 Hi, I'm Zor. Welcome to Unizor Education. Today I would like to continue talking about transformation of the heat. In particular, this is the form of transformation, which is called radiation. Now, this lecture is part of the course called Physics for Teens, and it's presented on Unizor.com. I suggest you to watch this lecture from the website, because it contains not only the lecture itself, but also textual description of the material and some problems and some exams, etc. The site is completely free, no advertisement, so it presents basically a course, a logically connected chapters, lectures, whatever, with each other. Now, speaking about radiation, well, first of all, let me start from basically mentioning two different physical terms, radiation and radioactivity. They are related, and I will talk about what kind of a relationship between them exists later in this lecture. But for now, radiation is something much broader, and radioactivity is a particular aspect which is related to radiation and certain danger for health, etc. So, radiation. We have actually covered two different kinds of heat transfer. One is conductivity or conduction. When there is no movement of particles, no movement of mass. So, if you have a solid, let's say steel rod and you are heating it in one particular place, the molecules are relatively stable in their position, but they're oscillating. So, the oscillation of the molecules is related to kinetic energy, obviously, and the heat which we are supplying to this end results in more intense oscillation of these molecules, which in turn force the neighboring molecules to be agitated. And then the next one, and that's how the heat is distributed, transferred through conduction. Now, conduction is, primarily in liquids and gases, is if you have, for instance, a pot with water, so you're heating this layer of the water, but then it moves, it freely moves, because the liquid actually is, liquid consists of molecules which are very movable, they're not fixed in the same place. So, molecules are moving in all directions, thus transferring the heat, same thing with gases. Now, radiation is significantly different from these two, primarily because we don't see any material particles, like molecules or whatever, which are moving from the source of the heat to a receptor of the heat. The heat somehow moves just by itself without any medium. This is the most fundamental property of the radiation. It doesn't look like it has certain medium which basically helps to transfer the heat. So, that's very, very important, so there is no medium. However, radiation is present. Any object which has a temperature greater than absolute zero emits radiation in some way or another, depending on, obviously, its temperature, etc. But anyway, the emittance of the radiation is present everywhere. Well, the same way as convection is present. Whenever I'm just standing here, there is definitely convection of air molecules around me, and if I'm holding this particular marker, then the heat from my hand is definitely transferred, although slowly because it's plastic, not very conductive, but anyway, it's transferred. So, in this particular way, heat is always transferred through some kind of different types or kinds or whatever. And again, radiation is present everywhere. So, right now, I'm not only producing certain amount of heat which is transferred through conductivity or through convection of the air molecules, I'm also radiating certain amount of energy towards the air and the walls actually absorb it. Okay. Next, how can I actually exemplify the fact that radiation does not require any medium which we can feel and see and somehow detect? Well, consider the sun and the earth. There is basically nothing in between, vacuum, so somehow the heat from the sun is hitting the earth and that's basically the source of life on earth. So, the radiation is extremely important and it exists and somehow it travels through space where seemingly nothing exists. So, how does it do it? Well, this is a very interesting question about how does it do it. Physicists had different theories about this. Now, all these theories were just models. They were thinking, for instance, there is some kind of a substance which is filling the whole space. It's called ether and the light is the oscillation of particles of ether in the same way as, for instance, sound is oscillation of the air particles they're oscillating or conductivity, conduction is an oscillation of the molecules of a solid object. But this model did not really explain certain things which experiments show and starting from the beginning of the 20th century, physicists declined this particular model. So, what to do? I mean, we have to have some kind of understanding, right? Why radiation, heat, whatever is somehow transferred from sun to earth? Well, the most significant theory which is basically related to this is the field theory. So, there is a concept of field. So, what is the field? Well, first of all, let's talk about gravity. We all know that we feel the gravity wherever we are. If we are on the surface of the earth, the force of gravity actually pulls us down and only the resistance of the floor prevents us from falling down to the center of the earth. Now, the earth circulates around sun and what keeps