Engineering MAE 91. Intro to Thermodynamics. 

presumably you are all here for thermodynamics introduction

it is a pretty full class actually fill up the lecture hall because the lecture hall I think holds

over 300 but I think that is about 200 almost 250 of you enrolled most of you

should be mechanical engineering majors or aerospace engineering majors but I'm

sure there are a few others for example from environmental engineering who also

have to take this class I just pulled this up from the from the Tripoli web

site so I can go quickly over a couple of this items the first thing I guess

you all know how to get here so we will need to talk about that the class has two teaching assistants and they're both

named their friends and the Rinne friends is here but the ring is not but

you'll get to see her this Friday when you go to the first discussion section so this Friday will start with

discussion sections you go to the bottom where it says information a couple of

things I want you to be aware of you need to enroll through the distance

learning center and I think if most of you tube dynamics how many of you took dynamics from Professor Jabari so you

are already familiar with the system through the distance learning center I

won't use it as extensively as he does but I will use it to collect homework so

do not turn in any homework in the boxes in the engineering gateway building

because your homework if you turn it in there will stay there for the whole quarter and nobody will ever see it so

the way you will turn in your homework is once you enroll in the class in MA 91

at the distance learning center then you can upload your homework by the deadline

as a PDF file into that website I'll go a little bit into the details in a

moment so that's number one item that is important the second one item that is important is that you'll

need an iclicker so if you don't yet have an iclicker you need to get one

from the bookstore we will use the I clickers to do a couple of to achieve a

couple of goals once one is participation so just showing up will

get you a few miscellaneous points but also quizzes via the I clicker and so we

won't start doing that until next week so make sure you get an iclicker before you come back to the lecture next week

let's see what else do I have here the text a few you have already asked questions about the text we're using the

eighth edition which looks like this at least the hard copy version of it so

and this is a brand new edition I think it actually has a copyright of 2013 and this is the one we're using the

publisher told me that maybe there weren't as many in the bookstore so if

you if you go to this link that is on the Triple E website

there are two links there the first one just takes you to a place where you can buy the book directly from the publisher

and the second one which perhaps is most important more important for you now you

can actually download the first three chapters so if you don't have the text

yet you can get the first three chapters from that second link and the first

three chapters will carry you probably about at least three weeks into the

course we're roughly going about a chapter a week except there are a couple of chapters that are long and will take

more than a week to cover them but you can get the first three chapters for

free from this link right here the second link right here

okay let me go a little bit into the

syllabus which is also there

first part I guess it's not important you may have seen this already so you

know how the the course is graded is heavily graded on exams as you can see

the last two entries 40% and 45% are the midterm and the final respectively but

there are miscellaneous points on the homework 10% and participation which I described a moment ago which is 5% you

also have the dates for the MIT remand the final there so make sure you are

going to be around those dates the

homework doesn't really count as much ten percent is not really a whole lot

considering the amount of time that you will probably spend on the homework so I

want to warn you about a couple of things the idea of the homework really in addition to that 10% which is really

pretty much like a token number of points is for you to practice so make

sure that that's what you're getting out of doing the homework you practicing on the homework problems will prepare you

for the exams and that's the main goal more than the 10% if you just copy and

every year students do copy if you copy and you don't practice you will get

maybe close to the 10% but you'll do really bad on the exams because you won't have developed that practice so

keep that in mind you can certainly collaborate you can work on the homework in groups as long

as you know yourself that you are actually thinking and putting the time to learn and solve the problems yes I

will do that yeah thank you for reminding me can you make a note of that we need to put a book on the on the

reserve and usually I just do the shortest period I forget which one of these two hours or whatever the shortest

period is another thing that is very

important is that we don't take any late homework so the homework is due at a certain time

as again via upload you have to upload it by the deadline if you don't that's

it don't come back saying that whatever excuse you have for that day we won't take any late homework you will do have

the benefit at the end of the course we usually drop your worst scores so if you

happen to miss one then you don't get heavily penalized make sure you know the

policy of an academic dishonesty you can

read an it there is a link on the main website for the campus academic

dishonesty information best thing is just to be honest if you're honest then

you won't have any problems as far as the content here is up here's the

content broken down into weeks actually a subject and then the amount of time that we're going to spend on each topic

it should add up to what is that two

times two lectures per week ten weeks or 20 weeks I'm sorry 20 lectures and you can see

the breakdown there so whatever you see two lectures that's one full week so we'll spend the first week in item

