The isolated system is theopposite of the open system,

24700:15:17 --> 00:15:22no mass and no energy canflow through the boundary.

24800:15:22 --> 00:15:26The closed system allows energyto transfer through the

24900:15:26 --> 00:15:28boundary but not mass.

25000:15:28 --> 00:15:32So a closed system would be,for instance, a glass of ice

25100:15:32 --> 00:15:35water with an ice cube init, with the lid on top.

25200:15:35 --> 00:15:37The glass is notvery insulating.

25300:15:37 --> 00:15:41Energy can flow across theglass, but I put a lid on top,

25400:15:41 --> 00:15:43and so the water can't get out.

25500:15:43 --> 00:15:45And that's the closed system.

25600:15:45 --> 00:15:50Energy goes through theboundaries but nothing else.

25700:15:50 --> 00:15:52Important definitions, eventhough they may sound really

25800:15:52 --> 00:15:55kind of dumb, but they arereally important, because when

25900:15:55 --> 00:15:59you get the problem, figuringout whether you have an open,

26000:15:59 --> 00:16:01closed, or isolated system,what are the surroundings?

26100:16:01 --> 00:16:02What's the boundary?

26200:16:02 --> 00:16:04What is the system?

26300:16:04 --> 00:16:08That's the first thing tomake sure that is clear.

26400:16:08 --> 00:16:10If it's not clear, theproblem is going to be

26500:16:10 --> 00:16:14impossible to solve.

26600:16:14 --> 00:16:18And that's also how people findways to break the second law,

26700:16:18 --> 00:16:24because somehow they've messedup on what their system is.

26800:16:24 --> 00:16:27And they've included too muchor too little in the system,

26900:16:27 --> 00:16:30and it looks to them that thesecond law is broken and

27000:16:30 --> 00:16:35they've created more energythan is being brought in.

27100:16:35 --> 00:16:39That's usually the case.

27200:16:39 --> 00:16:42Questions?

27300:16:42 --> 00:16:44Let's keep going.

27400:16:44 --> 00:16:48So, now that we've gota system, we've got

27500:16:48 --> 00:16:49to describe it.

27600:16:49 --> 00:16:57So, let's describethe system now.

27700:16:57 --> 00:17:02It turns out that when you'retalking about macroscopic

27800:17:02 --> 00:17:06properties of matter, you don'tneed very many variables to

27900:17:06 --> 00:17:11describe the system completelythermodynamically.

28000:17:11 --> 00:17:14You just need a few macroscopicvariables that are very

28100:17:14 --> 00:17:18familiar to you, like thepressure, the temperature, the

28200:17:18 --> 00:17:22volume, the number of moles ofeach component, the

28300:17:22 --> 00:17:25mass of the system.

28400:17:25 --> 00:17:27You've got a magnetic field,maybe even magnetic

28500:17:27 --> 00:17:30susceptibility, theelectric field.

28600:17:30 --> 00:17:32We're not going to worry aboutthese magnetic fields or

28700:17:32 --> 00:17:34electric fields in this class.

28800:17:34 --> 00:17:37So, pretty much we're goingto focus on this set

28900:17:37 --> 00:17:40of variables here.

29000:17:40 --> 00:17:42You're going to have to knowwhen you describe the system,

29100:17:42 --> 00:17:48if your system is homogeneous,like your coffee with milk in

29200:17:48 --> 00:17:52it, or heterogeneous, likewater with an ice cube in it.

29300:17:52 --> 00:17:55So heterogeneous meansthat you've got different

29400:17:55 --> 00:17:56phases in your system.

29500:17:56 --> 00:17:59I'm the heterogeneoussystem, soft stuff, hard

29600:17:59 --> 00:18:02stuff, liquid stuff.

29700:18:02 --> 00:18:04Coffee is homogeneous,even though it's made

29800:18:04 --> 00:18:06up of many components.

29900:18:06 --> 00:18:08Many different kinds ofmolecules make up your coffee.

30000:18:08 --> 00:18:11There are the water molecules,the flavor molecules, the

30100:18:11 --> 00:18:12milk proteins, etcetera.

30200:18:12 --> 00:18:14But it's all mixed uptogether in a homogeneous,

30300:18:14 --> 00:18:17macroscopic fashion.

30400:18:17 --> 00:18:20If you drill down at the levelof molecules you see that

30500:18:20 --> 00:18:22it's not homogeneous.

30600:18:22 --> 00:18:25But thermodynamics takesa bird's eye view.

30700:18:25 --> 00:18:27It looks pretty, beautiful.

30800:18:27 --> 00:18:32So, that's a homogeneoussystem, one phase.

30900:18:32 --> 00:18:35You have to know if yoursystem is an equilibrium

31000:18:35 --> 00:18:38system or not.

31100:18:38 --> 00:18:40If it's an equilibriumsystem, then thermodynamics

31200:18:40 --> 00:18:41can describe it.

31300:18:41 --> 00:18:44If it's not, then you're goingto have trouble describing it

31400:18:44 --> 00:18:46using thermodynamic properties.

31500:18:46 --> 00:18:51Thermodynamics talks aboutequilibrium systems and how

31600:18:51 --> 00:18:53to go from one state ofequilibrium to another

31700:18:53 --> 00:18:55state of equilibrium.

31800:18:55 --> 00:18:56What does equilibrium mean?

