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How do we feel heat?

When we "feel" heat, is it because electromagnetic radiation is exciting water molecules in our bodies?

First, a quick warning: I am not a biologist, but I've supplemented my vague memories from my high school Anatomy & Physiology class by reading some websites, so hopefully the biological part of my answer will be fairly accurate.

Now, let me distinguish between two uses of the word "heat." Heat is a sensation that occurs when temperature-sensitive nerves in our skin detect a difference between the temperature at the skin surface, and temperature deeper in your body. However, the term "heat" also has a specific meaning in physics, meaning thermal energy.

The sensation of heat comes from nerve-endings that detect the temperature of the skin. The temperature of the skin increases when heat energy flows into the skin. For moderate ranges of temperature, the nerve endings tend to adapt; this is why when you first get into a hot shower, it can seem VERY hot, but as time goes by you get used to it. For this reason, the nerve-endings are most sensitive to changes in temperature.

In general, there are three ways for heat to flow from one place to another: convection, conduction, and radiation.

Convection occurs in fluids, when parts of the fluid that are warm tend to rise--but it is not relevant here.

Conduction occurs when heat flows between two objects that are in direct contact. For example, when you wrap your hands around a warm coffee mug, the heat flows directly from the warm mug to your hands. This raises your skin temperature, and you feel the sensation of heat. This all occurs without any exchange of photons--just molecules banging into one another.

Radiation can carry heat in the form of photons. There doesn't have to be direct contact between a hot object and the person for radiation to carry heat, because photons can travel through air, or even a vacuum.

We often think of infrared as "heat radiation" because many of the objects that we have daily contact with (anything with a temperature less than about 500 degrees centigrade) radiate most of their energy in the infrared. However, all wavelengths of light carry heat. The Sun is so hot that it radiates most of its light in visible wavelengths, and these photons heat the Earth (including the people on it).

Also, any object can absorb the photons, not just water molecules. For example, as anyone who likes to go barefoot knows, a perfectly dry sidewalk can get very hot on a sunny day. You may be thinking of a microwave oven, which radiates photons which are absorbed very effectively by water molecules (and also other molecules common in foods, like fats). Your body can absorb microwaves, but they are not produced in great quanties by the Sun or other objects.

Now, when photons strike your skin, some of them are reflected. That's how we can see people! We see the photons visible that are reflected from their skin. But not all the photons are reflected--if they were, then people would look pure white. The photons that aren't reflected are absorbed. The absorbed photons transfer their energy to the skin, increasing its temperature, and again, we feel the sensation of heat.

May 2002, Britt Scharringhausen (more by Britt Scharringhausen) (Like this Answer)

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