the earth on the orbit? Well, the gravity of sun, moon, etc. I mean, there are many, many examples of the force which exists in the space and there is no immediate contacting body which actually is the source of this force. You know, if I'm holding this particular marker, I am pushing it up while gravity pushes us down. So, this particular object feels the force from my hand. But the earth doesn't seem to apply any visible object towards this. So, why doesn't it really go all the way up because I'm pushing it up? Well, because there is a gravitational field. The field is basically a model which describes certain space where certain forces act on certain objects. For instance, gravitation is the force which acts on all the objects which have certain mass. Now, we all know about magnetic field, for instance. We are using compass and the compass is working, the arrow is pointing north because around the earth there is a magnetic field and this magnetic field acts on this magnetic arrow and it actually forces this to go into a certain direction. And don't ask me why earth has magnetic field. I don't know. Whatever the source of this is, it exists. And don't ask why sun has gravitational attraction. It's something which people are basically talking about. They are suggesting certain theories why it happens. But in most of the cases, all these questions why remains unanswered. What is answered is the properties of the different fields. So, we have magnetic field, we have gravitational field, we have electrostatic field or electric field. If you have certain charged object, electrically charged object, then some other electrically charged object can either attract or repulse from it depending on polarity, plus, minus, etc. So, the fields do exist. In most of the cases we have certain theories about how and why they exist. But most importantly, and something which we really can study, is the properties of these fields. So, let's, well, unfortunately, I'm not saying that's the good thing, unfortunately, let's just consider that the concept of the field really exists. It reflects certain factors in space, certain forces which act in certain areas of the space, act on certain objects in certain way. And all we can do is, we can basically study the properties of all these fields, objects in the fields, forces which are acting on these objects, etc. So, the concept of field, fine. Now, let's talk about specific field, and this specific field is called electromagnetic. Now, we all know about magnetic field separately, and we all know about electrostatic, or I will call it electric field. So, what is electromagnetic field? Now, here I can refer to certain experiments. If you have a conductor through which electricity is going, then around this conductor there is a magnetic field. And if this is changing, magnetic field is changing. It's a fact. It's an experimental fact which has been detected, and people can talk about this. And what I'm saying is now, what is the current? Current is movement of the electrons. So, if electrons are changing their position in space, then it creates certain magnetic field, which is changing in time, basically. So, electricity causes the magnetism. And there is a reverse kind of experiment. If you have two magnets, north and south, and you have certain conductor here, which you will move back and forth, there will be current detected in this conductor. So, this is a very simple experiment which basically demonstrates how changing electricity causes the creation of the magnetic field. Changes in the magnetic field are causing electric current. So, electricity and magnetism are very much related. If you are changing one, it causes the change of another, and that in turn causes back-changing to the magnetic, and again to electric, magnetic, electric, etc. So, they are interconnected and they are actually acting like oscillations. So, let's just consider, however unsatisfactory you think about this model, that the field is something which exists, and in this case it's electromagnetic field. The oscillations of this field is something which is observable, experimentally observable, studied, and we can actually have certain numerical calculations, etc., etc. Now, and here is the very important piece of the whole, this explanation, that these oscillations of electromagnetic field, and now I'm calling it electromagnetic, because there is an electrical component, which is causing the magnetic component, which is causing electrical, magnetic, etc. So, these oscillations of this field are very much similar to whatever I was just picturing in the beginning, when the molecules of the solid object, these are agitated, intensely oscillating, they are transferring these oscillations to the next molecules, those to the next, etc., etc. So, in some way you can just think about oscillations of the electromagnetic field in the same way as you are viewing oscillations of the molecules, and that's how the heat is transferred within the solid using the conductivity. And this is exactly how the radiation actually transfers the heat using electromagnetic oscillations, or waves, or whatever you want to call them. So, let me start from this particular very important point again. We have electromagnetic field, we don't really think about what it consists of, how