31900:18:56 --> 00:18:59It means that the properties ofthe system, the properties that

32000:18:59 --> 00:19:06describe the system, don'tchange in time or in space.

32100:19:06 --> 00:19:10If I've got a gas in acontainer, the pressure of the

32200:19:10 --> 00:19:12gas has to be the sameeverywhere in the container,

32300:19:12 --> 00:19:14otherwise it's not equilibrium.

32400:19:14 --> 00:19:17If I place my container of gason the table here, and I come

32500:19:17 --> 00:19:20back an hour later, thepressure needs to be the

32600:19:20 --> 00:19:22same when I come back.

32700:19:22 --> 00:19:25Otherwise it's not equilibrium.

32800:19:25 --> 00:19:29So it only talks aboutequilibrium systems.

32900:19:29 --> 00:19:30What else do you need to know?

33000:19:30 --> 00:19:32So, you need to knowthe variables.

33100:19:32 --> 00:19:36You need to know it'sheterogeneous or homogeneous.

33200:19:36 --> 00:19:39You need to know if it's anequilibrium, and you also need

33300:19:39 --> 00:19:47to know how many componentsyou have in your system.

33400:19:47 --> 00:19:52So, a glass of ice water withan ice cube in it, which is a

33500:19:52 --> 00:19:54heterogeneous system, has onlyone component, which

33600:19:54 --> 00:19:57is water, H2O.

33700:19:57 --> 00:20:00Two phases, but one component.

33800:20:00 --> 00:20:04Latte, which is a homogeneoussystem, has a very, very large

33900:20:04 --> 00:20:07number of components to it.

34000:20:07 --> 00:20:09All the components thatmake up the milk.

34100:20:09 --> 00:20:12All the components that makeup the coffee, and all the

34200:20:12 --> 00:20:15impurities, etcetera. cadmium,heavy metals, arsenic,

34300:20:15 --> 00:20:22whatever is in your coffee.

34400:20:22 --> 00:20:26OK, any questions?

34500:20:26 --> 00:20:29All right, so we'vedescribed the system

34600:20:29 --> 00:20:31with these properties.

34700:20:31 --> 00:20:33Now these propertiescome in two flavors.

34800:20:33 --> 00:20:37You have extensive propertiesand intensive properties.

34900:20:37 --> 00:20:43The extensive properties arethe ones that scale with

35000:20:43 --> 00:20:44the size of the system.

35100:20:44 --> 00:20:47If you double the system,they double in there

35200:20:47 --> 00:20:48numerical number.

35300:20:48 --> 00:20:51For instance, the volume.

35400:20:51 --> 00:20:54If you double thevolume, the v doubles.

35500:20:54 --> 00:20:55I mean that's obvious.

35600:20:55 --> 00:20:58The mass, if you doublethe amount of stuff

35700:20:58 --> 00:21:02the mass will double.

35800:21:02 --> 00:21:06Intensive properties don't careabout the scale of your system.

35900:21:06 --> 00:21:10If you double everything in thesystem, the temperature is not

36000:21:10 --> 00:21:12going to change, it'snot going to double.

36100:21:12 --> 00:21:14The temperature stays the same.

36200:21:14 --> 00:21:16So the temperature isintensive, and you can make

36300:21:16 --> 00:21:19intensive properties out ofthe extensive properties by

36400:21:19 --> 00:21:24dividing by the numberof moles in the system.

36500:21:24 --> 00:21:27So I can make a quantity thatI'll call V bar, which is the

36600:21:27 --> 00:21:32molar volume, the volume of onemole of a component in my

36700:21:32 --> 00:21:37system, and that becomesan intensive quantity.

36800:21:37 --> 00:21:41A volume which is anintensive volume.

36900:21:41 --> 00:21:51The volumes per moleof that stuff.

37000:21:51 --> 00:21:54So, as I mentioned,thermodynamics is the science

37100:21:54 --> 00:22:05of equilibrium systems, and italso describes the evolution

37200:22:05 --> 00:22:07of one equilibrium toanother equilibrium.

37300:22:07 --> 00:22:09How do you go fromone to the other?

37400:22:09 --> 00:22:13And so the set of propertiesthat describes the system --

37500:22:13 --> 00:22:16the equilibrium doesn't change.

37600:22:16 --> 00:22:20So, these on-changingproperties that describe the

37700:22:20 --> 00:22:23state of the equilibriumstate of the system are

37800:22:23 --> 00:22:24called state variables.

37900:22:24 --> 00:22:37So the state variables describethe equilibrium's state, and

38000:22:37 --> 00:22:41they don't care about how thisstate got to where it is.

38100:22:41 --> 00:22:44They don't care about thehistory of the state.

38200:22:44 --> 00:22:48They just know that's if youhave water at zero degrees

38300:22:48 --> 00:22:53Celsius with it ice in, thatyou can define it as a

38400:22:53 --> 00:22:58heterogeneous system with acertain density for the water

38500:22:58 --> 00:23:01or certain density for theice, etcetera, etcetera.

38600:23:01 --> 00:23:04It doesn't care howyou got there.

38700:23:04 --> 00:23:06We're going to find otherproperties that do care about

38800:23:06 --> 00:23:09the history of the system, likework, that you put in the

38900:23:09 --> 00:23:11system, or heat that you put inthe system, or some

39000:23:11 --> 00:23:13other variables.

39100:23:13 --> 00:23:18But you can't use those todefine the equilibrium state.