number one which is introduction and some preliminary concepts and then keep

moving along the the main two items in an introduction to the thermodynamics course are the first law of

thermodynamics and the second law of thermodynamics so we'll spend about half of the course developing material that

has to do with the first law of thermodynamics and then the second half of the course we're not real it has to

do with the second law of thermodynamics I already told you about the text the

last page here just tells you how to do your homework the main thing again is that you submit

it as a PDF file so whatever whichever way you do it in whichever way you get

it into a digital format make sure you convert it to a PDF file

before you uploaded the rest of the items there are fairly straightforward

you don't need to copy the statement from the text just go ahead and start solving the problem do it in an

organized manner so the person who is great in your homework doesn't have to

guess what you're doing so you list your unknowns if it's useful draw some

problems are very simple especially at the beginning that you don't need to draw a schematic but later on as the

problems get a little bit more complicated we want you to draw a schematic or a sketch of the problem

that you're solving list your assumptions write everything down all

your equations so that they can be understood and any important answer just

put it in a box it takes a lot of time to grade 250 problem sets so that's the

reason really for all of these details

okay I think that's all I have here does

anybody have any questions about any of these no questions okay yes

yes in fact let me go back here the next

link home or assignment if you go here you can actually see all of the problems

for the whole quarter so you'll see there are total of eight assignments or

more assignment one through homework assignment eight they all come from the book and the sort of problem numbers for

the first week are these ones here you also have a reading assignment we

usually just goes parallel to my lectures the first week it is chapter one and so on and again these are from

the these are the problems from this eighth edition a common question from

the students is can I use the seventh edition or can I use the sixth edition

and so on the answer is a cautious yes

you may use an earlier edition because the material is probably 95 percent the

same what happens from Edition to Edition is that some of the stuff gets

shifted around chapters get recombined and that has happened this time so if

you go by an old edition you're the chapters won't match exactly and also

the problem numbers won't match so you got to do this problems from these

numbers from the eighth edition if you turn in the problem numbers from an old edition they'll be the wrong problems so

that's the only thing you have to be careful about so yes you can with that

caution and it is your responsibility to make sure that you're doing the right problem so that's all don't ask me or

the TAS you know which ones are the problems in the old edition you know you have to find that out on your own all

right any other questions yes

yes yes you need to enroll in ma 91 so

once you go to the distance Learning Center just look for this class I'm

sorry you have to take you have to enroll in the new one it's a different

website so you probably see still somewhere there they all want from last

year but make sure you enroll in this year's I'm sorry to hear that

are you taking it again I hope it wasn't my fault all right

okay let's see let me start by let me

make sure this is working before I do anything else I may all right it seems

kind of dark let's put a piece of paper here all right so we may have to is that

readable is the glare a little let's see

okay that's not bad I'll come back to that let me start by showing you a video

and we're not going to see the whole video but at least a portion of it

okay we don't need to watch the whole thing it's on YouTube so if you want to

watch the complete video you can just go to youtube and type jet-engine and

you'll come up with not only this one but many other videos that have to do with jet engines and I just picked this

as an example of a device that is

obviously heavily based on thermodynamics and in the course of

watching that video you actually heard many of the terms that we're going to be

working with as we go along in the course the engine itself the it's

obviously an important aspect of thermodynamics but you also heard about compressors combustion chambers nozzles

turbines and we by the time we're done thing weeks from now those words should

be very familiar to you and you should be able to know what are were the thermodynamic principles that control

those devices now really to get to something like that to understanding

everything about a jet engine particularly if you're an aerospace engineering major obviously it's not

just the thermodynamics this course really is the first in a series of courses that guide you through the

process of understanding how something like a jet engine works there is not only the thermodynamics but clearly

there is a lot of fluid mechanics you hear about the guy the air coming in the

combustion process the combustion gases the combustion products coming out there is a lot of fluid mechanics in there

there is a lot of heat transfer so these are not surprisingly the names of courses in your program of study as you

go along and then of course if you are nervous by engineering major then there is a course

on propulsion where perhaps you are finally able to put some of these

fundamental ideas together now thermodynamics is not just for aerospace

engineers obviously it's extremely important for mechanical engineers as

well so what do you think of when you

see the word thermodynamics what comes to your mind if you were to do a quick

word association what word comes up right there

rocket engines energy your heat that's

one usually that comes up quite often any of those answers are obviously

correct the word heat is is a common one

somebody else has said energy more important perhaps and just energy itself

is energy transformation how do we change energy from one form to another

form thermodynamics will help us with that interestingly enough the word

dynamics is associated with one huh well a little bit more narrow than just

physics say that again movement right in

particular what about the difference between kinematics and dynamics can imagine is also movement well in physics

