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Video transcript

- [Tutor] Let's talk about one of the most fundamental ideas in science and that is the notion of energy and energy definitely has some meaning in our everyday life, if we kind of imagine things that are moving or hot or bright as being energetic, but what I wanna talk about in this video is a more formal definition of it, a more scientific definition and the most typical one, that's often given is the ability to do work, ability to do and I'm gonna put work in quotes, because the notion of work here isn't the everyday notion of work, where you go to your job and you work nine to five and you get paid, work in a physics context is a little bit, it's not completely unrelated to our everyday notion of work, but I'll give you an example just to get a better idea of it, so let's say that you have some object here and you were to apply a force, you were to apply a force in that direction and the magnitude of that force, let's say it's 10 newtons and if the units, newtons in force isn't too familiar to you, don't worry too much, but you can also review it on those videos on Khan Academy, but you apply a force to the right on this object and by doing that you're able to move the object, you're able to displace the object in the same direction as that force, so you're able to displace it, let's say 10 meters, so after you've done it, the object, the object is right over here, so when you do this, you apply a force and it's causing the object to be displaced in the direction of that force, you would say that work has been performed and the amount of work that has been performed would be 10 newtons times 10 meters and so 10 times 10, it would be equal to 100 and then the units are newton meters of work, 100 newton meters, 'cause you're multiplying newtons times meters of newton meters of work and newton meters, that has been defined as the joule, which is the unit of work and also the unit of energy, so this is the same thing as 100, I could write it out as joules, 100 joules or we could just abbreviate it with a J, so 100 joules of work has been performed here by moving this, so we've done something here and this is considered to be work, 100 joules of work, if we move this twice as far, then it would be 200 joules of work and so energy is the ability to do this type of work. Now let's look at these pictures here, which are depicting different forms of energy and let's see if we can identify the forms of energy and then think about how they can relate to actually doing work. So if we look at the fire here, there's some maybe obvious forms of energy, we have some thermal energy, fires are definitely hot, so thermal energy, but we should think about what is thermal energy fundamentally? A system's temperature is really about the average kinetic energy of its molecules, so thermal energy is really about the energy of movement, it's really about all these little molecules here, because of the combustion reaction going on, they're getting excited and they have higher kinetic energy and so the temperature goes up, their average kinetic energy goes up, so thermal energy is really a form of energy due to movement and the general term for energy due to movement is kinetic energy, so thermal energy is really a form of kinetic, kinetic, kinetic energy. You also have light being emitted, that has energy as well, we call that radiant energy, so that light being emitted, that's the reason why we can see this fire, radiant, radiant energy. Now you might say, "Okay, maybe that's all of the energy in the system," but I'll say no, there's another form of energy and actually even in this picture, that's probably where most of the energy is and that's potential energy, so where is the potential energy? Well, it's sitting in the bonds of the fuel over here, so these are either chips of wood or charcoal of some kind, but these are formed by carbon-carbon bonds, so you have these carbon-carbon bonds and they could be bonded to other carbons or other things and they're also going to be bound to some hydrogens here and there, so you're gonna have, you're gonna have bonds like this, that actually store energy in them, they have the potential to be released, if you're able to break these bonds, those electrons are gonna get into a lower energy state, or they might bond with other things and in the process, they're going to release energy, that's gonna be radiant energy and thermal/kinetic energy, so how does this happen? How do these bonds actually get broken? Well, that's our good old friend, the combustion reaction, that's our good old friend, the combustion reaction, where you take some oxygen, you take some heat or we could say some energy, so it takes a little energy to get started, that's why you might have to light this with a match to begin with, so oxygen plus energy, plus energy and then you could say plus these carbon-carbon bonds and you could say plus you know, whatever it is, these fuels, which are made out of carbon, either charcoal or wood, so plus, I'll do it like this, I'll draw some carbon-carbon bond right over here, that's going to combust and I'll do this in a color, so that is going to, I'm really having trouble changing colors, this is going to combust, combust and it's going to