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Thermal conduction, convection, and radiation

Video transcript

when we observe what we call fire we have this combustion reaction going on and then we see these these flames what we're really observing are the three forms of thermal energy transfer we're observing conduction conduction convection convection convection and thermal radiation all at the same time so this is thermal thermal radiation thermal radiation and I could say thermal conduction thermal convection and thermal radiation the word thermal is just relating to things dealing with temperature so what is the conduction going on well what we have going on in the in the fuel for our fire so down here the fuel for our fire we have our classic combustion reaction I encourage you to watch the video on that in our in our chemistry playlist or chemistry section if you're interested in it but you're taking you're taking carbon molecules and and they're bonds and then in the presence of oxygen and some heat you have a combustion reaction producing carbon dioxide and producing water and even more energy than you put into it so it's producing a lot of energy and that energy is going to excite the molecules that around it so you have you have these molecules you have these molecules that end up getting a lot of kinetic energy and remember temperature is proportional to average kinetic energy and so the ones that just combusted or the the molecules right near the reaction or maybe the ones that were part of the reaction they're going to have this really high they're going to have this really high average kinetic energy because all of this energy was released from that combustion reaction and then they're going to bump into other molecules that might not have quite as high kinetic energy but then they're going to transfer that kinetic energy through these collisions and we have a whole video on thermal conduction but that's what's happening things with higher kinetic energy are bumping into things with lower kinetic energy and transferring some of that kinetic energy and they're transferring some of that momentum so that is definitely happening when we look at a fire like this neighboring molecules are bumping into other molecules and transferring energy now convection this is around the idea that that hot air is less dense and it's a lot so if we have a bunch of air particles here so let's say this is the cool air up here and it makes sense why cool air is going to be more dense than hot air because hot air these these particles have a much higher kinetic energy and so they're going to bump into each other and they're going to push each other much further away and and and and and get more separation from each other because of that high average kinetic energy but because the hot area is less dense this area right over here is less dense less dense then the white area these might all be the same molecule I just made them in different colors to show the less dense area in the more dense area it is going to rise it is going to rise or you could even think of it is the more dense area the more dense area is going to fall around it is going to fall around it or try to go under it because it is more dense and then the less dense area is going to rise and by doing that you have the hotter the the hotter molecules are moving upwards and then the cold molecules can go down too may be the source of heat the source of heat the source of energy right over here to get heated up more so this right over here so hot air rises let me write this the hot hot air rises and one way to think about it makes a room for the cooled air come down and then get heated up again and that's what's going on in the fire here you see the combustion reaction combustion reaction occurs right over there let me just in a different color combustion reaction is occurring in the fuel that super hot air it's going to rise and that's why it looks like these flames are kind of they're moving upwards they're kind of flickering upwards and you'll also see and actually we're going to talk about this in thermal radiation is that they also change color as they move upwards but this whole this whole idea of fought a fire moving upwards in the hot air if you put your hand up here it's going to be much hotter than if you were to put your hand then if you were to put your hand say right here even though in theory this is closer to the flame than up here and that's because the hot air the hot air is rising and it's making room for cold air or cold cooler air I guess I could say come down here and be part and get heated up again and then it would rise up again and so this this convection this idea of the hot air rising or the cold air falling this is another form of heat transfer now the last form that we're observing when we talk when we're looking at fire is thermal radiation and this is all around the idea that if you have charged particles being accelerated they're going to release electromagnetic radiation and so you might say when we charged particles where where are those being accelerated here well we have these molecules that are constantly being accelerated as they bump into each other the ones going to transfer kinetic energy from let's say this one bumps into that it's going to accelerate that in a different direction and even this one might go into another direction and acceleration is a change in velocity so it could be the magnitude of velocity or your direction of velocity so as they're colliding you have all of these acceleration of charged particles so let me write this down you have acceleration acceleration of charged particles charged particles particles and then that releases that releases electromagnetic radiation that releases electromagnetic radiation I know what you're thinking wait wait hold on a second so like okay I can buy that we have we have molecules that are being accelerated but where is this where is this charged where you getting this charged particles business well we have to remember these molecules are made up of atoms and atoms are made up of charged particles are made up of protons and electrons and so as you accelerate these and the more that you accelerate these the more radiation you are going to release and you might say way okay you said I'm observing that in fire where am i observing radiation well just a faery fact that you can see the wire the light emitted from the fire that is electromagnetic radiation that is electromagnetic radiation it's just the electromagnetic radiation in the wavelengths that your eye considers to be visible lighter that your eye considers to be light even the particles up here so even the particles up here that are still quite hot they are also emitting electromagnetic radiation because they're all bumping into each other and their electrons and their protons are all getting accelerated in different ways they're also releasing electromagnetic radiation but it is at a slower wavelength than your eye is capable of perceiving as light if you had if you had an infrared camera you would see the flames being much larger you would see them go all the way all the way up here and if you look at the flame if you are look at the flame closely you would see down here right where the combustion reactions is happening the flame looks blue and that's because the blue light is higher energy light and that's because the particles are being accelerated more down here and then it goes from blue to kind of white to a yellow to a red or to an orange to a red and then it to your bit to your regular eyes it disappears but everything everything that has some temperature is releasing electromagnetic radiation and you're like okay well that's that's all fine I can see it but how is that a form of energy transfer well if you've ever sat next to a flame you will feel the heat in fact even if the air between you and the flame is cold you would still feel you would still feel like you're getting warm so if this is if this is a flame right over here so that is fire now let's say you have cold air cold air let's say you're at a campfire right over here maybe it's thirty this air right over here is 30 degrees if you are if you are standing right over here you would still feel heat you feel still feel like you're getting warmed up and that's because that electromagnetic radiation is being emitted from the air particles that we perceive as fire and then that can actually excite particles on your skin and it'll transfer energy to your skin and so you feel like you are actually getting warmed up I remember once I this is kind of a strange story but I was on I was on the highway and there was a car on fire and I was literally you know we drove to the to the Far Lane because it was on fire were three lanes away from it and it kind of exploded I I don't think anyone was in it hopefully no one was in it but I remember right when it exploded it was an intense immediate heat that we felt through the window of the car and that was electro magnetic radiation that was thermal radiation being released by these accelerated particles in the air around that explosion which we perceived as an explosion or fire but then it was warming up particles on my on my skin