 So in doing calculations involving radiative heat transfer, there are some complex things that you can get into where you're looking at one surface with respect to another. However, what we're going to do is we're going to present now kind of a simplified equation where if you have a smaller object in a much larger surroundings and the surroundings temperatures are not changing, we'll look at an equation that can be used for that type of scenario. So we'll refer to this as being radiative heat transfer. Now in the theory of radiative heat transfer, we can consider an object to be an ideal radiator if it behaves as a black body. And for that it would be emitting, we quantify the amount of radiation that it is emitting using the following equation. So what this equation presents would be the amount of energy that is released from a body at a particular temperature and the temperature of the body would be Ts. And the proportionality constant in this equation is the Stefan-Boltzmann constant. Now one thing that I should say that you have to be very, very careful of when you're using these equations in radiative heat transfer is notice we have this raised to the power 4 and so therefore you need to ensure that your temperature is always in Kelvin. If you try putting degrees Celsius in here, you will get the incorrect value. So be very careful with that. Now an ideal radiator, that's kind of a theoretical maximum that an object would emit at a given temperature. In reality no objects emit at an ideal radiator, they emit at a slightly lower value than that. And in order to characterize that we have a term called the emissivity. So let me show you the equation for how a real object would respond. And so that would be a real surface in comparison to a black body. Notice when we have black body, we have the B subscript here. We do not have it for the rate at which energy is released, the term on the left hand side. And we also have this new term that we've introduced, this epsilon. This is the emissivity. And the emissivity is going to range between zero and one. And depending upon the object that you're doing the calculation for looking at, there are tables with emissivities and surface finish is very important for emissivity, be it dull or very, very polished, that can have a big impact on the emissivity. So that is in terms of what an object would emit. We're all radiating energy right now, radiative heat transfer given the temperature of our bodies versus the surroundings. At the same time, the surroundings around us are radiating and we're absorbing the radiation from the surroundings. And so an object may also absorb radiation. And we use this equation to characterize the amount of energy that an object is absorbing, where we will define the terms. So that is the amount of energy that an object will absorb. And what we have here, we have G abs, that would be the rate at which energy is absorbed. And on the right hand side then we have a new term, alpha, that is called the absorptivity. And just like the emissivity, it ranges between zero and one. And then we also have this term here, this is the incident radiation, sometimes called the irradiation. And that would be the amount of radiative energy coming on to a particular object. And so a very simplified case of this, we could look at a smaller area versus a larger surrounding. So let's take a look at that. And so if we had a scenario like this, and we were considering some smaller object within a very large surroundings, where the surroundings temperature is at T surrounding, and then the object itself is at T s for the surface of the object, we would have radiative interchange between these objects. And so this object would be radiating out, and then this surrounding would be radiating back, and you get reflection and everything. But essentially, you eventually get to a balance. And the net rate of exchange for the surface. So let's take a look at this surface right here. We can come up with an equation for the net rate of exchange. So the net rate of exchange is going to be what the object is emitting. And then we compare that or take off what is being irradiated. And so with that, we can sub in the two equations that we've looked at, the one with emissivity. So that is how much a body would be emitting. So you or I sitting in a room where irradiate or emitting radiation, it would be that amount. But we're also absorbing radiation from these surroundings. And so we characterize it with this term here. Now, in order to simplify this, and this is all very, very simplified, because in reality, these terms are wavelength dependent, and it gets much more complex. You have to look at projections of one surface versus another. But another simplification that we can say is if the emissivity is equal to the absorptivity, we call this a gray surface. And with that, the equation simplifies even a little more. And what we end up with is the following. And so this is an equation that we can use in very simplified calculations for radiative heat transfer, where if we know the emissivity of the surface of the object, and we know the surface temperature of the object and the surroundings, we can then estimate the amount of radiative heat transfer that is occurring. So what we're going to do in the next segment, we're going to take a look at an example, applying this equation. And that will then conclude our very, very brief overview of radiative heat transfer.