was the difference when you talk about kinematics versus dynamics in dynamics

where's that word somebody said here who said forces

she said forces in dynamics usually you pay attention to the forces that cause

that motion in kinematics you only want to quantify the motion itself so I

always say that this thermodynamics is a little bit of a misnomer at least for

the type of thermodynamics that we're going to be looking at because it's really closer to what you

would call thermostatic s' which doesn't exist at least as a basic

course because as you will see as we go along we're really not going to be

paying too much attention to how processes occur or how things change

during that motion but actually more about before and after we will pay some

attention to the during but most of the thermodynamics that you see in an

introductory course like this is like this is how things were at the beginning

and this is how things are at the end of a certain process and then we're trying to make some calculations to see maybe

how energy was converted me ask you another question can you hear me well

back there because I can speak a little louder and if it doesn't work I can wear

another microphone he's gonna keep putting Burlinson me so let me know if

at any time during the lecture you're having a hard time hearing way back there you can also move closer there's

plenty of space here near the front let me continue with this one important

aspect of course of our study of thermodynamics is based on an important

conservation law from physic which is conservation of energy for us energy is a quantity that is conserved

we're not talking about any relativistic effects of thermodynamics where energy

could be converted to mass and and vice-versa for us energy is a conserved

quantity and so of course is mass alright so when we talk about energy we

obviously think of different forms of energy and part of understanding thermodynamics is understanding that

conversion process how do I go from say having chemical energy into having at

least a portion of that chemical energy converted to mechanical energy the concept of heat itself by by itself

is important we're going to be looking at systems that transform energy that

energy conversion process so here at least some examples a power plant if

you're a mechanical engineer then a power plant perhaps is more of a common

device for you than say the heat engine the let me sorry the jet engine that we

were looking at in the video refrigeration systems designed to move

heat around right to move energy in the form of say temperature heat from one

place to another the internal combustion engine which operates in your in your

car's fuel cells also devices that

convert energy in one way to another we just saw an example of turbines I'm just

throwing words here down I could add of course a few more but just so that you

can get a little bit of an idea and of course the rocket engine that we saw in the video so as I said the video was

just one example but here's another one

here's a typical schematic of a power

plant what is this power plant doing for us

generating electricity and you see that here right therefore this is a simple

schematic so it's represented by this one electric generator that produces

electricity where does the generator get

the energy from the turbine so there are

several things happening here at the same time let's just and I of course II many of the components of a typical

power plant there is a boiler here the turbine that I just mentioned there

might be pumps in this case that we're just representing all of the pumps by

one there is a condenser right here which is

this rectangular box and of course there are some auxiliary systems like a COO

maybe some mechanism to cool the working fluid in this case they have a cooling

tower stack for the combustion products to go out so for example if we just

focus for a moment on this part so where we have the boiler the turbine the

condenser and the pump we can see that

here and this other which is just that part of the of the plant it's not really

that other one blown up it's a different schematic but you see the the four components that I was just referring to

the boiler in this case called a combustor unit the turbine the condenser

and the pump so one of the things that you see here that is very important in thermodynamics is that many of these

processes that are important to us as engineers operating cycles so what you

have here is a cycle you can see here cycle working fluid there is a fluid

that is going around through each of these devices over and over and over and

over and over again in the case of a power plan what is that fluid

water right water is typically the fluid that would be what we call the working

fluid in a power plant such as this one typically it would be water so what you

have you know pick any starting point for example let's pick this point here

between the pump and the combustor unit you're bringing that water as a liquid

into that combustion unit in that combustion unit you have like some sort

of a high temperature source maybe you're burning something as you can see here we're feeding fuel and air and of

course producing a chemical reaction and that chemical reaction will generate high temperatures those high temperature

combustion products then bring that

water into steam so boil it and bring it into high temperature steam so that's

what comes out if you follow that process through the combustion unit then

you come out with say steam some high pressure high temperature steam then you take that steam run it through a turbine

wort is produced by that turbine so the steam just goes to the turbine moves the

blades and produce some sort of a rotating power output which then will

drive your electric generator now as the steam goes to the turbine it loses

energy so the pressure of that steam and the temperature too will drop and it may

