Daniel Gabriel Fahrenheit chose as the zero of his temperature scale the temperature at which ice melts when it's mixed 50/50 with salt. He then set the temperature at which pure ice melts to be 30° above zero and normal body temperature to be 90° above zero. These values were adjusted several times over the years as temperature measurements became more accurate and are now 32° and 98.6° respectively. Having established the temperature scale based on these various situations, he had no choice about water's boiling temperature. Water's boiling temperature at normal atmospheric pressure simply turns out to be roughly 212° on his temperature scale.
The main mechanism by which heat is transferred to food in a normal oven is convection. In this mechanism, air heated by the gas or electric burner at the bottom of the oven rises because of buoyant forces (i.e., hot air rises) and carries heat to the food. But natural convection is slow and imperfect—if you overfill the oven, you block convection and the food cooks unevenly. In a convection oven, a fan stirs the air rapidly. Heat flows quickly and evenly from the burner to the food. Cooking occurs more quickly and you can also put more food in the oven without danger of uneven cooking.
Heat naturally flows from hotter objects to colder objects. As a result, you can heat food by putting it in hotter surroundings and cool food by putting it in colder surroundings. However, you can also heat food by converting an ordered form of energy into thermal energy, right inside the food. For example, microwaves can penetrate the food and their energy can become thermal energy inside the food, speeding up the cooking process.
However, there is no analogous way to reach inside the food and extract its thermal energy. You must wait for the thermal energy inside the food to drift to its surface and to be transferred to the colder surroundings. This requirement is the result of the laws of thermodynamics, which govern the interconversions of work and heat. While it's easy to turn mechanical work into heat (just rub your hands together), it's very difficult to turn heat into work. Because of this difficulty, thermal energy must usually be transferred elsewhere. You can't build a "microwave refrigerator" that turns thermal energy into microwaves inside the food.
Thermal radiation consists of electromagnetic waves. These waves are emitted and absorbed by the movements of electrically charged particles, usually electrons. Since all materials contain electrically charged particles, any of them can interact with thermal radiation. However, these interactions differ from material to material. The electrons in some materials are extremely effective at absorbing and emitting thermal radiation and these materials appear black. When the sun's thermal radiation strikes a black material, that material absorbs the sunlight and nothing reflects. That's why the material appears black. When you heat a black material to high temperatures, it also emits thermal radiation extremely well—for example, a hot piece of black charcoal glows brightly with its own red thermal radiation.
Materials in which the electrons are not able to absorb or emit thermal radiation have one of several familiar characteristics. Some are clear, meaning that thermal radiation passes right through them. Others are white, meaning that thermal radiation that strikes them is scattered uniformly in all directions. Still others are mirror-like, meaning that thermal radiation that strikes them is reflected in specific directions. All of these materials are virtually unable to emit their own thermal radiation: clear glass, white sand, and mirror-like aluminum emit very little thermal radiation even when they're "red hot."
Since black objects are best at emitting and absorbing thermal radiation, they are best at transferring heat via radiation. A black object will receive more heat from the hotter sun than a white object of similar dimensions and temperature. A black object will also radiate more heat to its colder environment than a white object of similar dimensions and temperature, although here "black" and "white" refer to the object's behavior regarding its own thermal radiation. Near room temperature, thermal radiation is in the infrared, and many objects that appear white to visible light are actually rather black to infrared light.
There are two parts to this question: how does thermal energy (or heat) reach the food and what does that thermal energy do when it arrives. I'll start with the first part, but first let me define thermal energy as a form of energy associated with the random jittering about of the atoms and molecules in a material. The hotter a material is, the more average thermal kinetic energy (energy of motion) each atom has—in effect, the more vigorously the atoms and molecules jiggle. Thermal energy naturally tends to flow from hotter objects to colder objects, so that when you put cold food on a hot stove or in a hot oven, thermal energy will flow toward the food. This moving thermal energy is called heat.
There are three main mechanisms for heat transfer: conduction, convection, and radiation. Heat that flows via conduction is being passed from atom to atom inside a solid or liquid. In metals, conduction is greatly assisted by mobile electrons (the same electrons that allow metals to carry electricity) that carry heat between atoms far away from one another. Conduction is important on the stovetop, where the food touches the pot and the pot touches the hot stovetop. Heat that flows via convection is carried by a moving gas or liquid. Convection is important in an oven that's heated from below so that hot air rises to touch the food. Heat that flows via radiation is carried by electromagnetic waves (forms of light). Radiation is important in an oven that's heated from above (as in a broiler) so that thermal radiation travels downward to the food's surface.
