The next method of heat transfer is convection

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It only works with gases and fluids. Let's use air as an example. Suppose we have a heat source, such as a stove. The air near the stove burner will increase in temperature through a heat conduction interaction. This warmer air will now have a lower density than the cooler air above it. It will rise and colder air will take its place. Then the hot air can have another heat conduction interaction with things above it, like perhaps the ceiling. That indirect transfer of heat from the stove to the ceiling is convection.

The third type of thermal interaction is radiation , which is what really inte Phone Number List rests us. When a hot object emits infrared radiation, it can be absorbed by other objects. This is exactly how your oven works. You put the things you want to cook inside and the heating elements get very hot, producing thermal radiation. The food absorbs it and increases its temperature.

Now imagine that you preheat the oven, turn it off and put a potato inside. The hot oven emits thermal radiation and the potato absorbs most of it. The result is that the potato heats up and the oven cools. This isn't really a normal way to bake a potato, but the thing is that when objects produce thermal radiation, they cool down.

Although if everything around us emits electromagnetic radiation in the infrared, shouldn't everything cool down ? Well no . If we take an apple and place it on a table, it emits thermal radiation. But it also absorbs radiation from everything else: the table, the air, the walls. Therefore, if all nearby objects are at the same temperature, they will not be cooled by radiation.

Reflectivity vs. Emissivity
There is another very important property that must be taken into account to understand how radiative cooling works : the difference between reflectivity and emissivity. Imagine that you have a perfect mirror. All the light that hits it is reflected. That mirror would have a reflectivity of 1, meaning that 100% of the light that hits it bounces back.

A sheet of aluminum foil also reflects quite a bit of light, but not all of it . It could have a reflectivity of around 0.88, which means it reflects 88%. The other 12% of the light that falls on the sheet is absorbed, increasing its temperature.



Let's now imagine an object that does not reflect any light . Of course, it still emits light, but only because of its temperature, and not because light is reflected from it. This object would have an emissivity of 1 and we would call it a "perfect black body", meaning that it absorbs all electromagnetic radiation. Thus, emissivity is essentially the opposite of reflectivity.

Both reflectivity and emissivity depend on the wavelength of light. Just because something is not very reflective in the visible spectrum (400-700 nm wavelength) does not mean that it acts the same for infrared wavelengths (around 10 micrometers). Look again at the infrared image of my dog ​​above. Have you noticed that you can see his reflection in the ground? The ground is not very reflective in the visible spectrum, but it is is in the infrared.