come out still as vapor but at a much lower pressure and temperature and

remember we started by putting liquid into the combustion chamber and we have

a vapor here so what we are missing to close the cycle is a condenser where we

can bring that steam that came out of the turbine back into liquid state by

taking more energy out of it how do we take more energy out of it we

have another water system which is the cooling water that is coming in removing

energy from the steam so that it condenses and then now I have a liquid

but I have another problem which is that this liquid is at a pressure which is not high enough to be fed into the

combustion chamber that's why there is the fourth ingredient in the cycle

devices is the pump so now I need to take that liquid that comes out of the condenser at a low pressure and pump it

up to a pressure that is high enough to go into the combustion unit so you see a

combustion unit turbine condenser and pump are the four main components of a

power plant such as this one more

examples by the way if you have any questions just raise your hand and I'll

try to see your hand if it comes up and try to answer your question here's another example what is this well it's

just a cylinder right and and the crankshaft of an internal combustion

engine so here is the cylinder this is another example of an engineering device

where we need thermodynamics to understand how it operates so what is

happening here well of course we're putting fuel and air in there is a spark

to light it so that it burns that high energy then pushes the piston down the

crank mechanism converts that linear displacement into rotation so of course

then we can drive the wheels by rotation

and then of course then the combustion products need to be exhausted to the exhaust valve so that's another example

there's a different completely different device from what we saw earlier in the

powerplant but still as you will see later is governed

by the same thermodynamic principles and so suppose that I say well let me actually show you a little bit how this

works and we're going to see all of these things in detail later but for the time being I'm just going to show you

some examples this is what we call it so I'm gonna take this cylinder and I'm

gonna flip it on its side like this for this next plot so here it is down here

is my schematic of the cylinder so it's moving between this the top of this of

this piston is moving between this dash line and the lower dash line we call

this one the highest position we call it top dead center you may have seen that

here so the highest point of the top of the cylinder is called top dead center whereas when it's all the way down at

the bottom we call it bottom dead center so top dead center bottom dead center so

this piston is just in this schematic it's moving horizontally but what I have

done here just to show you something of one of the things that we're going to be learning is what we call an indicator

diagram for this cylinder for this internal combustion engine and what

we're doing is we're plotting pressure versus volume and we're going to see lots of plots of pressure versus volume

temperature versus volume and so on so pressure on the vertical axis volume on

the horizontal axis and of course the volume is the volume in here to the left

of this piston so the minimum volume is the V sub C when the piston is at top

dead center and the maximum volume of course when the piston is at bottom that

Center is back here the V sub a and of course as the piston moves up and down

then the volume of course changes between those two volumes so take for example the point that is denoted here

by our when we are at R we have

the piston top dead center right so then

suppose now we we draw the piston we pull it out and if we do that of course

the volume increases and if we open a valve which is not scared here but if we

were to open about to let air in then

that's what would happen between R and say B when we reach bottom that Center

and now say we close the valves and then we start pushing the piston back towards

top that Center since we closed about now we have a restricted volume that is

decreasing there is no escape for the air that is in here then it's obvious that it's pressure will I'm sorry I

shouldn't I shouldn't have told you B we're going from R to a right so when

we're withdrawing the piston we're going from R to a and now from a we're pushing

it back in the the valves are closed the is not surprising that the pressure will

start to go up because we're compressing that air and at some point here which is

maybe what is denoted here by C prime maybe that's when the spark comes off it

ignites the mixture so the pressure continues to rise the volume continues to decrease we reach top that Center

still the pressure continues to go up and then we reach a point Z there where the pressure is the maximum at some

point we would have to then open the exhaust valves and then of course not

notice that what is happening here is that the piston is already going back you know between C and Z we're already

starting to go back that's the souq that's the driving part of the of the

process that's when we're pushing the piston back with the force produced by

the combustion products by the explosion of the mixture and we go back to B and

then that at sample then we have to open the exhaust valve and then the piston goes back and pushes

all the combustion products out so if we started at are going first to a then to

Z then to B in the back to our we have completed a cycle so that's a typical

cycle of an internal combustion engine let's look at one more any questions

let's look at one more which is the one we saw in the video so here is that this

is not a great picture but this is the

jet engine that we saw in the video so the air comes in from the left side he

goes through the compression through the compressing part of the engine which is this bluish greenish area here once that

