I’ve read through these posts and seen a lot of gum flapping, opinions, generalizations and few posts based on basic physics explaining the difference between radiation and the other two ways heat reaches equilibrium.
Here’s an old goat’s interpretation on how part of our world works.
First, realize “heat” is the result of motion between minute particles of matter and that it is constantly being exchanged in an endless circle wanting and seeking thermal equilibrium. “Temperature” measures intensity of heat and, indirectly, the amount of motion in the particles.
CONVECTION
Gases, like the air we breathe, exchange heat mostly by convection, a means of flow, whereby cold gas particles have less movement and thus are closer together (read more dense) than warm air particles (less dense) accounting for the fact warm air rises and cold air sinks. When a warm air particle (molecule) contacts a cold air particle, its higher heat energy in the form of molecular vibration is transferred to the cold air particle, which then becomes warmer and rises with its less dense cousins. This energy transfer, per molecular vibrations, moves like a wave through the cooler particles as they warm up. There is significant space between air molecules, so this process of molecular collisons passing on energy takes time and causes air currents as warmer molecules rise and cooler ones sink. In the process, as the wamer particles contact fixed objects in the room, these absorb the vibration energy and begin to warm. This method of heat transfer (convection) is indirect since the heat transfer goes through air containing much space between molecules. This continual movement of air from a hot stove causing convective air currents can result in drafty indoor conditions with temperature zones (aka “indoor weather").
Liquids also exchange heat chiefly via convection. Particles are closer together than in air but the same phenomenon occurs explaining why cooler water is near the bottom of a lake and warmer water on the surface. But in this example, wind can blow warm surface water away resulting in the temporary surface feeling cold as colder water rises to replace the displaced warmer surface water.
CONDUCTION
Solids exchange heat by radiation and, with direct contact, conduction. In its quest for equilibrium, heat transfers via conduction when two solid objects are in contact. Vibrations from the warmer object molecules spread by contacting the cooler objects molecules thereby increasing their vibrations and heat which, spread like a wave through the object. The degree of vibrations, from the amount of heat, can be substantial. If a very hot object (hot metal stove) with considerable heat energy (lots of vibrating molecules) is touched by a finger, the molecules on the surface of the finger can vibrate so fast that those molecules separate resulting in a burn to the finger.
Friction is another way to increase particle movement resulting in heat by conduction. For example, experience the warmth generated by rubbing your hands together.
RADIATION
Heat transfer by radiation is a slightly different story. To understand it, realize that any matter with a temperature above Absolute Zero (0* K, or -273* F) gives off “infrared radiation”, not seen in the visible spectrum of light, but explained by quantum mechanics as a stream of extremely small photons having properties of waves and particles, maybe both, maybe alternating between the two. So small are these photons that it is debated (when I was in school, please, update me) whether they are pure energy or particles, either travelling at phenomenal speeds. When these photons collide with other molecules or particles of matter, they cause increased particle movement and more heat.
As mentioned, gases, like air, have relatively enormous amounts of space between the molecules of the gas. Liquids have less space between molecules and solids even less (solids being generally more dense than liquids or a gas). The tighter the molecules are packed together in a piece of matter, the easier it is to absorb any radiant heat photon which may strike it, making it warmer. Conversely, in air, since the molecules are far apart making the chances of a radiant energy photon hitting the gas particle much smaller.
Hot objects radiate photons of greater amplitude (like a sine wave) than cool objects. A photon from a very hot source has a greater chance of colliding with a particle of matter, like an oxygen or nitrogen molecule in room air, in a given distance than a photon from a cooler source.
This explains why a hot metal stove at 550* F tends to heat the air around it and induce convection air currents versus a cooler stove at 180* F which will radiate photons with less amplitude, have less chance of colliding with air molecules over a given distance and can heat objects further away from the source than the hot metal stove.
That’s the story and I’m stickin’ to it.
Aye,
Marty
Albert Einstein once said, “Nothing happens, until something moves.”
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