1 - 5 - 01.05 - Thermodynamic Properties (09-04)

7/18/2019 1 - 5 - 01.05 - Thermodynamic Properties (09-04)

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[BLANK_AUDIO]Welcome back.So we were considering our coffee cupsystem, and whether or not it shouldbe treated as open or closed, or a controlmass, or a control volume.So recall a control mass is one with afixed mass, or closed system,and a control volume is an open system,where mass can cross the system boundary.So I said, well I want you to consider thecoffee in the mug.So the answer is a little bit tricky inthat,okay what do we mean by the coffee in themug?Are we pouring the coffee into the mug?Then I'd argue that's best treated as anopen system.If we're looking at maybe, is there anyevaporation of the coffee from the mug?Yeah, again that would be considered anopen system.On the other hand, if we don't think

there's going to be any evaporation, andall we'relooking at is let's say the coffee coolingin the mug, that would be a closed system.So it's kind of,it requires a little bit of thoughtsometimes.I mean that's a little bit of a funexample, butsome systems it may be just like that, alittle bit tricky.And we'll cover some of those trickierexamples as we go through the class.

The second system is pretty straightforward.So we want to know is the microcheck,microchip inyour computer best treated as an open orclosed system?Well, if the system is just the CPU,then we only want to, then it is a closedsystem.But if we're considering air around theCPU, thenobviously the air has got some movementassociated with it.

And we'll use this as an example a littlelater in the class too.But, as I think I've mentioned already,themicrochips that we use, themicrocomputers, the brainsthat are in our laptops in particular, ha,have to dissapate quite a bit of heat.And that puts an incredibledemand on the heat removal from that



system, the heat transfer from the system.So we can only generate as much power asthe waste heat can be removed from themicrochip.So we'll discuss the cooling of themicrochip inan example a little later on in thisclass.Okay, back to our definitions.So again, we need the vocabulary beforewe can actually start the quantitativeanalysis.So properties are the characteristics of asystem.And specifically we care about thethermodynamic properties.And those describe the state of thesystem, or the thermal properties of thesystem.So the state is the condition of thesystemas described by the thermodynamicvariables, so the thermodynamicproperties.

Steady state means we have a system wherethose properties are invarying [SIC].And that's important for us, because inthermodynamics, we are going to, inorder for us to define a state, theproperties have to be invarying.Okay.So, what types of properties are there?There are extensive properties, and thoseare properties which dependon the extent, or the amount, of thematerial that'spresent in the system.

These properties are additive.And the best and easiest example is mass.So, if I have so much material in thesystem, ifI have twice as much material, I havetwice as much mass.If I have, volume is another good exampleof an extensive property.If I have twice as much mass in them asto, here in the system, I have twice asmuchvolume in the system.Intensive properties do not depend on the

amount of material present.These are properties which are notadditive.Things like pressure or temperature, whichare not additive.If I have my brick, and then I haveanother brick at the same temperature,the mass of the system, let's say is twiceas much.Let's say they're two bricks of the



same size, but the temperature of thebricks are the same.So temperature's not an additive property.It's an intensive property.Equilibrium, which I've already mentioneda little bit once, is when asystem is unchanging in terms ofits thermal, mechanical, phase, andchemical characteristics.Some of that gets into a lot ofdetail that we're not going to cover inthis class.But the thermal equilibrium meansessentially thereare no temperature gradients in thesystem.Mechanical equilibrium is in terms of theforce balance in the system.That the system is in equilibrated, orbalanced,in terms of the forces on the system.Phase is with respect to phase change.So solid, liquid, vapor, things like that.Chemical means there's no spontaneous

chemicalreaction that's occurring in the system.It's chemically equilibrated.There's no change in the composition.And a process,is the path that connects betweendifferent states, between two states inparticular.So process is the path that's followed toconnect onestate of the system to another state inthe system.So some of these may be a little bit odd

right now, againI'd encourage you to use yourreading material to help support yourunderstanding.But it'll make a lot more sense when westart doing some examples.We can say this is a system,this is the state, this is the path theprocess that the system undergoes.So let's move on.Let's start talking about some specificthermodynamic properties.Well we've already mentioned temperature.

We can tell that's going to be veryimportant, because we know it drives heattransfer.It's used to define the state, or it is apropertythat we can use to define the state of asystem.But there are some otherproperties that I expect you are quitefamiliar with already.



Density, which is the mass per unitvolume.Specific volume, which is the inverse ofthe density, so it's volume per unit mass.Pressure, and there's both absolutepressure, and relative pressure.And again, good reading material in thereferences if youwant to learn more about the differencesbetween those and temperature.Those are four intensive thermodynamicproperties.So let's talk about temperature a littlebit more.Now pressure, we already talked about.You know, all the you, issues associatedwiththe many different units that there arefor pressure.So be aware, there are many units, andyou're going to have tobe able to move between the differentcon-, units, using conversion factors.temperatures, there really only four units

for temperature.And those are on a relative scale and anabsolute scale.And inthermodynamics, we only want to use theabsolute scale.You can use, there are times where youcan use the relative scale, and you'll befine.But if you always use the absolute scale,you'llalways be using the correct analysis, thecorrect tools.

So I strongly encourage you to alwaysconvert into absolute units.And in British and SI, there are twodifferentabsolute scales.And there's a conversion factor we'll gothrough right now between the two scales.So, the SI Relative scale, the SI unitsare Celsius.The absolute scale, are units of Kelvin,and we can convert betweenthe two just using this expression.[SOUND] So we just

take the temperature in Kel, in Celsiusunits, add273.15, and that's going to give us unitsof Kelvin.So this again, is the conversion from therelativescale to the absolute scale for the SIsystem.And if we're looking at the Britishsystem,



we know the relative scale is degreesFahrenheit,and the absolute scale are degreesRankine.And we don't often use the British units.We're trying to standardize towards SIunits around the world.So I'd strongly encourage you to converteverything into the SI standard.And the conversion between Fahrenheit andKelvin is just a multiplicationfactor of 1.8.Okay.So, these are just a few of the propertiesthat we're going to use in this class.These are all intensive properties.In other words, each one of theseproperties is independentof the, of the amount of mass present inthe system.We'll build additional thermodynamicproperties into our skill set aswe move along, but let's start with thesefor right now.

Okay so I know you're familiar with theseproperties, and you've seen them before.So, temperature and pressure we alreadydiscussed and we talked all about theirunits, so what I want to do in thisquestion to answer more specifically.What are the units of density, and whatare the units of specific volume?And I want you to consider those units,and then in the secondquestion here, I want you to identify arethese properties intensive or extensive?I've already told you.

But I want you to consider why thepressure and the specific volume areintensive properties.And we'll cover that next time.Thank you.

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1 - 5 - 01.05 - Thermodynamic Properties (09-04)