Okay, so everywhere I've read, I hear the main difference is the requirement of a medium. But for example, if you take the case of heat 'radiating' from a red-hot iron, isn't that actually convection and not radiation? I mean, isn't the temperature difference between the surrounding air and the iron causing the air to gain heat?
[Physics] What exactly is the difference between radiation, conduction, and convection
convectionradiationterminology
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Heat transfer texts clearly indicate that empirical equations are involved, due to the fact that turbulence is often encountered. You may have to do a search for your particular geometry to find the equation that estimates your convective heat loss.
For example, let's say we have a big turkey. Now, we surround the turkey with air heated to 350F, that is convection cooking. The air transfers its heat by contact to the turkey.
This is what we would typically call roasting in a conventional oven. When the turkey absorbs heat from the surrounding air, the air cools down; because air is a very bad conductor of heat, the (still) air develops a temperature gradient near the turkey which means that the air that the turkey is actually in contact with is substantially below $350^\circ$.
If we equip the oven with a fan system which circulates hot air to ensure that this layer of cool air near the turkey does not develop, then this is what we would call roasting in a convection (sometimes called fan-assisted) oven. Obviously this will cook the turkey faster because the air in contact with the turkey will actually be $350^\circ$, in contrast to the conventional oven.
In both of these cases, we are not cooking the turkey via the radiant heat from the (extremely hot) heating element. The heating elements are cycled on and off to maintain the temperature inside the oven, but they spend most of their time off. If we do cook with the radiant heat from the heating element, we keep the heating element on the whole time; this type of cooking is called either broiling or grilling, depending on where you live.
To test this I put some chicken in the oven and right next to it a block of wood. Both were about 12 inches from the heating element. After some time, the surface of the chicken was 250F and the surface of the wood was 450F.
Wood is dry, chicken is not. Water has a massive heat capacity - far larger than wood or almost anything else we use in a kitchen - which means it takes a lot of energy to raise the temperature of something which has a lot of water in it. The surface of the chicken is in direct contact with the interior of the chicken, which keeps it cool.
Best Answer
To pretty much everything you stated in your question, "no".
That convection requires a medium is not the main difference, it is simply the most obvious aspect of what is a fundamentally different mechanism for transferring energy. Convection is the transfer of energy by movement of a medium, whereas radiation is the transfer of energy by, well, thermal radiation. Conduction also requires a medium, but, again, it is a fundamentally different mechanism than either convection or radiation; in this case it is the transfer of energy through a medium.
Unfortunately, analogies are hard but if you can visualize the particles involved, it would help. Picture the red hot iron you mentioned. On a molecular level, the material is emitting lots and lots of photons (hence why it is glowing red). The creation of these photons takes energy; energy from the heat of the iron. These photons leave the iron, pass through the environment, and eventually collide with some other object where they are absorbed and deposit their energy. This is radiative heat transfer. If that energy is deposited on your retina or a CCD (like in a digital camera), an image forms over time. This is how infrared goggles work and they would work equally well in high vacuum as here on earth.
In conduction, the next simplest example, there is no generation of photons (physics nerds forgive me for the sake of simplicity). The individual atoms in the object are vibrating with heat energy. As each atom gains energy from its more energetic neighbors, so it gives up energy to its less energetic ones. Over time, the heat "travels" through the object.
In convection, the molecules of gas near the object gain energy, like in the conduction case, but those same molecules that gained energy then travel through the environment to some other location where they then give off their heat energy.
In summary: