Mechanical Energy and Thermal Energy (1959)

[Music] it's easy enough to see that the kinetic energy of the car near the bottom of its motion is being converted on the way up into potential energy and is all potential energy at the top of the path the potential energy now converts back into kinetic energy and at the bottom there it's all energy of motion the same kind of thing is happening here watch it in slow motion kinetic energy the energy of motion now becomes the energy of separation between the ball and the Earth and down at the bottom it all goes into potential energy of compression now all along all the time the ball is losing energy what about the principle of conservation of energy you know it loses some by friction with the air but it also loses some when it hits the glass plate it's a lot easier to photograph the compression of a rubber ball than a ball bear but the idea is the same as you can see the energy loss at impact is important but it's easier to discuss the uh energy loss due to friction with the air and once you've understood that you'll understand the effect at the solid surface too let's go take a look at a model of a gas we've got over here first Style drop in uh one molecule and now I'll add another and now another and even with only three you see how ridiculous it would be to try to keep all these motions in mind even in slow motion even though it's easier to see you'd still have to be outrageously patient to keep track of all of these motions [Applause] but with enough molecules this motion this chaotic motion takes on a certain kind of order we'll use this U barrier for a piston the effect of the impacts of many molecules tends to average out and we see a smoothing effect now in a real gas we substitute average quantities for instance for the uh bouncing of the molecules against the wall for the collisions that impact against the wall we take an average quantity called the pressure the force per unit of area you read it on a pressure gauge like this and for the motion of the molecules inside the gas what we do is to substitute for their average kinetic energy of motion average kinetic energy of linear motion a simple quantity called the temperature you know what you read on a thermometer now what gives us the privilege of substituting a concept like temperature for the average kinetic energy of motion of the molecules temperature you've grown up with all your life and uh the kinetic energy of a molecule is something that uh maybe you just hear about the argument is really simple it goes something like this the kinetic theory of gases the theory says that the pressure in a gas is proportional to the average kinetic energy of the molecules the average kinetic energy of the linear motion of the molecules the text has it all worked out but uh let's write it this way one of these quantities MV uh comes from the momentum change at impact the molecule hits and it bounces back and the momentum changes of course proportional to the momentum of the molecule and the other V comes in because the faster the molecules are going the more molecules hit the wall that's easy enough and that's why we get the MV s now experiments say that the pressure in a gas is proportional to the absolute temperature the pressure is proportional to the absolute temperature this is a little more complicated than I make out here simply because we use this relation to define absolute temperature in order to clarify this notion let's go do some experiments over here we're going to observe the uh pressure of a gas as a function of the temperature here we have a container with uh two atmospheres of helium gas Andy would you take this we're going to immerse it first in boiling water then in melting ice then in dry ice and alcohol and finally in a uh bath of uh liquid nitrogen okay Andy can we start taking some readings 2.49 [Applause] 1.81 1.38 0.55 now let's take a look at the pressure readings they go all the way from above two atmospheres from 2 and 1/2 atmospheres all the way down to uh half an atmosphere at the temperature of liquid nitrogen so let's try the experiment again with another gas this time we'll pick oxygen okay Andy let's go 2.43 1.81 1.39 0.31 let's take a look at the oxygen data compare the two sets of readings remember we forced them to fit at room temperature at two atmosphere spes for both but at boiling water temperature and at melting ice and at the dry ice temperature they agree perfectly as well as we could read them but down here at the temperature of liquid nitrogen the oxygen reading is very different from the helium reading some of you may have uh guessed already what went wrong there let's take a look here uh here's some uh liquid nitrogen in a uh tin can and uh let me hold this so you can see you can see the oxygen from the air condensing on the outside and if you don't believe that's oxygen Just Watch What Happens so you see what happened in the experiment is that the oxygen partly liquefied partly liquefied in the bulb if we had taken a lot less gas for instance if we had put in a less than a tenth of an atmosphere in both bulbs then the readings would have agreed all the way down right down through the nitrogen Point what I'm trying to say is this that for gases that are um thin or tenuous like the gases in the upper atmosphere if the agre at one temperature if you take a certain amount of gas so that they agree at one temperature they will agree all the way up and down so we take this Universal Property of gases as a way of defining the temperature scale in order to clarify that let's go look at the graph right here let's plot uh the uh pressure against temperature but we've got to pick a temperature scale let's set the temperature of melting ice at 273° arbitral and we put our point there now what we do is to draw a straight line right through that point and the origin then we can put our other data on that gra and say by definition that the pressure is proportional to the temperature and that defines our temperature scale now if we want to read the temperature of boiling water say we put its pressure at 248 right there and uh read its temperature it comes out of course 373 de that gives us 100° between the temperature of melting ice and the temperature of boiling water and that's exactly why we chose the 273 for the ice point so we'd get that 100° difference between those two temperatures now if we want to find the temperature of U the uh carbon dioxide slush we plot its uh pressure at 1.4 right there so it comes out 20 uh uh 20° absolute and the liquid nitrogen temperature the pressure from the helium graph of course we put right at .55 there so it is at a temperature of about 80° absolute now let's plot the oxygen Point here that. 29 falls off the graph you knew it would and you know why we started with too much gas in both of the the bulbs tooo much helium too much oxygen they weren't tenuous enough for them to agree all the way up and down let's go back to the logic uh what we said was simply this from the theory from the kinetic theory of gases the pressure is proportional to the average kinetic energy of linear motion of the molecules and we've just discussed that the pressure in a gas is proportional to the absolute temperature we simply identify one with the other and say that t the temperature is proportional to the average kinetic energy of motion of the molecules we simply measure the average kinetic energy of translation of the molecules by observing the absolute temperature when a gas is hot the molecules are moving fast when it's cold the molecules are moving slowly but all of that energy all of that energy is chaotic it's random now in a mixture of gases with heavy molecules and light molecules the heavy ones are moving relatively slowly and the light ones are moving fast to keep their kinetic energy up even in the extreme case of these particles of smoke in this Brownie and motion experiment the average kinetic energy of the smoke particles is the same as the average kinetic energy of the molecules of the air even though these smoke particles contain millions of atoms okay let's get back to our original question what happened to the bouncing ball when it lost energy to the gas in order to show what we think happened we'll have a model here's a frictionless disc which will let slide down an incline gaining kinetic energy as it goes until it hits now what would happen to that if it went through some gas the best way to show that is to bring on some gas [Applause] and the disc comes to a browni and motion [Applause] hle let's try it again the orderly kinetic energy of the disc is transferred into disorderly random motion of what we have called the molecules that were hitting it it may not be perfectly apparent but every time a molecule hit that disc some of the energy of the disc was transferred over to it in that way the dis lost its energy to that gas of molecules in exactly the same way when this falling ball that we were talking about falls through the air its energy is transferred into the random energy of motion of the gas random energy of motion of the air and when that ball hit the table H the plate then the same sort of thing happened the orderly energy that you could see both potential energy and kinetic energy were converted over into energy of motion of the molecules of the plate and of the ball and of some of the air so we have to ask the question is the energy concerned it certainly is then where does it go at least you know what it doesn't do it doesn't just go backwards like this what usually happens is that we get what we call thermal conduction heat flow energy flows from a hot body to a cold one let's go take a look at a model of that over here here is our old friend the marble machine let me turn it on first [Applause] the marbles in the lower compartment have no effect on the ones in the upper compartment they are separated by two bars with an insulating airspace between them now suppose we take away that insulating barrier it's a little clumsy but let me try it right just like that with the insulating barrier removed the marbles in the lower compartment are now able to transfer their random thermal energy to the marbles in the upper compartment and that's exactly what happens when we heat something with a flame here's a flame here's something bulb of gas and the molecules in the flame that are moving very rapidly transfer their kinetic energy first to the bulb and then to uh the gas now some of the energy of the flame goes into the energy of the gas and the temperature goes up and the pressure goes up now what do we mean by the principle of conservation of energy energy appears in various forms as kinetic energy and potential energy and uh sometimes it's orderly and sometimes it's random and when it's random enough we call it thermal energy some people call it heat what we mean by the principle of conservation of energy then is simply this we believe that energy is never created never destroyed we may convert it from one form to another from orderly motion to random motion we may Fritter It Away by warming up the air by warming up the ocean but it's still somewhere now there's one more thing we've got to discuss before we finish now when we transport energy we might transport the whole material body complete with the energy in all like uh throwing hot rivets or carrying coal or we might have energy transported by molecular bombardment we've just been discussing that we call it heat conduction or energy might be transported by radiation usually this takes the form of electromagnetic radiation say infrared light or visible light in fact in fact most of our energy comes to us that way it comes to us from a star our own personal star called the sun in the Sun the energy is stored by the atomic nuclei and this energy is converted into random thermal energy in the Sun and that random thermal energy into radiation and the radiation comes through space and some of it is captured here on the Earth and some of that by the process of photosynthesis is converted into trees and the trees were converted into coal or oil and some of that fuel was converted into hot steam or something that turns a turbine or runs a motor and we get orderly mechanical energy out of that but regarding the conversion of thermal energy into mechanical energy and then mechanical energy back into thermal more of that in the next film for