air is at a sufficiently high pressure then it goes into the combustion chamber

which you can see here it burns and then the combustion products go out through

the turbine and the turbine of course rotates and in fact it is the turbine

which drives the compressor so some of the work that is generated by the

turbine has to be used to drive the compressor and a few other systems in

the engine and then the exhaust comes back out and that's the end

look notice a big difference there doesn't seem to be a cycle here because

the air is coming here and then going out there is a little harder to visualize a cycle but let's do another

plot and they have to move this back down here so there are you can see

another cycle here let me go and make this a little bigger there are two lines

and if you can see that two different colors there is a black one which is actually closed

that is a cycle from one to two to three to four and then back to one and then there is a blue one which has the same

one but then goes to two prime instead of two goes to three prime and then goes

to four prime but that one is not closed that blue one is representative or more

representative of the real situation we're plotting here temperature versus

entropy which is a property we're going to talk about later in the course so

temperature versus entropy just to give you a different type of a plot instead

of pressure versus volume which we saw in the other one so so if we started one

then as we go let's look at the black one first as we go from one to two that

would be the compression notice that there are two other lines here that are

important one is this one here at the bottom this curve which I denote the constant pressure this is a constant

pressure line so any point along this line has the same pressure and then

there is a similar line here which is also a constant pressure line any point on this line here has the same pressure

obviously this one here is higher than this one here so you can see as we go

from one to two we're going from the lower pressure to the high pressure from one to two so that's the compression

that's as we go say from we go back here from the inlet to just outside of the

compressor that would be one to two so one to two is what we call the ideal process whereas one to two prime is more

of the real process then we go from two to three would be in the combustion

chamber whether it is from two to three or from two prime to three prime well that's the difference between ideal and

real but that process between two and three or between two Prime and three prime would be during the combustion

process you can see how that happens at nearly constant pressure in fact for the

black cycle it is constant pressure because we're following exactly that constant pressure

line for the real one you can see there is a little bit of a decrease from two prime to three prime

we're not exactly following that pressure line but we're dropping off a little bit and then finally from three

to four we're going through an expansion we're going now from the high pressure

to the low pressure that would be the process through which part of the engine

sorry no to the turbine right so as we go

through the turbine we drop in the pressure from three to four in the ideal scenario or let me reduce this a little

bit again or from 3 prime to 4 Prime in the in the in the real one three to four

in the ideal one three three point four Prime in the ideal one now in the real engine there is no going from four prime

to one because the exhaust go out you

don't really bring the exhaust back out into the front you just leave it there behind but in the ideal cycle when you

first studied this type of a of a cycle for a heat engine I'm sorry for a jet

engine you actually pretend that you are actually going from four to one and you

close the cycle that way this cycle is called a Brayton cycle just so you know

this type of a cycle is a cycle for air because the main substance the main

fluid that is going through is air sure we are adding fuel at some point but

when we're talking about an ideal Brayton cycle we're thinking of only the air thinking what the air is doing so

the area is undergoing a compression then a constant pressure process in the

combustion chamber an expansion to the turbine and then somehow another

compression which in the real cycle doesn't exist

all right so that's just another example

and maybe let's look at one more this is

as that you can see at the top there a refrigeration cycle move it up there in

a refrigeration cycle such as the one that is operating in your standard

household refrigerator the idea of course is to take energy from someplace

and put it somewhere else and of course we do this for average in a refrigeration cycle we're thinking of

Heat so we're taking heat from somewhere and we're putting it somewhere else and it also has four components or four

devices that are important and there are of course variations over this basic

schematic the four components you can see them here an evaporator a compressor

a condenser and an expansion bulb we

already saw a compressor even though it's a different type of a compressor

when we were looking at the jet engine a moment ago so here's another compressor

we saw a condenser also when did we see a condenser when we're looking at the

power plant we saw a condenser there so those we have seen at least versions of

them for different equipment the two new

ones here will be the expansion valve and the evaporator all right so what is

happening here again let's start anywhere suppose we start here at number one is this here we're starting with

saturated or superheated vapor those are key words that we're going to learn about later for the time be ingesting

vapor we're starting with vapor we put it to the compressor so now the

compressor increases the pressure so when we come out to we have a higher pressure is still a

vapor put them at a higher pressure and then what happens then we go through a

condenser that's a key word for us it means that this vapor is going to condense so when it comes out at 3 it

comes out as liquid and of course in order to do that you will have to remove

heat out of the fluid in the condenser that's what you see here Q out is the

heat that is leaving the fluid but is the working fluid by the way in here

some refrigerant that's enough for now we'll see about which refrigerants later

but some refrigerant as opposed to water which was the fluid in the power plant

so remove he'd leave the condenser now

as a liquid and then go through an expansion valve purpose of this valve is

to drop the pressure that's the main purpose of it so you will drop the pressure from the high pressure in the