39200:23:18 --> 00:23:20You can only use the statevariables, independent

39300:23:20 --> 00:23:22of history.

39400:23:22 --> 00:23:25And it turns out that for a onecomponent system, one component

39500:23:25 --> 00:23:30meaning one kind of molecule inthe system, all that you need

39600:23:30 --> 00:23:37to know to describe the systemis the number of moles for a

39700:23:37 --> 00:23:43one component system, and todescribe one phase in that

39800:23:43 --> 00:23:46system, one component,homogeneous system, you

39900:23:46 --> 00:23:53need n and two variables.

40000:23:53 --> 00:23:58For instance, the pressureand the temperature, or the

40100:23:58 --> 00:24:00volume and the pressure.

40200:24:00 --> 00:24:05If you have the number of molesand two intensive variables,

40300:24:05 --> 00:24:07then you know everything thereis to know about the system.

40400:24:07 --> 00:24:11About the equilibriumstate of that system.

40500:24:11 --> 00:24:17There are hundreds ofquantities that you can

40600:24:17 --> 00:24:19calculate and measure that areinteresting and important

40700:24:19 --> 00:24:23properties, and all you need isjust a few variables to get

40800:24:23 --> 00:24:26everything out, and that'sreally the power of

40900:24:26 --> 00:24:29thermodynamics, is that ittakes so little information to

41000:24:29 --> 00:24:32get so much information out.

41100:24:32 --> 00:24:42So little data to get a lot ofpredictive information out.

41200:24:42 --> 00:24:51As we're going on with ourdefinitions, we can summarize a

41300:24:51 --> 00:24:56lot of these definitions into anotation, a chemical notation

41400:24:56 --> 00:25:00that that will bevery important.

41500:25:00 --> 00:25:04So, for instance, if I'mtalking about three moles

41600:25:04 --> 00:25:08of hydrogen, at one bar100 degrees Celsius.

41700:25:08 --> 00:25:12I'm not going to write, giventhree moles of hydrogen at one

41800:25:12 --> 00:25:14bar and three degrees,blah, blah, blah.

41900:25:14 --> 00:25:17I'm going to write itin a compact notation.

42000:25:17 --> 00:25:21I'm going to write it likethis: three moles of hydrogen

42100:25:21 --> 00:25:28which is a gas, one bar100 degrees Celsius.

42200:25:28 --> 00:25:30This notation gives youeverything you need to

42300:25:30 --> 00:25:31know about the system.

42400:25:31 --> 00:25:33It tells you thenumber of moles.

42500:25:33 --> 00:25:34It tells you the phase.

42600:25:34 --> 00:25:37It tells you what kind ofmolecule it is, and gives

42700:25:37 --> 00:25:41you two variables thatare state variables.

42800:25:41 --> 00:25:44You could have the volumeand the temperature.

42900:25:44 --> 00:25:47You could have the volumeand the pressure.

43000:25:47 --> 00:25:48But this tells you everything.

43100:25:48 --> 00:25:51I don't need to writeit down in words.

43200:25:51 --> 00:25:55And then if I want to tell youabout a change of state, or

43300:25:55 --> 00:25:57let's first startwith a mixture.

43400:25:57 --> 00:26:02Suppose that I give to amixture like, this is a

43500:26:02 --> 00:26:06homogeneous system with twocomponents, like five moles of

43600:26:06 --> 00:26:13H2O, which is a liquid, at onebar 25 degrees Celsius, plus

43700:26:13 --> 00:26:21five moles of CH3, CH2, OH,which is a liquid, and one

43800:26:21 --> 00:26:26bar at 25 degrees Celsius.

43900:26:26 --> 00:26:30This describes roughlysomething that is fairly

44000:26:30 --> 00:26:36commonplace, it's 100-proofvodka 1/2 water, 1/2 ethanol

44100:26:36 --> 00:26:41-- that describes thatmacroscopic system.

44200:26:41 --> 00:26:43You're missing all theimpurities, all the little the

44300:26:43 --> 00:26:47flavor molecules that go intoit, but basically, that's the

44400:26:47 --> 00:26:50homogeneous system we weredescribing, two component

44500:26:50 --> 00:26:53homogeneous systems.

44600:26:53 --> 00:26:56Then you can do all sortsof predictive stuff

44700:26:56 --> 00:26:59with that system.

44800:26:59 --> 00:27:01All right, that's theequilibrium system.

44900:27:01 --> 00:27:04Now we want to show a notation,how do we go from one

45000:27:04 --> 00:27:07equilibrium state like thisdescribes to another

45100:27:07 --> 00:27:16equilibrium state?

45200:27:16 --> 00:27:18So, we take our two equilibriumstates, and you just put an

45300:27:18 --> 00:27:24equal sign between them, andthe equal sign means go

45400:27:24 --> 00:27:25from one to the other.

45500:27:25 --> 00:27:31So, if we took our three molesof hydrogen, which is a gas at

45600:27:31 --> 00:27:38five bar and 100 degreesCelsius, and, which is a nice

45700:27:38 --> 00:27:40equilibrium state here, and wesay now we're going to change

45800:27:40 --> 00:27:43the equilibrium state tosomething new, we're going to

45900:27:43 --> 00:27:48do an expansion, let's say.

46000:27:48 --> 00:27:50We're going to drop thepressure, the volume

46100:27:50 --> 00:27:51is going to go up.

