Conduction

--The atoms in a solid vibrate about their equilibrium positions. As they vibrate, they bump into their neighbors. In those collisions they exchange energy with their neighbors. If the different regions of a solid object or of several solid objects placed in contact with each other have the same temperature, then all atoms are just as likely to gain energy as to loose energy in the collisions. Their average random kinetic energy does not change. If, however, one region has a higher temperature than another region, then the atoms in the high temperature region will, on average, loose energy in the collisions, and the atoms in the low temperature region will, on average, gain energy. In this way heat flows through a solid by conduction.--

** The microkinetic energy of the outermost electrons of any atom acts upon lesser-energy neighbors atoms' outermost electrons. This is 'conducting' heat. **

--The stiffness of the springs (strength of the chemical bonds) determines how easily the atoms can exchange energy and therefore determines if the material is a good or bad conductor of heat. Each atom has a nucleus, surrounded by electrons. In a solid metal all nuclei are bound to their equilibrium positions. But some electrons are free to move throughout the solid. They can easily pick up kinetic energy in collisions with hot cores and loose it again in collision with cooler cores. Since their mean free path between collisions is larger than the distance between neighboring atoms, thermal energy can move quickly through the material. Metals are, in general, much better conductors of heat than insulators.

** By definition, metals are good conductors. In fact, several artificial molecules made of nonmetallic atoms are good conductors, being therefore called metals. I find the paragraph unclear. **

Convection

--Convection transfers heat via the motion of a fluid which contains thermal energy. In an environment where a constant gravitational force F = mg acts on every object of mass m, convection develops naturally because of changes in the fluids density with pressure. When a fluid, such as air or water, is in contact with a hotter object, it picks up thermal energy by conduction. Its density decreases. For a given volume of the fluid, the upward buoyant force equals the weight of this volume of cool fluid. The downward force is the weight of this volume of hot fluid. The upward force has a larger magnitude than the downward force and the volume of hot fluid rises. Similarly, when a fluid is in contact with a colder object, it cools and sinks. When a volume of fluid such as air or water starts to move, the surrounding fluid has to rush in to fill the void. Otherwise large pressure differences would develop. This sets up a convection current and the looping path that follows is a convection cell. Since fluid can not pile up at some point in space without creating a high pressure area, it will flow in a closed loop. Convection can be increased if the fluid is forced to circulate. A fan, for example, forces the air to circulate.--

** Heated gases or fluids move upwards because their component molecules are less compact. In so doing, the "conduction" of heat is facilitated. This phenomenon is called convection. **

Radiation

**NOTE from JG: Radiation is considered to be the third manner for heat to be transferred. It is quite easy to understand that when objects interact in terms of heat, the hottest heats the coldest until both reach the same temperature (equilibrium). In the process of heating by radiation, the hot object emits an energy which is not microkinetic, but radiating (photonic). The object loses microkinetic energy in this way, with no relationship to the eventual fate of the photonic energy. Thus, there is no equilibrium.**

T:--Nuclei and electrons are charged particles. When charged particles accelerate, they emit electromagnetic radiation and lose energy. Vibrating particles are always accelerating since their velocity is always changing. They therefore always emit electromagnetic radiation. Charged particles also absorb electromagnetic radiation. When they absorb the radiation the accelerate. Their random kinetic energy increases. In thermal equilibrium, the amount of energy they lose to radiation equals the amount of energy they gain from radiation. But hotter objects emit more radiation than they absorb from their cooler environment. Radiation can therefore transport heat from a hotter to a cooler object.--

JG:** A very confusing paragraph: "Charged" and "vibrating" are undefined, as is "electromagnetic radiation" and "random kinetic energy." The last phrase sounds as if heat were a transportable object. **

T:--Electromagnetic radiation refers to electromagnetic waves which travel through space with the speed of light. The quantity that is "waving" is the electromagnetic field, an esoteric but quite measurable entity. A wave is characterized by a wavelength.--

J.G:** EM radiations are both waves and particles. They travel at the speed of light by definition, because in physics light refers to all EM radiations. **

T:--The wavelength is the distance from crest to crest or from trough to trough. We classify electromagnetic waves according to their wavelength. The visible part of the spectrum may be further subdivided according to color, with red at the long wavelength end and violet at the short wavelength end.--

JG:** Each "color" (an antropic definition), is composed of a large number of "hues," which are each one of the waves that are classified as color. The boundary hues are very difficult to distinguish by the human retina Thus, hues define the different energies of colors. Those that are toward the right side of the spectrum are shorter and more energetical (more per unit of time, viz., have a higher frequency). This rule applies to all EM vawes, i.e., to all "light." However, the effects of the radiations --emited from a given electron-- on other atoms' electrons depend not only on energy but also on the 'shape' of the wavelenght. Thus, for instance, infrared radiations are more calorific than the more energetic "color" ones. In a greenhouse, infrared radiations cannot penetrate nor exit the walls. 'Color' radiations can, and they start the heating process, which is intensified by the emited --and 'trapped'-- infrared, until the electrons' microkinetic energy called heat reaches equilibrium. This is the point where as much 'heat' is lost through the walls of the enclosed space as it is created by the described 'heating' process. **

T:--Hot objects emit radiation with a distribution of wavelengths. But the average wavelength of the radiation decreases as the temperature of the object increases. Most thermal radiation lies in the infrared region of the spectrum. We cannot see this radiation, but we can feel it warming our skin. Different objects emit and absorb infrared radiation at different rates. Black surfaces are generally good emitters and absorbers, while silvery metal surfaces are poor emitters and absorbers.

JG:** It was while studying a black box that Planck conceived light as composed of units, thus starting quantum theory. Infrared radiations, as all EM radiations in nature, are composed of 'hues,' i.e., of many wave lenght frequencies. The more energetic ones are borderline with the color ones, and yet, their wave shape is not so appropriate for 'heating' as are the middle range ones. By the same token, the lesser-energetic EM radiations called microwaves are more 'shape-appropriate' to heat in a way proper to them. That humans and other biological systems can or cannot see, and react in peculiar ways to EM radiations, is a result of evolution constrained by the physical laws applying to the different elements and derivatives that compose matter. **