condenser to the low pressure in the evaporator so we come out at 4 at a low

pressure this was a liquid here when it comes out in that process of being

throttled through the valve some of it evaporates so when it comes out at at 4

some of it is liquid some of it is vapor and then we put it through the evaporator and whatever remaining liquid

there was will be evaporated obviously in order to evaporate it we need to add

Heat and then we come out here where we started at all right so where is this

heat coming from in your household refrigerator from the inside this is

this here would be the inside and where is this heat going out

usually it's the back right but if it's going into whatever room whatever room the refrigerator is located so that's

where that heat is is he's going to are

these the same you think so whatever heat I take out of the refrigerator

compartment is that the heat that goes down into the room why not

right if you if you look carefully and again this we haven't learned this yet

but there is a little magic part of the cycle that I didn't talk about here and

I need to drive the compressor the compressor doesn't run by itself it

needs to be driven right which means essentially that it needs to be plugged in the refrigerator you'll need a source

of energy and that's also energy that is going into my entire system so if I

think of all of this as one device so if I put all of these four components into

a big black box then I see that I am adding heat here I'm adding something

here is not heat initially it doesn't look like he'd but its energy and I am

putting it out so if you just do a quick energy balance in your head the conclusion is that this has to be larger

than this because there was some other energy added here right we learned that

the details of this as we go along again this is just to give you a sort of an

idea of the type of devices that we're going to be looking at but let's step

back and go back to the basics you've heard a lot of words today

and there's gonna be a lot more it will be important for you to understand the

concepts very well so you we sort of have a system definitions and you know

things may mean something in the outside world but for us in thermodynamics they have specific meanings so let's go over

a few of those the first one is what we

in thermodynamics call a system and that's a very simple definition as you

can see they're a quantity of matter constitutes a system which means of

course that pretty much anything that you wish to study using thermodynamics

is a system you speak a certain amount of mass and make that to be your system

we do something very simple in thermodynamics which is once we define what the system is everything else that

exists that is not what we charge to be the system we just call it the surroundings and so when we're learning

thermodynamics then the world is very simple the world is the system that

we're interested in and the surroundings is everything else that's it that's all

we have to worry about but then of course what is the dividing surface or

line between your system and your surroundings and it will need a name

what do you propose we call it good for me

so we'll call it a boundary so or you know the system boundary or the boundary

will be what separates the system from the surroundings so it's most likely the

system will occupy a certain volume then the boundary will be a surface with some

sort of a surface that separates the system from the surroundings now it is very important to understand that the

boundary might be real it may be a tangible surface or it might be an

abstract surface so it might not really exist depending on the system that

you're looking at it might not be something that is really tangible but you can define it you will see that as

we work on problems now we can also call that surface the control surface that's

another name for the boundary the control surface if the system is closed

and we'll see what that means in a second then we can call it a control mass - what does that mean that a system

is closed and I call it a control mass what do you think that means no mass

transfer across the boundary so if the system is closed it is a control mass

and I put it right there in red it means that no mass crosses the boundary so a

closed system is one for which no mass

crosses the boundary and therefore the opposite an open system which by the way

we can also call a control volume an open system or a control volume is one

in which mass is allowed to cross the boundary all right these are some of the

preliminary concepts there is another one

that will come up once in a while what we call an isolated system and you can

see the definition there nothing crosses the boundary in particular what do we

mean when we say nothing so what's

particularly important for us energy right we say nothing to make it very

general nothing crosses the boundary but if you know really for us in addition to

mass as something that can cross the boundary the other very important quantity that can cross the boundaries

energy so if there is no energy crossing the boundary no mass crossing the boundary essentially there is no

interaction between that system and the surroundings then that's an isolated system you'll see that that's an

important concept for us later as we start to develop some other concepts

here's here is a very simple problem of

which you're going to see a lot of these type of problems are typical in thermodynamics so the cylinder for

example think of the internal combustion engine that we were just looking at a moment ago so here's a cylinder it's

fitted with a piston there is a gas in there and there is a weight on top and

say that I can control that weight if I control the weight more or less less weight remove weight add weight I can

move the piston up and down so what do you think is the system here or one

choice when one good choice for the system here if I'm doing a

thermodynamics problem involving something like this what would you pick

as your system how about you

pick something no except for the what is the question you cannot pick that what

is the system okay but I want something more specific than that I'm sorry couldn't get that word what