it's physically created or whatever, it exists, and you don't like the way that you don't really feel it, I don't like this as well, but that's the reality, which we just have to accept. And the oscillation of the electromagnetic oscillations of this field are actually the mechanism using which the radiation is transferred from, let's say, sun to earth or from me to all these surrounding walls. So, we are all emitting certain electromagnetic waves, electromagnetic oscillations, and any object which has a temperature greater than absolute zero emits radiation in this particular sense. Okay? All right. Now, next is, think about, again, the solid object and conduction as the way of transferring the heat. If the heat is more intense, how is it physically reflected in this particular object on a microscopic level? Well, the molecules are oscillating with a higher frequency, higher amplitude, etc., right? So, that's what intensity actually means. So, that's how you get the greater kinetic energy. And the more heat, obviously, is more kinetic energy to these molecules, and they are stronger, shaking the whole lattice of all these molecules, and the radiation actually is going forward. But what's important is that the frequency and amplitude of these oscillations are very much related to the amount of heat, intensity of the heat which we are applying to this end of this object. Exactly the same thing is with oscillation of the electromagnetic field. So, these waves of electricity and magnetism converging into each other, they have their own frequency. It can be greater or it can be lower. Now, what's important is that the spectrum of all the different frequencies of these oscillations is very broad. The object which has a temperature slightly more than absolute zero emits a very low frequency, electromagnetic waves. The greater the temperature, the greater the frequency, and at a certain moment we can actually even feel it with our skin. Because if radiation is within certain range, we feel the warmth, the hardness or whatever of this radiation. I mean, if you will just look at the sun, you will feel with your hand, with your body, you will feel the warmth, right? Now, in certain frequency ranges, not only we can feel it with our skin, we also see it with our eye. So, the light itself is also electromagnetic oscillations within certain range from this frequency to that frequency, whatever our eye is capable to accept. Just terminology-wise, we are very much concerned about this range spectrum of frequencies which we can see with our naked eye. And everything which is below that, we call infrared, and everything above the range of visibility we call ultraviolet. Why do we call it this way? Well, because if you will look at the spectrum where the eye actually can sense the electromagnetic waves, then the lower end of this spectrum we will see as red light, and the high end of this spectrum, higher frequency, we will see as violet. So, that's why it's infrared, which is below red, and ultraviolet if it's above the violet. Now, why do we actually see it in different colors? Well, I don't know. It's all very much dependent on how our eye is arranged, how our brain actually senses all these, takes the signals from the eye, etc. But we see different colors, and these different colors are actually different frequencies. Now, on the infrared part of the spectrum, we do see the worms with our skin, for instance. And again, we feel warmer if it's hotter, right? Or colder if it's back to the absolute zero, right? So, it's all different kind of senses which we have. But in any case, it's all exactly the same physical concept. It's oscillations of electromagnetic field, just different frequencies. Now, if you're going even higher than that, the frequency becomes very high, and that means that intensity is very high. Now, why do we actually talk about sun shades which are covering ultraviolet lights? Because high intensity will damage your eye. So, that's why it's important to wear these sun shades which have certain property of covering, shielding from ultraviolet lights. Now, sun as a source of light, it's a source of all kinds of electromagnetic oscillations in all the spectrums, from very low to very high. So, all I'm saying is that the higher frequencies become actually dangerous, in this case, for your eye. But if you go even further, that actually becomes just dangerous for the entire health. Because these high frequency oscillation of electromagnetic field can actually destroy the cells of our body. And that's where we are talking about radioactivity. So radioactivity is also electromagnetic oscillations, electromagnetic waves, but very, very high frequency. We call it gamma rays, for instance. Gamma rays. So, these are infrared rays, ultraviolet rays. But if you go further and further to the upper frequencies, you will get so-called gamma rays, which become very, very dangerous for the health. Obviously, it depends on dosage, how long you're exposed to radioactivity. But this is where radiation actually kind of morphs into radioactivity. So, radiation is all the different kinds of frequencies. Radioactivity is a specific, very, very high intensity radiation. Okay, what's next? Well, next we have a very interesting formula. The only one formula which I would