release, it's going to release water, because the fuel has hydrogens in it, it's going to release carbon dioxide and it's going to release a lot more energy, I'll do that in caps, it's going to release a lot more energy and that energy we see in the form of the kinetic energy of the molecules and the radiant energy being emitted. Now you might say, "Okay, I can buy that, "I have this potential energy here," and this potential energy that's in the bonds between these atoms, we call that chemical potential, so we have chemical, chemical potential energy, potential, potential energy is right over there, but you might say, "Okay, I buy that, "the chemical energy is being converted "into the thermal energy and the radiant energy," and this is actually an interesting point, energy, this is the law of conservation of energy, energy cannot be created or destroyed, it can only be converted from one form or another, but you might be saying, "Okay, I can convert from one form to another, "but how can this actually do work "in the way that I've even depicted here?" Well, the entire Industrial Revolution is all about trying to convert from one form of energy to another and also to do work, so a steam engine is fundamentally based upon combustion, heating up some steam and then that steam can expand and then it can push a piston to do all sorts of things, including move a train, combustion is what's going on in your car engines, where the pistons are expanding due to the thermal energy and then that helps drive the drive train of the actual car, so it can clearly do work. So here we have some other examples, this is lightning and so when you see the lightning, there's something clearly very kinetic is going on, you have electrons, you have electrons moving from the cloud, from the cloud to the ground and you might say, so this right over here, that is, you could say that's kinetic energy, kinetic, kinetic energy and you might say, "Well, how can I do work with that?" Well, that's what the whole electronics industry is all about, that's what power lines are all about, movement of electrons, that's current and current can be used to do all sorts of amazing things, you can actually have an electric motor is one way to actually do it, so that's kinetic energy there, there's clearly radiant energy going on, we can see the lightning and that radiant energies do, because the air gets ionized and gets heated and so there's also thermal energy, as the electrons go down, there's heat that is actually being generated. Now, where did this energy come from? It just doesn't come from anywhere, well, you have all of this potential energy, that starts building up in these clouds as the water vapor rises and the mechanism isn't fully understood of how this happens, but because of energy from the sun, you have water vapor rising, as the water vapor rises through the clouds, the bottom part of the cloud becomes more negative, it becomes more electron rich and the top part of the cloud become more positive and so you have these electrons, that really wanna get down here onto the ground, because the air above the ground becomes more negative, the ground starts becoming more positive and so you can imagine these electrons, more and more wanna get down here, but this air isn't a natural conductor, but once the electric potential gets high enough, these electrons find a way, the air essentially gets ionized and the electrons are able to find a path, so while this is all building up, you have this electrostatic potential building, so this is electrostatic, static, you can't see it that well, electrostatic potential and how this forms, once again, it's an area that people are still, there's some good theories out there about how this forms, but it's not 100% well established and over here in this third old drawing of this person doing a handstand dive, this is probably the most typical example of potential energy being converted into kinetic energy that you might find in a physics textbook. Over here at the top of the diving board, this gentleman has potential energy by virtue of his position and over here, it's very clear he has the potential to fall and it has the potential to turn it into kinetic energy and so once he falls over, at this point, most of his potential energy has been converted into kinetic energy, so here it's potential, here it's potential and here it is kinetic, kinetic energy. So the big takeaway is energy has the ability to do work, it cannot be created or destroyed, but it can be converted from one form to another and all of the forms at their essence, you can really think about them in two big buckets, you can think about them as potential energy or kinetic energy, or kinetic energy and as a last example, you might say, "Well, how can this guy do work?" Well, you can imagine if there was some type of system here, new system here, where, you know, we'll create some machinery, maybe there's a pulley right over here and then it's lifting a weight right over here, well, if he jumped on this, he won't fall down as fast, but then if, as long as he's heavier than this weight, it's gonna pull this down and then this weight is gonna go up, so he has the potential to do work by virtue of his position, there just isn't this pulley system there to get that actual work done.