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RC
Numerade Educator

# Heat transfer is the exchange of thermal energy between physical systems. The rate of transfer is dependent on the temperature difference (thermal gradient) between the systems, and conductive, convective, and radiative properties of the material. Conduction is the transfer of energy between objects that are in direct physical contact. Convection is the transfer of energy between objects that are not in direct contact through the movement of fluids. Radiative heat transfer is the transfer of energy by radiation, which occurs due to a temperature difference between the systems.

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University of Washington  ##### Aspen F.

University of Sheffield

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#### Physics 101 Mechanics

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### Video Transcript

welcome to our third example video Looking at heat transfer mechanisms In this video, we're going to start talking about radiation. So if we have our son, which is a meeting radiation in every direction and some of that is hitting us at Planet Earth and we are some distance away from the sun and then the sun has some radius will assume that the radiation that's getting to us is coming from the surface of the sun. Now, given this, if we want to figure out thesis AFIS temperature of the sun, what we need to know is how much is getting test? Well, we know that the intensity of sunlight arriving at our atmosphere from the sun is approximately 1400 watts per meter squared Now intensity, if you recall, is equal to power divided by area, which means we could say P is equal to I times A where a is four pi times the distance between us squared because that's the area of a sphere all the way around the sun with the radius at our orbit. So it's important to have power here because when we go back to the equation that we're really after which is D Q T t equals E sigma a t to the fourth notice here. That Deke DT that's jewels per second. That's power. So what we can do is we can set this equal to that. So we have i times for pie R s C squared is going to be equal to in Mississippi Time Sigma times a times t to the four. Awesome. So a couple things that we need to know about this situation because we can quickly solve for t, of course, which it will just be four pi r s c squared, multiplied by I divided by E Sigma and a all to the 1/4 power. But what don't we know so far? Well, we don't know. First of all, we can write down the distance between us and the sun. Just remind ourselves you can look this up on the Internet or pretty much any table. It's going to be 1.5 times 10 to 11 m and then for the area down here that's going to be equal to four pi r s squared. The radius of the sun is approximately 6.96 times 10 to the 8 m. So we've got those taking care of Sigma is the bolts mons constant, the Stefan bolts Mons Constant which we wrote down previously as being equal to 5.67 times 10 to the negative eight watts per meter squared proc held into the fourth. Okay, and then also we need the imbecility Now. M acidity is a major in some sense of how much of a black body you are. So a black body is a perfect emitter of radiation. It follows a particular curve. Um, in some introductory physics classes, they will get into black bodies a lot and others they won't. We will get more into it in a later unit. As we start to talk more about modern physics. For now, you can just know that the sun is a perfect black body, which means it's imbecility is one for most objects that absorb light that hits it. Instead of reflecting it back, the imbecility is actually going to be very close toe one. So even you probably have a festivity close toe, one generally for a human being, we'd estimated at about 10.95 or so. So for the sun, though imbecility is just one. So now we can plug everything in and we'll be able to calculate it. Notably, You wouldn't be able to do this if you didn't know Thean passivity. So it's something that either would have to be given to you or you'd have to look up in a table or you'd have to calculate from the equation.

RC
University of North Carolina at Chapel Hill

#### Topics

Kinetic Theory Of Gases

The Second Law of Thermodynamics

##### Top Physics 101 Mechanics Educators  ##### Marshall S.

University of Washington  ##### Aspen F.

University of Sheffield