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First Law of Thermodynamics introduction

The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. For example, kinetic energy may be converted into thermal energy, or potential energy may be converted into kinetic energy. Energy is never "lost"—it is transferred or converted in some way.

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  • primosaur ultimate style avatar for user nickeldime88
    Why can't energy be created or destroyed? Doesn't the sun create light energy or when we are coasting on a bicycle don't we create kinetic energy?
    (24 votes)
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    • starky ultimate style avatar for user kesamnro
      The sun doesn't create energy. It simply chemically changes hydrogen atoms into helium atoms through a process of Nuclear Fission. The byproduct of this reaction is a massive volume of light and heat energy.

      Bicycles don't create kinetic energy. We give the bike kinetic energy by pedaling the bike.
      (54 votes)
  • mr pants teal style avatar for user Nina
    energy can be destroyed or created.does it mean that "heat energy" can be transformed into a more useful type of energy that has the ability to do work? thus decreasing the level of entropy in the universe after all energy can be transformed from one form to another
    (25 votes)
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    • piceratops ultimate style avatar for user RowanH
      It depends on what you mean by 'heat energy'. If you mean there is a temperature difference between things, then yes, you can use that energy and convert it to a different form (this happens for example in a power station). However, when we talk about thermodynamics, 'heat' often describes energy lost to surroundings by increasing the random motions of molecules. Although energy can be converted from one form to another, it cannot be converted back and forth any way you want. For things to go forward, you need to have an increase of entropy in the universe (this is the second law of thermodynamics), and there is no way you could collect back all the heat energy that has been dissipated into lots of molecules. If you think about a fridge, you can decrease energy of molecules inside it, but at the cost of increasing entropy by increasing heat outside it.
      (10 votes)
  • starky tree style avatar for user sumayya wafapoor
    At , if the light heats up the glass bulb, then how is it intact? Shouldn't the glass overheat and explode?
    (7 votes)
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    • aqualine ultimate style avatar for user Yellow Dragon
      The light doesn't actually heat up anything too much, and since the glass is transparent, barely any heating will occur. The light will simply pass through the glass. If you were talking about the heat from the filament, it would disperse slowly through the collisions of molecules and get more and more dffused with each collision, so it wouldn't heat up the glass much. Even if it did, the heat would quickly disperse out through the glass, without heating it much, as Sal mentioned at .
      (3 votes)
  • aqualine ultimate style avatar for user Just Awesome99
    If energy can not be created or destroyed, what energy was there before the Big Bang?Energy has to be created,but only once.Because the energy has to be there for the big bang to happen, right?
    (7 votes)
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    • male robot hal style avatar for user Manan Shah
      Well if you agree that there was absolutely nothing before the Big Bang, we may well say that none of the laws of conservation were applicable (in this context the law of conservation of energy which says energy can neither be created nor be destroyed), hence we may say that matter could be formed and that's exactly where the matter that exploded during Big Bang came from.
      (5 votes)
  • starky sapling style avatar for user Ansel
    Once you start running there will be air friction towards you then why don't we feel hot while we run?
    (3 votes)
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    • leaf red style avatar for user Richard
      I mean, I don’t know about you, but I feel hot after running for a while. However, the hotness isn’t really due to air friction to a noticeable degree, but rather by the heat released by your body expending chemical energy to move. Air fraction, or drag, does cause an increase in temperature to an object in motion, but you have to be going fast, like jet plane fast, for the effect to be noticeable. This is why returning spacecraft reentering earth’s atmosphere have heat shields to protect from the air friction burning craft up.

      As mentioned in another reply, your body also sweats too to try to release that excess heat and help prevent your body from overheating. So there are heating and cooling effects at play when running.