46200:27:51 --> 00:27:53I don't need to tell you thevolume here, because you've

46300:27:53 --> 00:27:56got enough informationto calculate the volume.

46400:27:56 --> 00:28:01The number of moles staysthe same, a closed systems,

46500:28:01 --> 00:28:03gas doesn't come out.

46600:28:03 --> 00:28:05Stays a gas, but now thepressure is less, the

46700:28:05 --> 00:28:06temperature is less.

46800:28:06 --> 00:28:10I've done some sort ofexpansion on this.

46900:28:10 --> 00:28:12I've gone from 1 equilibriumstate to another equilibrium

47000:28:12 --> 00:28:15state, and the equal signmeans you go from this

47100:28:15 --> 00:28:16state to that state.

47200:28:16 --> 00:28:17It's not a chemical reaction.

47300:28:17 --> 00:28:20That's why we don't have anarrow here, because we could

47400:28:20 --> 00:28:22go back, this way too.

47500:28:22 --> 00:28:24We can go back and forthbetween these two

47600:28:24 --> 00:28:25equilibrium states.

47700:28:25 --> 00:28:25They're connected.

47800:28:25 --> 00:28:27This means they're connected.

47900:28:27 --> 00:28:30And when I put this, Ihave to tell you how

48000:28:30 --> 00:28:31they are connected.

48100:28:31 --> 00:28:33I have to tell you thepath, if you're going

48200:28:33 --> 00:28:34to solve a problem.

48300:28:34 --> 00:28:36For instance, you want to knowhow much energy you're going to

48400:28:36 --> 00:28:39get out from doingthis expansion.

48500:28:39 --> 00:28:42How much energy are you goingto get out, and how far are you

48600:28:42 --> 00:28:44going to be able to drive a carwith this expansion, let's

48700:28:44 --> 00:28:46say, so that's the problem.

48800:28:46 --> 00:28:49So, I need to tell you howyou're doing the expansion,

48900:28:49 --> 00:28:51because that's going to tellyou how much energy you're

49000:28:51 --> 00:28:53wasting during that expansion.

49100:28:53 --> 00:28:56It goes back to the second law.

49200:28:56 --> 00:28:57Nothing is efficient.

49300:28:57 --> 00:28:59You're always wasting energyinto heat somewhere when you

49400:28:59 --> 00:29:03do a change that involvesa mechanical change.

49500:29:03 --> 00:29:07All right, so I need to tellyou the path, when I go from

49600:29:07 --> 00:29:09one state to the other.

49700:29:09 --> 00:29:12And the path is going to be thesequence, intermediate states

49800:29:12 --> 00:29:15going from the initialstate the final state.

49900:29:15 --> 00:29:22So, for instance, if I draw agraph of pressure on one axis

50000:29:22 --> 00:29:27and temperature on the otheraxis, my initial state is at a

50100:29:27 --> 00:29:34temperature of 100 degreesCelsius and five bar.

50200:29:34 --> 00:29:42My final stage is 50 degreesCelsius and one bar.

50300:29:42 --> 00:29:46So, I could have twosteps in my path.

50400:29:46 --> 00:29:49I could decide first of all tokeep the pressure constant

50500:29:49 --> 00:29:53and lower the pressure.

50600:29:53 --> 00:29:55When I get to 50 degreesCelsius, I could choose to

50700:29:55 --> 00:29:59keep the temperature constantand lower the pressure.

50800:29:59 --> 00:30:01I'm sorry, my first step wouldbe to keep the pressure

50900:30:01 --> 00:30:04constant and lower thetemperature, then I lower the

51000:30:04 --> 00:30:07pressure, keeping thetemperature constant.

51100:30:07 --> 00:30:08So there's my intermediatestate there.

51200:30:08 --> 00:30:12This is one of many paths.

51300:30:12 --> 00:30:15There's an infinite numberof paths you could take.

51400:30:15 --> 00:30:19You could take a continuouspath, where you have an

51500:30:19 --> 00:30:24infinite number of equilibriumpoints in between the two, a

51600:30:24 --> 00:30:28smooth path, where you drop thepressure and the temperature

51700:30:28 --> 00:30:30simultaneously inlittle increments.

51800:30:30 --> 00:30:35All right, so when you doa problem, the path is

51900:30:35 --> 00:30:38going to turn out to beextremely important.

52000:30:38 --> 00:30:43How do you get from the initialstate to the final state?

52100:30:43 --> 00:30:44Define the initial state.

52200:30:44 --> 00:30:45Define the final state.

52300:30:45 --> 00:30:47Define the path.

52400:30:47 --> 00:30:50Get all of these reallyclear, and you've basically

52500:30:50 --> 00:30:51solved the problem.

52600:30:51 --> 00:30:56You've got to spend the time tomake sure that everything is

52700:30:56 --> 00:30:58well defined before you starttrying to work out

52800:30:58 --> 00:31:02these problem.

52900:31:02 --> 00:31:04More about the path.

53000:31:04 --> 00:31:08There are a couple ways youcould go through that path.

53100:31:08 --> 00:31:10If I look at thissmooth path here.

53200:31:10 --> 00:31:14I could have that path be veryslow and steady, so that at

53300:31:14 --> 00:31:18every point along the way,my gas is an equilibrium.

