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Heat transfer

Understanding conductive, convective, and radiative heat transfer using a thermal camera. Created by MIT+K12.

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  • leafers sapling style avatar for user Peter Collingridge
    Is it normal for a thermal camera to represent hot objects as blue and cold objects as red? It seems the wrong way round to me.
    (11 votes)
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    • blobby green style avatar for user James Cameron
      No, not necessarily.

      A thermal camera is using a technique called "False color" (you can find that in Wikipedia) to convey the temperature. The colors might be swapped, or more than two colours used. It depends on the display attached to the camera.

      What colours you choose depends on what you want to look for; finding someone from a search helicopter at night; choose black for below zero, and choose white for human body temperature. For finding air leaks in a kitchen fridge, maybe blue for cold and red for room temperature?
      (17 votes)
  • blobby green style avatar for user irtza_rehman
    a person sitting infront of fire so heat transfer to that person through??
    convection through air
    (9 votes)
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    • blobby green style avatar for user Christopher Dayton
      The heat you feel while standing back from the fire is radiation. If you hold something between you and the source, it will be blocked.

      Convection is the heat rising off the fire . . . what you feel if you were to put your hand over the fire.

      Conduction would be if you put a a poker in the fire to heat it up . . . actually . . . if you put a poker in the fire . . . it'd be a combination of all three.
      (7 votes)
  • orange juice squid orange style avatar for user Abraham George
    How can radiation give you cancer? In history we learned that people got cancer from the radiation from the atomic bomb in World War 2.
    (6 votes)
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    • mr pink red style avatar for user aneetta
      as radiation hits a cell, it can either have no affect, can damage it slightly, but the cell can repair itself, can damage it beyond repair, or it can destroy the cell. When a cell is damaged beyond repair, the amino acid that tells a cell when to start reproduction and when to stop it is damaged and the cell just continues to reproduce. This damaged amino acid is copied and transfered to all the other cells it makes therefore, causing a chain reaction of uncontrollable cells. This is how cancer starts..it is the abnormal growth of cells.
      (8 votes)
  • leafers ultimate style avatar for user Dave
    What exactly is radiation? Energy carried on light waves? I don't understand how metal foil can act as a great reflector of radiation energy, even though we can clearly observe its reflective properties. The outer electrons in a foil like that are loose and move about freely; that's what makes them great conductors, but then how come they don't get energized by light waves? What happens on the atomic level?
    (6 votes)
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    • blobby green style avatar for user wolmot
      The conceptual background pertaining to your question delves first into the quantum physics branch of the 20th century. I suggest reading the works of Einstein and his Noble prize winning paper on the photo-electric effect. With some general chemistry and physics background into the life works of Niels Bohr and Thomas Young you can begin to understand what happens at the atomic level. The general jist of the matter is that electrons are excited by stray energy travelling in the form of photons of light. The wavelengths, frequencies and intensity with which they travel determine the amount of energy they contain. Knowing that electrons of an atom orbit on different levels or quantum states, the manner in which electrons transcend their energy or quantum state within atoms is through these exchanges of energies. If a given electron is exposed to a threshold frequency or energy of incoming photon light, the electron may break free from the atom in the form of a specific type of radiation (most commonly: alpha, beta or gamma). Referring to the video's aluminum example, the optics branch of physics gives insight to the refractive and reflective indices of material. If you're really interested in the effect of light waves, this has vast applications to black-body (for example our sun) radiation as well.
      (3 votes)
  • aqualine ultimate style avatar for user ValiantKnight
    What is the exact definition of thermal radiation?
    (3 votes)
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  • leaf blue style avatar for user bhargava2001
    water is a bad conductor of heat but it becomes hot when kept over a flame.why?
    (2 votes)
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    • blobby green style avatar for user Christopher Dayton
      Water isn't necessarily "bad" conductor of heat . . . it is far more effective than air . . . which is why most modern engines are "water cooled" . . . the heat is conducted out of the engine cylinders and into the water, and off to the "radiator" which optimizes the release of the excess heat.