      Hope that helps.
      (8 votes)
  • duskpin ultimate style avatar for user C4LOwenZ
    At , Sal said the 1st Law of Thermodyamics is that energy cannot be created or destroyed, it can only be converted. Isn't that the Law of Conservation of Energy? Are these two the same? What is going on here?
    (3 votes)
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  • starky sapling style avatar for user Aslan
    What kind of energy, does our body generate on a normal basis?
    Like, when we walk or jump.
    (3 votes)
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    • piceratops ultimate style avatar for user RowanH
      We are not really 'generating' energy, we are using energy (chemical energy stored in food) and converting it into another form. As Davin commented, we convert a lot of energy to heat. But since you ask about walking or jumping, if you are moving, you are converting energy to kinetic energy. And if you jump or climb up a hill, you need to use a source of energy to gain gravitational potential energy. Does this answer your question?
      And in case you are wondering where the energy from the food comes from, it all (indirectly), comes from the light energy from the sun. Plants capture this energy and use it to drive chemical reactions that make sugars from CO2 and water, producing sugars and oxygen. Normally that chemical reaction would go the other way, but it is the light energy that makes allows the plant to produce sugars. Then by 'burning' the sugars, we can release energy again, this time not as light, but in a way the lets our body do chemical reactions, and somewhere down the line this for example makes our muscles move. But we can't use all the energy in a productive way, some is always lost as heat.
      (6 votes)
  • starky sapling style avatar for user Akshat Khandelwal
    what is the difference between Radiant energy & Light energy??
    (2 votes)
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  • duskpin ultimate style avatar for user jasonmoses05
    So the First Law of Thermodynamics is the Law of Conservation of Energy?
    (3 votes)
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    • male robot hal style avatar for user Charles LaCour
      Essentially the First law of Thermodynamics and the Law of Energy Conservation imply the same thing but the First law of Thermodynamics is focused on thermodynamics and the transfer of heat energy where as Law of Energy Conservation is more generic in its scope.
      (5 votes)
  • blobby green style avatar for user Melinda
    is thermal energy and kinetic energy the same thing ?
    (3 votes)
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Video transcript