53400:31:18 --> 00:31:21So I've got, this piston hereis compressed, and I slowly,

53500:31:21 --> 00:31:25slowly increase the volume,drop the temperature.

53600:31:25 --> 00:31:29Then I can go back, thegas is included at

53700:31:29 --> 00:31:33every point of the way.

53800:31:33 --> 00:31:35That's a reversible path.

53900:31:35 --> 00:31:36That can reverse the process.

54000:31:36 --> 00:31:39I expand it, and reverseit, no problem.

54100:31:39 --> 00:31:49So, I could have a reversiblepath, or I take my gas, and

54200:31:49 --> 00:31:53instead of slowly, slowlyraising it, dropping the

54300:31:53 --> 00:32:00pressure, I go from five barto one bar extremely fast.

54400:32:00 --> 00:32:01What happens to my gas inside?

54500:32:01 --> 00:32:04Well, my gas inside isgoing to be very unhappy.

54600:32:04 --> 00:32:06It's not going stayin equilibrium.

54700:32:06 --> 00:32:08Parts of the system aregoing to be at five bar.

54800:32:08 --> 00:32:10Parts of it at one bar.

54900:32:10 --> 00:32:14Parts of it may be even at zerobar, if I go really fast.

55000:32:14 --> 00:32:15I'm going to create a vacuum.

55100:32:15 --> 00:32:21So the system will not bedescribed by a single state

55200:32:21 --> 00:32:23variable during the path.

55300:32:23 --> 00:32:27If I look at different pointsin my container during that

55400:32:27 --> 00:32:31path, I'm going to have to usea different value of pressure

55500:32:31 --> 00:32:32or different value oftemperature at different

55600:32:32 --> 00:32:35points of the container.

55700:32:35 --> 00:32:38That's not an equilibriumstate, and that process

55800:32:38 --> 00:32:42turns out then to be inirreversible process.

55900:32:42 --> 00:32:43Do it very quickly.

56000:32:43 --> 00:32:46Now to reverse it and get backto the initial point is going

56100:32:46 --> 00:32:50to require some input fromoutside, like heat or extra

56200:32:50 --> 00:32:53work or extra heat orsomething, because you've done

56300:32:53 --> 00:32:54an irreversible process.

56400:32:54 --> 00:33:04You've wasted a lot of energyin doing that process.

56500:33:04 --> 00:33:09I have to tell you whether thepath is reversible or

56600:33:09 --> 00:33:13irreversible, and theirreversible path also defines

56700:33:13 --> 00:33:15the direction of time.

56800:33:15 --> 00:33:19You can only have anirreversible path go one way

56900:33:19 --> 00:33:20in time, not the other way.

57000:33:20 --> 00:33:24Chalk breaks irreversiblyand you can't put it

57100:33:24 --> 00:33:25back together so easily.

57200:33:25 --> 00:33:29You've got to pretty much takethat chalk, and make a slurry

57300:33:29 --> 00:33:32out of it, put water, and dryit back up, put in a mold, and

57400:33:32 --> 00:33:34then you can have the chalkagain, but you can't just

57500:33:34 --> 00:33:35glue it back together.

57600:33:35 --> 00:33:36That would not be thesame state as what

57700:33:36 --> 00:33:40you started out with.

57800:33:40 --> 00:33:42And then there are abunch of words that

57900:33:42 --> 00:33:43describe these paths.

58000:33:43 --> 00:33:47Words like adiabatic, whichwe'll be very familiar with.

58100:33:47 --> 00:33:50Adiabatic means that there's noheat transferred between the

58200:33:50 --> 00:33:51system and the surrounding.

58300:33:51 --> 00:33:55The boundary is imperviousto transfer of heat,

58400:33:55 --> 00:33:56like a thermos.

58500:33:56 --> 00:33:59Anything that happens inside ofthe thermos is an adiabatic

58600:33:59 --> 00:34:03change because the thermos hasno connection in terms of

58700:34:03 --> 00:34:04energy to the outside world.

58800:34:04 --> 00:34:06There's no heat that cango through the walls

58900:34:06 --> 00:34:07of the thermos.

59000:34:07 --> 00:34:10Whereas, like isobaricmeans constant pressure.

59100:34:10 --> 00:34:15So, this path right herefrom this top red path

59200:34:15 --> 00:34:18is an isobaric process.

59300:34:18 --> 00:34:21Constant temperature meansisothermal, so this part

59400:34:21 --> 00:34:23means an isothermal process.

59500:34:23 --> 00:34:28So then, going from the initialto final states with a red

59600:34:28 --> 00:34:32path, you start with anisobaric process and then you

59700:34:32 --> 00:34:34end with an isothermal process.

59800:34:34 --> 00:34:36And these are words that arevery meaningful when you read

59900:34:36 --> 00:34:42the text of a problemor of a process.

60000:34:42 --> 00:34:44Any questions before wegot to the zeroth law?

60100:34:44 --> 00:34:49We're pretty much done withour definitions here.

60200:34:49 --> 00:34:49Yes.

60300:34:49 --> 00:34:53STUDENT: Was adiabaticreversible?

60400:34:53 --> 00:34:56PROFESSOR: Adiabatic can beeither reversible or not, and

60500:34:56 --> 00:35:01we're going to do that probablynext time or two times.

60600:35:01 --> 00:35:02Any other questions?

60700:35:02 --> 00:35:06Yes.