all closed system no no I want to know what the system is define the system for me I wouldn't use the word volume the

mass as a better choice the mass of a gas or simply the gas take the gas as

your system remember a system is a quantity of matter so it's really mass

more than boil the volume can change right where's the math wand if it's

closed system right if I don't let any other gas escape so let's say that we say the the gas is our system whatever

gas is there then of course you see a dashed line there which is the boundary the control surface you may argue well

that's not very precise because I see some of the guys outside that boundary well yeah that's because that's the

schematic right ideally you would like to draw that boundary so that it contains all of the dots that represent

the gas but you get the point so if you pick the gas to be your system

then of course you would think that maybe the inner lining of that cylinder and the bottom surface of the piston

constitute your boundary but in a simple schematic like this we just draw a dashed line like that and that's usually

sufficient so so now you somebody said

something about a closed system if this a closed system it appears to be that

way I don't see any way for the gas to go out you know assuming that this is properly sealed you know that this no no

gas escaping of course if some gas does leave that cylinder then it'll become a

question what my system is so suppose I think let me let me go over that very

quickly suppose I say that the system is the gas the system is this gas

but now there is a leak and maybe some of these gas is escaping here is that a

closed system or an open system

openopenopen anybody closed nobody closed so here's

one of the situations where you have to be careful suppose I say this system is

the gas that was initially in the cylinder and some of the gas escapes

open or closed you still think is open huh

now he thinks it's closed why the system

I said the way I Ward that it is the gas that was initially in the cylinder it

doesn't matter where the gas goes after if well the piston could be moving up

and down so the volume question the volume is not that important for this particular discussion but if the system

is all of the gas that is initially in the cylinder it doesn't matter if it escapes

I see the escapes is still part of my system it just happens to be a very complicated system now because where do

I put the boundary if some of the gas has escaped so I could treat this either

way I could say that the system is the gas that is in the cylinder at any time

which case I might treat it as an open system with some of the mass crossing

the boundary or I can continue to think of the entire gas regardless of what it

is ask my system and that's then a closed system some of those distinctions

become important when you're solving problems when you have to make that decision this of course is a very simple

problem but in some problems you'll have to make that decision as to what exactly is your system so you will see how

that's the thing you have to do when you're solving a thermodynamic problem is decide what your system is well if I continue to say

that the boundary is the same boundary that we had at the beginning so if I say the boundary of the system is the inner

line the inner lining of that cylinder and the bottom of the piston that defines the boundary whatever is inside

of it is in my system that's an open system now right because there is gas crossing through whatever it's leaking

any other questions okay let's see how

are we doing here in time let me make a

quick distinction between two ways of looking at thermodynamics there are

generally two ways to study thermodynamics one is called classical

thermodynamics and the other one is called statistical thermodynamics the

one we're looking at is classical so we're looking at what we know as classical thermodynamics the main

difference is this in classical

thermodynamics we are taking what we call a macroscopic view of our systems

whereas in statistical thermodynamics we're talking about a microscopic view

so this is a very simple distinction but

it's one that I hope will be obvious to you so in the macroscopic approach to

thermodynamics we are measuring and working with quantities or properties

such as temperatures and pressures so we say we have in the previous schematic we

have the gas in the cylinder I could certainly measure its temperature I could certainly measure its pressure and I'll be here on my left

side in my macroscopic view two

properties that I could measure now in reality where are those properties that I measure in the macroscopic approach

where are they coming from the you have the answer on the right hand side is there really atoms and molecules that

are moving around right what is pressure if I measure the pressure say that this

the air in this room is exerting on that wall where is that pressure coming from

molecules of the air that are hitting that wall when I'm looking at

macroscopic thermodynamics I am NOT concerned with the details of what's going on with those molecules I just

take a measurement of pressure and that's what I need or I make a measurement of temperature and that's what I need we know of course that in

the in the in the microscopic approach those are due to atomic and molecular

and in the case of solids lattices that are vibrating and so on that are

producing those quantities so that's why I wrote here at the bottom this

quantities on our macroscopic world are really measurable results in the

macroscopic world of what's happening at the level of atoms and molecules a huge

term for us it has already come up a couple of times is energy we talked in

one of the earlier slides about forms of energy chemical energy make

chemical energy and so on as we start going into the details we think of

energy si total energy could have the sum of all the possible energy modes in

my macroscopic approach will you say the symbol capital letter e to denote that

energy that energy would really be made up of different types of energy so when

I say this gas in the previous example

if I say this guy's has energy I can break down that energy into different

types of energies so by far the most important energy for us will be

something that we call internal energy internal energy is in fact associated

with those microscopic properties of the system that I mentioned up here which

which we measure in our macroscopic view by means of say pressure and temperature

