Introduction to energy (video) | Khan Academy (2024)

Video transcript

- [Tutor] Let's talk about oneof the most fundamental ideas in science and that isthe notion of energy and energy definitely has somemeaning in our everyday life, if we kind of imaginethings 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 definitionand the most typical one, that's often given isthe ability to do work, ability to do and I'mgonna put work in quotes, because the notion of work here isn't the everyday notion ofwork, where you go to your job and you work nine tofive 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 youhave some object here and you were to apply a force, you were to apply aforce in that direction and the magnitude of thatforce, let's say it's 10 newtons and if the units, newtons in force isn't too familiar toyou, don't worry too much, but you can also review it onthose videos on Khan Academy, but you apply a force tothe right on this object and by doing that you'reable to move the object, you're able to displace the object in the same direction as that force, so you're able to displaceit, 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 theobject to be displaced in the direction of that force, you would say that work has been performed and the amount of workthat has been performed would be 10 newtons times 10 meters and so 10 times 10, itwould be equal to 100 and then the units arenewton meters of work, 100 newton meters, 'cause you're multiplyingnewtons times meters of newton meters ofwork and newton meters, that has been defined as the joule, which is the unit of workand 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 workhas been performed here by moving this, so we'vedone something here and this is considered tobe 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 abilityto do this type of work. Now let's look at these pictures here, which are depictingdifferent forms of energy and let's see if we canidentify 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 maybeobvious forms of energy, we have some thermal energy, fires are definitelyhot, so thermal energy, but we should think about what is thermal energy fundamentally? A system's temperature is really about the average kineticenergy of its molecules, so thermal energy is reallyabout the energy of movement, it's really about all theselittle molecules here, because of the combustionreaction going on, they're getting excited andthey have higher kinetic energy and so the temperature goes up, their average kinetic energy goes up, so thermal energy is really aform of energy due to movement and the general term forenergy due to movement is kinetic energy, so thermalenergy is really a form of kinetic, kinetic, kinetic energy. You also have light being emitted, that has energy as well, wecall 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 ofthe energy in the system," but I'll say no, there'sanother form of energy and actually even in this picture, that's probably wheremost of the energy is and that's potential energy, so where is the potential energy? Well, it's sitting in thebonds of the fuel over here, so these are either chips ofwood or charcoal of some kind, but these are formedby carbon-carbon bonds, so you have these carbon-carbon bonds and they could be bonded to other carbons or other things and they'realso going to be bound to some hydrogens here and there, so you're gonna have, you'regonna 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 getinto a lower energy state, or they might bond with otherthings and in the process, they're going to release energy, that's gonna be radiant energyand thermal/kinetic energy, so how does this happen? How do these bonds actually get broken? Well, that's our good oldfriend, the combustion reaction, that's our good old friend,the combustion reaction, where you take some oxygen, you take some heat or wecould say some energy, so it takes a littleenergy 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 plusthese carbon-carbon bonds and you could say plusyou know, whatever it is, these fuels, which are made out of carbon, either charcoal or wood, soplus, I'll do it like this, I'll draw some carbon-carbonbond right over here, that's going to combust andI'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 fuelhas hydrogens in it, it's going to release carbon dioxide and it's going to releasea 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 youmight say, "Okay, I buy that, "the chemical energy is being converted "into the thermal energyand the radiant energy," and this is actually an interesting point, energy, this is the lawof conservation of energy, energy cannot be created or destroyed, it can only be convertedfrom one form or another, but you might be saying, "Okay, I can convertfrom 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 convertfrom one form of energy to another and also to do work, so a steam engine is fundamentallybased upon combustion, heating up some steam andthen that steam can expand and then it can push a pistonto do all sorts of things, including move a train, combustion is what's goingon in your car engines, where the pistons are expandingdue to the thermal energy and then that helps drive the drive train of the actual car, soit can clearly do work. So here we have some otherexamples, this is lightning and so when you see the lightning, there's something clearlyvery kinetic is going on, you have electrons, you have electrons moving from the cloud, fromthe cloud to the ground and you might say, sothis right over here, that is, you could saythat's kinetic energy, kinetic, kinetic energy and you might say, "Well, how can I do work with that?" Well, that's what thewhole electronics industry is all about, that's whatpower lines are all about, movement of electrons, that's current and current can be used to doall 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 andthat radiant energies do, because the air getsionized and gets heated and so there's also thermal energy, as the electrons go down, there's heat that isactually being generated. Now, where did this energy come from? It just doesn't come from anywhere, well, you have all ofthis potential energy, that starts building up in these clouds as the water vapor rises and the mechanism isn't fully understoodof how this happens, but because of energy from the sun, you have water vapor rising, as the water vapor risesthrough the clouds, the bottom part of thecloud becomes more negative, it becomes more electron rich and the top part of thecloud become more positive and so you have these electrons, that really wanna getdown here onto the ground, because the air above theground 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 electricpotential gets high enough, these electrons find a way, theair essentially gets ionized and the electrons are able to find a path, so while this is all building up, you have this electrostaticpotential building, so this is electrostatic, static, you can't see it that well, electrostatic potentialand how this forms, once again, it's an areathat people are still, there's some good theories outthere 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 beingconverted 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 hasthe potential to fall and it has the potential toturn 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 energyhas the ability to do work, it cannot be created or destroyed, but it can be convertedfrom one form to another and all of the forms at their essence, you can really think aboutthem in two big buckets, you can think aboutthem 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 therewas 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 aweight right over here, well, if he jumped on this,he won't fall down as fast, but then if, as long as he'sheavier 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.