60800:35:06 --> 00:35:07STUDENT: Is there a boundarybetween reversible

60900:35:07 --> 00:35:08and irreversible?

61000:35:08 --> 00:35:12PROFESSOR: A boundary betweenreversible and irreversible?

61100:35:12 --> 00:35:14Like something isalmost reversible and

61200:35:14 --> 00:35:15almost irreversible.

61300:35:15 --> 00:35:16No, pretty much thingsare either reversible

61400:35:16 --> 00:35:19or irreversible.

61500:35:19 --> 00:35:28Now, in practice, it depends onhow good your measurement is.

61600:35:28 --> 00:35:35And probably also in practice,nothing is truly reversible.

61700:35:35 --> 00:35:43So, it depends on yourerror bar in a sense.

61800:35:43 --> 00:35:45It depends on what what youdefine, exactly what you

61900:35:45 --> 00:35:46define in your system.

62000:35:46 --> 00:35:50It becomes a gray area, butit should be pretty clear if

62100:35:50 --> 00:35:57you can treat something isreversible are irreversible.

62200:35:57 --> 00:36:03Other questions, It'sa good question.

62300:36:03 --> 00:36:06So the zeroth law we're goingto go through the laws now.

62400:36:06 --> 00:36:10The zeroth law talks aboutdefining temperature and

62500:36:10 --> 00:36:12it's the common-sense law.

62600:36:12 --> 00:36:14You all know how.

62700:36:14 --> 00:36:16When something hot, it's gota higher temperature than

62800:36:16 --> 00:36:18when something is cold.

62900:36:18 --> 00:36:21But it's important to definethat, and define something

63000:36:21 --> 00:36:23that's a thermometer.

63100:36:23 --> 00:36:26So what do you know?

63200:36:26 --> 00:36:28What's the empiricalinformation that

63300:36:28 --> 00:36:29everybody knows?

63400:36:29 --> 00:36:34Everybody knows that if youtake something which is hot and

63500:36:34 --> 00:36:40something which is cold, andyou bring them together, make

63600:36:40 --> 00:36:48them touch, that heat is goingto flow from the hot to the

63700:36:48 --> 00:36:56cold, and make them touch, andheat flows from hot to cold.

63800:36:56 --> 00:36:57That's common sense.

63900:36:57 --> 00:37:03This is part of your DNA, Andthen their final product is an

64000:37:03 --> 00:37:11object, a b which ends up at atemperature or a warmness which

64100:37:11 --> 00:37:13is in between thehot and the cold.

64200:37:13 --> 00:37:16So, this turns out to be warm.

64300:37:16 --> 00:37:19You get your new equilibriumstate, which is in between

64400:37:19 --> 00:37:28what this was, andwhat a and b were.

64500:37:28 --> 00:37:34Then how do you know that it'schanged temperature, or that

64600:37:34 --> 00:37:37heat has flowed from a to b?

64700:37:37 --> 00:37:41Practically speaking, you needsome sort of property that's

64800:37:41 --> 00:37:43changing as heat is flowing.

64900:37:43 --> 00:37:50For instance, if a weremetallic, you could measure

65000:37:50 --> 00:37:54the connectivity of a orresistivity, and as heat

65100:37:54 --> 00:38:01flows out of a into b, theresistivity of a would change.

65200:38:01 --> 00:38:04Or you could have somethingthat's color metric that

65300:38:04 --> 00:38:09changes color when it's colder,so you could see the heat

65400:38:09 --> 00:38:13flowing as a changes color orb changes color as

65500:38:13 --> 00:38:15heat flows into b.

65600:38:15 --> 00:38:17So, you need some sort ofproperty, something you can

65700:38:17 --> 00:38:20see, something you canmeasure, that tells you

65800:38:20 --> 00:38:21that heat has flowed.

65900:38:21 --> 00:38:26Now, if you have three objects,if you have a, b, and c, and

66000:38:26 --> 00:38:37you bring them together, and ais the hottest, b is the medium

66100:38:37 --> 00:38:41one, and c is the coldest, sofrom hottest to coldest a,

66200:38:41 --> 00:38:49b, c, -- if you bring themtogether and make them touch,

66300:38:49 --> 00:38:57you know, intuitively, thatheat will not flow like this.

66400:38:57 --> 00:38:59You know that's notgoing to happen.

66500:38:59 --> 00:39:03You know that what will happenis that heat will flow from a

66600:39:03 --> 00:39:07to b from b to cand from a to c.

66700:39:07 --> 00:39:08That's common-sense.

66800:39:08 --> 00:39:11You know that.

66900:39:11 --> 00:39:13And the other way in thecircle will never happen.

67000:39:13 --> 00:39:17That would that would give riseto a perpetual motion machine,

67100:39:17 --> 00:39:18breaking of the second law.

67200:39:18 --> 00:39:22It can't happen.

67300:39:22 --> 00:39:24But that's an empiricalobservation, that heat

67400:39:24 --> 00:39:27flows in this direction.

67500:39:27 --> 00:39:29And that's the zerothlaw thermodynamic.

67600:39:29 --> 00:39:32It's pretty simple.

67700:39:32 --> 00:39:39The zeroth law says that if aand b -- it doesn't exactly say

67800:39:39 --> 00:39:41that, but it implies this.

