Engineering MAE 91. Intro to Thermodynamics. Lecture 01.

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

Channel: UCI Open

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Keywords: UCI, UC Irvine, OCW, OpenCourseWare, Engineering, Mechanical, Aerospace, Thermodynamics, Course Information, Energy Transformation, Conservation of Energy, Power Plant system, Gas Turbine Engine, Refrigeration, closed system, open system, control volume, Isolated System, statistical thermodynamics, equilibrium

Id: rvZZYeouz_I

Length: 71min 7sec (4267 seconds)

Published: Wed Apr 03 2013