like to actually present here. Now, we all kind of suspect that amount of heat, which is transferred from hot to cold, let's say, is kind of dependent on the temperature. We were actually talking about certain laws which describe this kind of a dependency. Exactly the same thing with radiation. So, the radiation is basically amount of energy from unit of surface per unit of time. And what's interesting, it depends only on the temperature. And this is so-called Stefan-Boltzmann constant, which is a constant actually. Its value is 5.67 times 10 to the minus 8 watt per square meter and temperature Kelvin in the force degree. So, if we multiply this by area which emits radiation, we will have a total amount of radiation which is emitted by the body. As long as we know the body's temperature, we can actually come up with amount of energy. What is basically Joule per second? So, that's per unit of time and per unit of area. So, knowing this, knowing the temperature Kelvin degrees and obviously the constant, this is the constant, we know amount of radiation which is emitted by the body. So, the sun emits huge amount of energy because it has a very big area of the surface of the sun and temperature is something like 6000 degrees more or less. So, to the force degree. So, you can imagine how much energy the sun loses every second. Well, it's big so it probably will not turn dark during certain number of, probably billions of light, billions of years. But anyway, that's the fact. The fact is that the sun is losing this energy and obviously it's not infinite. So, this is the law of Stefan Boltzmann. What else? Okay, what's important is energy is radiated, but at the same time energy is absorbed. So, if my body for instance is emitting certain amount of radiation, I'm also absorbing radiation which is emitted by everything around me. And that's why my body is in thermal equilibrium, right? I'm not losing my temperature, I'm not gaining temperature, I'm more or less of the same temperature. Well, actually in most cases we are trying that the temperature of surrounding would be less than the temperature of the body, otherwise it would be very hot. Which means that I'm losing energy, just living being in this environment which has a temperature lower than my own. I'm losing energy and that means I have to eat to compensate for this energy. And that's basically the whole life cycle contains. You're spending energy, you're consuming energy. Now, so energy can be absorbed, but what's interesting is that the transfer of energy using conduction or conduction has only basically back and forth you're transferring to or you're accepting from. What's interesting with, so that's basically emitting and absorption. What's interesting about radiation it can also be reflected. Now, what's the example? Well, mirror. Regular mirror is actually the demonstration of the light being reflected from this mirror. So radiation can be reflected, not only mirror, for instance you are using this special thermos which has mirror walls and you put some hot liquid, you close it and it basically preserves the temperature. Why is it preserving the temperature? It does not let radiation to carry out the heat from inside. These mirrored walls inside this reservoir, they are reflecting all the heat back to the liquid, to your cup of tea or cup of coffee. So that's example of reflection which is specific for radiation. Okay, basically I think that's all I wanted to talk about. Well, example of absorption for instance. Well, if you're using a toaster for instance to toast the bread, you slice of bread. Now, what are these spirals doing inside the toaster? They are red hot and they radiate the heat from it. Now, there is also the air inside between the spirals and the bread, but let me tell you this. If you will make an experiment in the vacuum, you will pump out from some kind of a big reservoir all the air or as much air as possible and put the toaster inside. Will it work? Yes, it will work and the bread will actually be toasted. And there is no air, so there is no medium actually which transfers energy, heat in this case from the spiral to the bread. But nevertheless, it would work. Why? Radiation. So this is when spirals emit and bread absorbs radiation and that changes inside the internal structure of the bread. It actually has some kind of a brownish color becomes etc. So there is a structural change caused by the heat absorbed from the radiating spirals. So that's it basically. So again, I have to admit that I feel certain degree of dissatisfaction by not really explaining what the field actually is. All we know about the field, we know its properties. Now, there are many different scientists who have many different theories about the fields. There is a classical field theory. There is a quantum field theory. There is a string theory. There are many different kind of theories. All of them has certain degree of complexity and certain degree of correspondence to experiments. But look, it's a difficult kind of a concept. Just try to be in peace with this fact that you don't really maybe understand all the intricacies of what actually field is. But nevertheless, you can study the properties and that's kind of give some satisfaction, I hope. That's it for today. Thank you very much and good luck.