67900:39:41 --> 00:39:45It says that if a and b are inthermal equilibrium, if these

68000:39:45 --> 00:39:48two are in thermal equilibrium,meaning that there's no heat

68100:39:48 --> 00:39:51flows between them, so that'sthe definition of thermal

68200:39:51 --> 00:39:54equilibrium, that no heat flowsbetween them, and these two are

68300:39:54 --> 00:39:57in thermal equilibrium, andthese two are in thermal

68400:39:57 --> 00:40:02equilibrium, then a and c willbe also be in thermal

68500:40:02 --> 00:40:04equilibrium.

68600:40:04 --> 00:40:07But if there's no heat flowingbetween these two, and no heat

68700:40:07 --> 00:40:09flowing between these two,then you can't have heat

68800:40:09 --> 00:40:13flowing between these two.

68900:40:13 --> 00:40:16So if I get rid of thesearrows, there's no heat flowing

69000:40:16 --> 00:40:18because they're in thermalequilibrium, then I can't

69100:40:18 --> 00:40:20have an arrow here.

69200:40:20 --> 00:40:22That's what thezeroth law says.

69300:40:22 --> 00:40:24They're all thesame temperature.

69400:40:24 --> 00:40:25That's what it says.

69500:40:25 --> 00:40:28If two object are in the sametemperature, and two other

69600:40:28 --> 00:40:30object are in the sametemperature, then all three

69700:40:30 --> 00:40:34must have the same temperature.

69800:40:34 --> 00:40:37It sounds pretty silly, butit's really important because

69900:40:37 --> 00:40:44it allows you to define athermometer and temperature.

70000:40:44 --> 00:40:48Because now you can say,all right, well, now b

70100:40:48 --> 00:40:49can be my thermometer.

70200:40:49 --> 00:40:54I have two objects, I have anobject which is in Madagascar

70300:40:54 --> 00:40:59and an object which is inBoston, and I want to know, are

70400:40:59 --> 00:41:01they the same temperature?

70500:41:01 --> 00:41:05So I come out with a thirdobject, b, I go to Madagascar,

70600:41:05 --> 00:41:06and put b in contact with a.

70700:41:06 --> 00:41:11Then I insulate everything, youknow, take it away and see

70800:41:11 --> 00:41:12if there's any heat flow.

70900:41:12 --> 00:41:15Let's say there's no heat flow.

71000:41:15 --> 00:41:18Then I insulate it, get back onthe plane to Boston, and go

71100:41:18 --> 00:41:20back and touch b with c.

71200:41:20 --> 00:41:23If there's no heat flow betweenthe b and c, then I can say

71300:41:23 --> 00:41:27all right, a and c werethe same temperature.

71400:41:27 --> 00:41:29B is my thermometer thattells me that a and c are

71500:41:29 --> 00:41:30in the same temperature.

71600:41:30 --> 00:41:33And there's a certain propertyassociated with heat flow with

71700:41:33 --> 00:41:35b, and it didn't change.

71800:41:35 --> 00:41:37And that propertycould be color.

71900:41:37 --> 00:41:38It could be resistivity.

72000:41:38 --> 00:41:39It could be a lot ofdifferent things.

72100:41:39 --> 00:41:41It could be volume.

72200:41:41 --> 00:41:44And the temperature then isassociated with that property.

72300:41:44 --> 00:41:47And if it had changed, then thetemperature between those two

72400:41:47 --> 00:41:50would have changed in avery particular way.

72500:41:50 --> 00:41:57So, zeroth law, then, allowsyou to define the concept of

72600:41:57 --> 00:42:05temperature and the measurementof temperature through

72700:42:05 --> 00:42:09a thermometer.

72800:42:09 --> 00:42:12Let's very briefly gothrough stuff that

72900:42:12 --> 00:42:13you've learned before.

73000:42:13 --> 00:42:16So, now you have this objectwhich is going to tell you

73100:42:16 --> 00:42:19whether other things are inthermal equilibrium now.

73200:42:19 --> 00:42:21What do you needfor that object?

73300:42:21 --> 00:42:27You need that object to be asubstance, to be something.

73400:42:27 --> 00:42:31So, the active part of thethermometer could be water.

73500:42:31 --> 00:42:35It could be alcohol, mercury,it could be a piece of metal.

73600:42:35 --> 00:42:39You need a substance, and thenthat substance has to have a

73700:42:39 --> 00:42:41property that changes dependingon the heat flow, i.e.,

73800:42:41 --> 00:42:45depending on whether it'ssensing that it's the same

73900:42:45 --> 00:42:46temperature or differenttemperature than

74000:42:46 --> 00:42:46something else.

74100:42:46 --> 00:42:51And that property could be thevolume, like if you have a

74200:42:51 --> 00:42:54mercury thermometer, thevolume of the mercury.

74300:42:54 --> 00:42:55It could be temperature.

74400:42:55 --> 00:42:58It could be resistivity, ifyou have a thermocouple.

74500:42:58 --> 00:43:02It could be the pressure.

74600:43:02 --> 00:43:04All right, so nowyou have an object.

74700:43:04 --> 00:43:06You've got a propertythat changes, depending

74800:43:06 --> 00:43:07on the heat flow.

74900:43:07 --> 00:43:09It's going to tell youabout the temperature.

75000:43:09 --> 00:43:11Now you need to definethe temperature scales.

75100:43:11 --> 00:43:16So, you need some referencepoints to be able to tell you,

75200:43:16 --> 00:43:22OK, this temperature is 550degrees Smith, whatever.