- [Voiceover] Let's now explore the first law of thermodynamics. And before even talking about the first law of thermodynamics, some of you might be saying, "Well, what are thermodynamics?" And you could tell from the roots of this word. You have thermo, related to thermal, it's dealing with temperature. And the dynamics, the properties of temperature, how do they move, how does temperature behave? And that's pretty much what thermodynamics is, it's about, it's the study of heat and temperature, and how it relates to energy and work, and how different forms of energy can be transferred from one form to another. And that's actually the heart of the first law of thermodynamics which we touched on on the introduction to energy video. And the first law of thermodynamics tell us that energy, this is an important one, I'm going to write it down, energy cannot be created or destroyed. Cannot be created, or destroyed. It can only be converted from one form to another. It can only only be converted only be converted, I'm having trouble writing today. Converted from one form, from one form, to another. Or you could transfer it but you're not going to, you're not going to create or destroy it. And the whole thing that I, the rest of this video I just want to really have you internalize that, and I want to look at a bunch of examples and think about, well, what is the energy that we're observing, or that we're seeing in a system? And then thinking about where is that energy coming from, to appreciate that it's not just coming out of nowhere, and that it's not just disappearing, it's not getting destroyed either. And so let's start with this example of a lightbulb. And I encourage you to pause this video, think about the forms of energy that we can see here, and then think about where is that energy coming from, and where is it going? Well, the most obvious form of energy that you see here, and this, the whole point of a lightbulb, is you see the radiant energy, you see the you see the electromagnetic waves, the light, being emitted from it. And that light, so this is radiant energy. Radiant energy. And that radiant energy, is due to the heat in the filament right over here, as the electrons go through it, it generates heat, so you have thermal energy. So you have thermal energy as well. Thermal energy. But where does this radiant and thermal energy come from? Again, first law of thermodynamics it tells us, it's not just being created out of thin air, it must be converted or being transferred from some place. Well, I just gave you a hint, this thermal energy is due to the electrons moving through the filament. They're moving through the filament which has some resistance, and that generates heat. So the electrons are moving through this, and as they move through that resistor, they generate heat. So you actually have the kinetic energy of the electrons. I'll just write KE for short, kinetic energy of the actual electrons. Well, where is that kinetic energy coming from? Well that's coming from the potential energy. You know maybe this thing is plugged into, is plugged into a socket of some kind. So let me draw a little electric socket right over here. And the electric socket, I'll draw, the electric socket if this is the electric socket in your home, there is an electrostatic potential between these two terminals. And so when you make a connection, the electrons are able to move. And we'll get into the details of AC and DC current in the future, but there's an electrostatic potential, from this point to this point if we assume that's the direction that the electrons are going in. And so that, it's that potential energy we convert to this kinetic energy of the electrons, which is really in the form of a current, and then that gets converted into thermal energy and radiant energy. Now what happens after, let's say you unplug the light, the light goes dark, what happened to all of that energy? Is it still there? Well yeah, that thermal energy is going to continue to dissipate through the system. And this right over here would be an open system, it's going to, the air inside the lightbulb, you can't fully see the lightbulb right here, but it looks something like this. That's going to heat up, but then it's going to heat up the glass surrounding the lightbulb, and that's going to heat up the surrounding air. So the thermal energy is going to be transferred, and that radiant energy is going to move outward. And it could be used, it could be converted into other forms of energy, most likely thermal energy, it is also probably going to heat up other things. Well, what about a pool table? When I hit a, if I hit a pool, a billiard ball or a pool ball right over here, well, where is that energy going? Well some of that energy might be going to go hit the next ball, which might go to hit the next ball. But as we all know, if we've ever played pool, at some point they're going to stop. So what happened to all of that energy? Well, while they were rolling, there was some air resistance, so they're bumping against these, the air molecules, and it's really friction due to air. And that energy is essentially going to be converted to heat. And one trend that you're going to see very frequently, is as systems progress, a lot more of the energy tends to turn into heat, rather than doing useful work. And so you're going to have, as the billiard balls move, there's the air, and so that's going to be, that's going to be converted, some of that kinetic energy is going to be turned into heat energy. You're also going to have friction with the actual felt on the table. And that friction, you're going to have molecules rubbing up against each other, that's also going to be converted into heat. And so that, because that kinetic energy gets sapped off, gets keeping sapped away from the friction, which is essentially converting the kinetic energy to heat energy, eventually you won't have any more kinetic energy. Now what about this weight lifter here? He's using the chemical energy in his, in the ATP in his muscles, that converts into kinetic energy that moves his muscles, that moves this weight, but once he's in this position, what happened to all of that energy? Well, a lot of that energy is now being stored in potential. it's the potential energy, he's got this big weight, he's got that big weight above his head, and if he were to just let go, that thing would fall, I wouldn't recommend he do that, but that thing would fall quite fast. And so now it's all, or a lot of it has been stored up in potential energy. But he would have also generated heat, his muscles would have generated heat. Even the act of moving it through the air is going to be some heat in the air, some friction with it. And so I want you to appreciate that this energy is not coming out of nowhere, it is being converted from one form or another, or being transferred from one part of the system to another. Now we can look at these examples over here. Same thing with our runner, what happens after, you can buy the fact that okay, his chemical energy is allowing his muscles to move, and that's turning in his kinetic energy for his entire body, his body is moving, but at some point he stops, where did all that energy go? Well, some of it will be heat in his body that's being dissipated into the broader system, into the air. And also, when he was running, there was this contact with the ground, that's going to make the molecules of the ground vibrate a little bit, some of it will be transferred as sound, so the air particles moving through the air, and a lot of it will be heat. And we're going to see that over and over and over again. The diver up here, you have mostly potential energy. Then it converts to kinetic energy as he's, as he gets almost in the water. But what happens once he falls into the water? Well, then that energy's going to be transferred, as you're going to have these waves of water move away. And it will also increase friction, so, well actually he would have had friction as he fell down, so that would have generated some heat, and there would have been also some heat with the friction with the water, you normally don't think of friction with the water, but there is some friction with the actual water, and there's also, these waves, you have higher kinetic energy of the actual water being transferred outward from where he actually dropped in. And I could keep going on and on. You have the chemical potential energy of the fuel here being, you have combustion occurring, and then that gets converted into the thermal energy, and the radiant energy of what we associate with fire. And that doesn't disappear, it just keeps radiating outwards, the radiant energy just keeps radiating outward, maybe it might heat up something. And the thermal energy will just keep radiating outward, or I should say, the thermal energy will just dissipate outward, and heat up the things around it. Same thing with our lightning example. You start with the electrostatic potential, where the bottom of the clouds were more negative, and then the ground is positive as well, and at some point, that potential energy turns into kinetic energy as the electrons transfer through the air, and then that gets converted into, or a good bit is going to be converted to heat and radiant energy. So the whole point of this video is, no matter what example you look at, if you think about it carefully enough, and I encourage you to do this in your everyday life, the energy isn't just coming out of, you know, magically appearing, it's just being converted from one form to another.