Once the heat arrives at the food, it raises the food's temperature. As the food becomes hotter, chemical reactions begin to occur and molecules begin to change shape. Thermal energy makes it possible for chemical bonds within and between the molecules to come apart so that new bonds and new molecules can form. Water and other small molecules evaporate more and more rapidly until the water begins to boil. Sugar molecules rearrange to form caramels and carbon. Protein molecules rearrange and stiffen. These molecular changes, together with the increased temperature of the food, are what we associate with cooking.
In electric insulators, heat is carried by motions of the atoms themselves. You can think of this heat transfer as a bucket-brigade process—one atom jiggles its neighbor, which in turn jiggles its neighbor, and so on. If one end of an insulator is hotter than the other, this jiggling effect will gradually transfer thermal energy from the hotter end (more vigorous jiggling) to the colder end (less vigorous jiggling). Imperfections and weaknesses in most electric insulators make them relatively poor conductors of heat, although there are a few exceptional materials such as diamond that use the bucket-brigade mechanism very effectively and are excellent thermal conductors. In electric conductors, mobile electrons help out by carrying thermal energy from one atom to another over long distances. Even in a material that doesn't make good use of the bucket-brigade mechanism, the mobile electrons provide substantial thermal conductivity. Thus good electric conductors, such as copper, silver, and aluminum, are also good thermal conductors.
In a steam heating system, steam rises upward from a boiler in the basement and condenses in the radiators. As the steam transforms into water, it releases an enormous amount of heat and this heat is transferred to the air in the rooms. The condensed water than descends back to the boiler to be reheated. The beauty of this system is that the rising steam and the descending water can both pass through the same pipes, propelled by gravity alone. The low-density steam is lifted upward by the high-density water.
However, there are a few potential problems with this system. If there is air trapped in the pipes, the steam will have trouble reaching the radiators. Even though steam is lighter than air, it will diffuse slowly through the trapped air. That's why each steam radiator has a small bleeder valve. When the steam pressure exceeds atmospheric pressure, it should push the air in the pipes out the bleeder valves of the radiators. You ought to be able to hear the air leaving and the valves may continue to sputter a bit even when the pipes and radiators are essentially full of steam. I suspect that the bleeder valves on your upstairs radiators aren't functioning well so that steam isn't reaching them.
The wax molecules in the candle react with oxygen in the candle flame and are converted into water molecules and carbon dioxide molecules. That reaction is associated with combustion and it releases energy so that the candle produces light and heat. The molecules formed by this combustion drift off into the air.
Normal candle wax (paraffin wax) consists of relatively large hydrocarbon molecules. Each molecule in paraffin is a chain of between 30 and 50 carbon atoms that are surrounded by hydrogen atoms. Because its molecules are fairly long and they stick together reasonably well, paraffin is a firm, crystalline solid. If the chains were shorter, say 20 to 30 carbon atoms long, the material would be softer—it would be a liquid-like wax known as petroleum jelly. If the chains were much longer, say 2000 to 3000 carbon atoms long, the material would be firmer—it would be a solid known as polyethylene. Still shorter chains are used in machine oil, diesel fuel, unrefined gasoline, and finally petroleum gases such as propane and methane. The shorter the chain, the softer, thinner, and more volatile the hydrocarbon is at any given temperature. All of these hydrocarbon molecules can burn completely, leaving only water molecules and carbon dioxide. In a candle, the heat of the flame vaporizes the wax molecules—they become a gas—and they then burn completely in the flame itself. As long as the wax doesn't drip away from the flame, the flame will consume it all completely and leave no ash or wax. Although the structure of the molecules in beeswax is slightly different from that in paraffin, beeswax also vaporizes from the heat of the flame and then burns completely.
Heat is thermal energy that's flowing from one object to another because of a temperature difference between those two objects. Whenever an object contains thermal energy—which it always does—the atoms and molecules in that object are jittering about microscopically. Each atom or molecule isn't completely stationary; instead it is vibrating back and forth, and pushing or pulling on its neighbors. The object's thermal energy is the sum of the tiny kinetic and potential energies of those atoms and molecules as they move back and forth (kinetic energy), and push or pull on one another (potential energy). The hotter an object is, the more thermal energy each of its atoms has, on average, so this thermal energy tends to flow to a colder object when you touch the two objects together. When that thermal energy is flowing from the hotter object to the colder object, we call it "heat."
Fire is a chemical reaction in which a combustible fuel reacts with oxygen to release large amounts of thermal energy. Many atoms bind very strongly with oxygen atoms and these fuel atoms release energy when they bind with oxygen. Initiating these combustion reactions normally requires some thermal energy to get started. This starting energy is known as activation energy. That's why you have light the fire—you must provide the activation energy. After that, each oxidization reaction produces the activation energy needed to start another oxidization reaction and the fire keeps itself going until it has consumed all of its fuel.
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