75300:43:22 --> 00:43:27So, you assign values to veryspecific states of matter and

75400:43:27 --> 00:43:29call those the referencepoints for your temperature.

75500:43:29 --> 00:43:32For instance, freezingof water or boiling of

75600:43:32 --> 00:43:34water, the standard ones.

75700:43:34 --> 00:43:36And then aninterpolation scheme.

75800:43:36 --> 00:43:41You need a functional form thatconnects the value at one state

75900:43:41 --> 00:43:45of matter, the freezing pointof water, to another phase

76000:43:45 --> 00:43:47change, the boilingpoint of water.

76100:43:47 --> 00:43:51You can choose a linearinterpolation or quadratic,

76200:43:51 --> 00:43:54but you've got to choose it.

76300:43:54 --> 00:43:56And it turns outnot to be so easy.

76400:43:56 --> 00:43:58And if you go back into the1800's when thermodynamics was

76500:43:58 --> 00:44:02starting, there were a zilliondifferent temperatures scales.

76600:44:02 --> 00:44:06Everybody had their ownfavorite temperature scales.

76700:44:06 --> 00:44:09The one that we're mostfamiliar with is the centigrade

76800:44:09 --> 00:44:12or Celsius scale where mercurywas the substance, and the

76900:44:12 --> 00:44:13volume of mercuryis the property.

77000:44:13 --> 00:44:16The reference points are water,freezing or boiling, and the

77100:44:16 --> 00:44:19interpolation is linear, andthen that morphed into the

77200:44:19 --> 00:44:21Kelvin scale, as we'regoing to see later.

77300:44:21 --> 00:44:24The Fahrenheit scale isan interesting scale.

77400:44:24 --> 00:44:26It turns out the U.S. andJamaica are the only two

77500:44:26 --> 00:44:28places on Earth now thatuse the Fahrenheit scale.

77600:44:28 --> 00:44:33Mr. Fahrenheit, Daniel GabrielFahrenheit was a German

77700:44:33 --> 00:44:37instrument maker.

77800:44:37 --> 00:44:41The way he came up with hisscale was actually he borrowed

77900:44:41 --> 00:44:43the Romer scale, whichcame beforehand.

78000:44:43 --> 00:44:48The Romer scale was, Romer wasa Dane, and he defined freezing

78100:44:48 --> 00:44:54of water at 7.5 degreesRoemer, and 22.5 degrees

78200:44:54 --> 00:44:56Romer as blood-warm.

78300:44:56 --> 00:45:00That was his definition.

78400:45:00 --> 00:45:03Two substances,blood and water.

78500:45:03 --> 00:45:05Two reference points,freezing and blood-warm,

78600:45:05 --> 00:45:07you know, the human body.

78700:45:07 --> 00:45:09A linear interpolation betweenthe two, and then some numbers

78800:45:09 --> 00:45:12associated with them,7-1/2 and 22-1/2.

78900:45:12 --> 00:45:15Why does he choose 7-1/2 asthe freezing point of water?

79000:45:15 --> 00:45:18Because he thought thatwould be big enough that in

79100:45:18 --> 00:45:22Denmark, the temperaturewouldn't go below zero.

79200:45:22 --> 00:45:24That's how he picked 7-1/2.

79300:45:24 --> 00:45:25Why not?

79400:45:25 --> 00:45:28He didn't want to use negativenumbers to measure temperature

79500:45:28 --> 00:45:31in Denmark outside.

79600:45:31 --> 00:45:33Well, Fahrenheit came along andthought, well, you know, 7-1/2,

79700:45:33 --> 00:45:36that's kind of silly; 22-1/2that's, kind of silly.

79800:45:36 --> 00:45:40So let's multiplyeverything by four.

79900:45:40 --> 00:45:46I think it becomes 30 degreesfor the freezing of water and

80000:45:46 --> 00:45:4922.5 x 4, which I don't knowwhat it is, 100 or something

80100:45:49 --> 00:45:55-- no, it's 90 I think.

80200:45:55 --> 00:45:58And then for some reason, thatnobody understands, he decided

80300:45:58 --> 00:46:04to multiply again by 16/15, andthat's how we get 32 for

80400:46:04 --> 00:46:08freezing of water and 96 in hiswords for the temperature in

80500:46:08 --> 00:46:10the mouth or underneaththe armpit of a living

80600:46:10 --> 00:46:12man in good health.

80700:46:12 --> 00:46:14What a great temperature scale.

80800:46:14 --> 00:46:16It turns out that 96wasn't quite right.

80900:46:16 --> 00:46:21Then he interpolated and foundout water boils at 212.

81000:46:21 --> 00:46:24But, you know, his experimentwasn't so great, and, you know,

81100:46:24 --> 00:46:26maybe had a fever when he didthe reference point

81200:46:26 --> 00:46:28with 96, whatever.

81300:46:28 --> 00:46:31It turns out that it's not96 to be in good health,

81400:46:31 --> 00:46:34it's 98.6 -- whatever.

81500:46:34 --> 00:46:37That's how we got tothe Fahrenheit scale.

81600:46:37 --> 00:46:40All right, next time we'regoing to talk about a much

81700:46:40 --> 00:46:43better scale, which is theideal gas thermometer and how

81800:46:43 --> 00:46:45we get to the Kelvin scale.

81900:46:45 --> 00:46:45