so a certain pressure and a certain temperature will be the reflection of

molecular activity and that represents a certain amount of energy that exists

within my system as I said by far that will be the most important form of

energy for us the so called internal energy associated with the atomic

molecular energies of the system but we could have other forms of energy so I

wrote here a couple that are common in thermodynamics kinetic energy and potential energy and you're very

familiar with those so we don't need to spend a lot of time talking about those kinetic energies of course the motion

associated with the motion with the velocity that the system has you know in

this picture again it looks like the gases nicely sitting there doesn't seem

to be moving but if I start moving the piston up and down this gas is going to

move it's going to acquire some velocity that I can measure in the macroscopic

world and of course that velocity will be associated with some kinetic and this potential energy that you see

here is actually gravitational potential energy so the fact that if I if I take

my cylinder and I move it up somewhere higher in the lab I take it from a lower

shell to a higher shell it will have acquired some sort of gravitational potential energy because of the fact

that I moved it moved it up with respect to a a gravitational energy system okay

any questions okay let's keep marching along next concept with our

thermodynamic properties what do you

think our thermodynamic properties give me one example anything getting help

from here again fancy entropy right entropy what else

pressure what else have I mentioned a moment ago energy mass temperature yeah

what are they how would you define them then what is another word that you could use for property in this case then there

would mean more or less the same thing no very good characteristics that help

describe the state of a system so is a characteristic that help us describe the

state of a system and I underlined the word state because of course that is another concept that we need to learn

and use so a characteristic that describes the state of a system is a thermodynamic

property when I say stayed here I mean a

thermodynamic state so I underline state thermodynamic state is a

specific condition a system is in as the Thurmond might properties to a kind of a

circle definition right so the properties are the set of

characteristics that define the state and the state is the condition the system is in as the term in why's

properties in other words this is saying that if I have a certain certain values

for the properties those values of those properties will determine the state of

that system and a question that will be

important for us later on is well how many properties do I need to know to

determine the state of a system how many

do you think how many properties think of the gas in the cylinder again how

many properties would we need to know to uniquely determine the state of that gas

of that system that's a question whose answer you shouldn't know except for the

fact that you may have seen some thermodynamics in physics or somewhere

else any guess two three four I won't

answer that question today but we'll get to that this is similar to saying well

how many four how many properties do I need to define uniquely an individual in

this class this class is pretty large so if we just say for example well first

name right and I just say Mike how many Mike's one two or three so it's not

enough right it's not enough so I would have to throw in a last name most likely if I put a last name

most likely we'll be okay probably there'll be a unique person for that first name and

that last name because this is a large group but it's not a group that is in

the thousands if we were in the thousands that might not be enough they

might need middle name come I need three so think about that in terms of the

thermodynamic system how many do we need and we'll talk about that later when we come back to this topic for now just

know what properties and where the thermodynamic state is here's another very important one the concept of

equilibrium now maybe I'll end it there

after this after this so equilibrium it says there a condition of balance

characterized by the absence of driving potentials that's kind of a what you

might say well what does that mean so how would you define equilibrium with your own words

let's try somebody on this side yeah equilibrium it's still in a

thermodynamic sense but with your own words external forces equal internal

forces now give you some points for that but anybody else you can derive it from

this definition yes yeah but now you're getting too fancy

how about very plain now you're getting fancy using symbols and equations plain

work for somebody who is not an engineer not in thermodynamics you want explain it to a friend who is not coming to

school or nothing is happening right

nothing is happening that's probably the simplest definition nothing is happening

that's the queerly room so if nothing is happening that's equilibrium of course what she

said is true the is nothing forcing something to change there's kind of the same thing right

nothing forces something to change then nothing happens give me an example of

something that would break away from equilibrium in the properties that we have been talking about the properties

that we have discussed today people have thrown properties around give me an example of something having to do with a

property that would make something happen temperature in what sense no not exactly

I hold on hold on something with temperature what do I need to do with

temperatures for something to happen make them different right how the

temperatures say on this side of the room higher than the temperature in that side of the room what would happen heat

will move from this side to that side right so that's an example we'll pick it up on Thursday