      But to your initial question . . . the heating of the water, causes convection, which mixes the fluid, and "averages" the temperature.
      (3 votes)
  • old spice man green style avatar for user ☣WyattR.☣
    What is cold? Is cold just a loss of heat? Or is it its own kind of energy?
    (2 votes)
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    • leaf green style avatar for user Robby358
      When something feels cold to the touch, that means that it is accepting energy that your skin is releasing. Ordinarily, we are surrounded by air which has a relatively high specific heat, meaning that it won't "drain"/accept as much energy from us as quickly as something like metal, so it doesn't feel as cold. Metal has a low specific heat, meaning that it will quickly and easily accept vast amounts of energy from our bodies, so long as it is at a lower initial temperature (because heat energy flows from hot to cold always). So to sum up, hot and cold are all relative and mainly describe how we feel temperature. When we feel something cold, energy flows from us to it, and when it feels hot, energy flows from it to us.
      (2 votes)
  • hopper jumping style avatar for user Colton poole
    is radiation GOOD or BAD
    for your health
    (2 votes)
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    • blobby green style avatar for user Kelli Dantin
      The answer is not as simple as good or bad for your health. Radiation is hazardous to our bodies at high exposure rates. For example, when you get a sun burn that is mild radiation poisoning, just like if you drink too much you can get alcohol poisoning. Repeated exposure can cause cell replication errors and create cancerous growths. However, we also use radiation to kill cancers, so it can be good in that regard. In these cases, doctors intentionally bombard an area with radiation to poison and kill them. Your body is constantly exposed to low levels of radiation, which do not harm the body. In fact, some studies are testing whether that low level background radiation helps kill off our weaker cells keeping us healthier. It is the high and/or repeated exposure that causes issues.
      (1 vote)
  • aqualine ultimate style avatar for user Tejhasvi Jaikumar
    I want 2 make sure that I understood everything. Can anyone plz give the main differences between conduction, convection and radiation...some examples along with it which isn't there in the video would also be appreciated.
    (0 votes)
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    • blobby green style avatar for user http://facebookid.khanacademy.org/100001227312992
      Suppose you want to distribute apples among your friends , the same way things transfer heat among themselves.There are 3 ways to do it.Firstly you can pass each apple to your neighbour who passes it to his neighbour and so on.This is conduction where heat is transferred from one atom to another like heating an iron rod.Secondly each of your friends can come to you and take an apple from you.This is convection like heating water where each atom comes takes some heat and moves up.Thirdly you can throw each apple to your friends.This is radiation like the sun heating the earth.I hope this would help you.
      (5 votes)
  • marcimus pink style avatar for user Anna
    does cold air spread the same way as hot air
    (0 votes)
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Video transcript

[MUSIC PLAYING] Hello. My name is James White. And today we'll be investigating heat transfer. Heat transfer affects our everyday life. For example, it allows our computers to run without burning up. It helps us to design houses that use less energy. It makes cooking food possible. And very importantly, it helps keep us warm or cool, like this radiator. We will look at three different types of heat transfer-- conduction, convection, and radiation, and find examples where they occur around us. To do this we'll use a thermal camera. A thermal camera can see the radiation given off by all objects and use it to measure their temperatures. For example, cold objects normally give off less radiation than hot objects, which give off more radiation. The camera records this radiation and creates a picture. The colors in the picture represent temperatures. Hotter temperatures are lighter colors, and colder temperatures are darker colors. In this picture, my hand is warm. But the copper wire below it is cold. However, the cameras isn't always correct. Let's look at a video of a lighter flame. Although the flame appears to be the same temperature as my hand, it's actually about 2000 degrees Celsius. This is too hot for the video to show us the real temperature. Using the lighter and copper wire, we will observe the first type of heat transfer-- conduction. I'll show you our simple experiment. Conductive heat transfer occurs when there is a temperature difference between two materials in contact with each other. In this case, the right side of the wire is heated by the lighter. At this point, temperature in the material is very high in one area, and cold in other areas. Heat transfers from the high temperature side to the low temperature side. Let's watch as this happens. Over time, the temperature of the material becomes more and more uniform as heat transfer moves energy from hot to cold areas of the wire. When engineering a building, we often care about keeping warm or cold air inside. How well a wall can retain heat in a building depends on two things-- it's thickness and its thermal conductivity. Materials that are thicker will let less heat through. And materials with higher thermal conductivity will let more heat through. How do you think this metal door compares to this foam panel? Which lets through more heat? What about traditional materials like these adobe blocks? When the temperature of a wall or door is different than the surrounding air, a different type of heat transfer begins to occur, named convection. A lighter, a copper wire, and a cardboard backdrop make up our simple experiment. When a material is hotter or cooler than a fluid surrounding it, it causes the fluid to rise or sink. In the case of a hot wire, heated air rises as cold air fills its place. This cools down the wire faster than conduction. If we push the wire towards the cardboard, we see that the hot air rising off the wire heats the cardboard. This hot stream of air is called a convection current, and is part of natural convection. Sometimes natural convection isn't fast enough, so engineers will use forced convection. Your computer uses forced convection to keep itself from overheating. That's what your fans do. Using the thermal camera, we can see that this laptop has several hot components-- here, here, here, and here. When these components get too hot, fans in the laptop will start to pull cool air across the parts. This results in forced convection. The hot air is then blown out the side or back of the computer. Earlier I mentioned that all objects give off radiation. When radiation from a hot object hits another object, it warms it up. This process is called thermal radiation. Two bright lights, a piece of aluminum foil, and a cardboard backdrop make our simple experiment. When the lights are turned on, let's see what happens. We can see that radiation from the lights is absorbed by the cardboard. However, the radiation is reflected by the aluminum foil. When the lights are turned off, we can see that the cardboard under the aluminum foil was not heated because radiation never reached it. The sun is an extremely hot object, and its thermal radiation warms our entire planet. This means that when designing a building, window placement is a very important part of the thermal engineering. When the sun's radiation comes through windows, it will heat up the exposed cooler indoor materials. Conduction and convection then transfer this heat around the building. By considering thermal radiation in design, a building will be warmer during the winter and cooler during the summer. Radiators are a common way to keep buildings in cold environments. Although named a radiator, this device actually uses radiation and convection to transfer its heat to the room. Thermal radiation is sent out in all directions from the radiator and makes you very warm when you stand next to it. Natural convection then circulates the hot air from the radiator around the building. There are so many examples of heat transfer around us every day, and as engineers, we have to think about using them to make the world a better place. [MUSIC PLAYING]