Like everything in our world, glass does have electrons. Its atoms are built out of electrons. But those electrons are localized on the individual atoms or between them in such a way that they can’t move easily. When you try to push these electrons through the glass, they won’t go. Thus neither heat nor electricity flows easily through glass. In a metal, some of the electrons are mobile and can carry heat and electricity.
The fire of a burning candle begins with vaporized wax. Heat from the flame melts wax, which then flows up the wick because of its attraction to the fibers. The wax then becomes so hot that it turns into a gas and this gas mixes with air at the bottom of the flame. When the temperature becomes high enough, the wax molecules begin to decompose into fragments that react chemically with oxygen molecules. Water and carbon dioxide molecules are produced in the reaction and chemical potential energy is released as thermal energy. This thermal energy provides the candle’s light and also the heat needed to sustain the combustion. The glow that the candle emits comes primarily from hot particles of carbon in the flame. These particles emit thermal radiation with a color spectrum that is characteristic of the flame’s temperature.
Radiation. Convection will take the heat up toward the ceiling rather than down toward the food. But radiation travels in straight lines, from the lamps to the burgers below them.
The colors that you see are determined by the visible light absorbed by a surface. Thus, while the whitest skin reflects most visible light and appears white, it does absorb light that you can’t see: ultraviolet light. This ultraviolet light is what damages the skin and causes sunburn. Darker skin absorbs most of the ultraviolet before it gets to sensitive skin cells while lighter skin lets that ultraviolet in far enough to cause injury.
A flame should have serious problems in a weightless environment because it normally uses convection to carry burned gas away and to bring fresh air in. Since convection depends on gravity, there will be no tendency for the burned gas to leave and fresh air to replace it.
I talked with Kathryn Thornton, a former NASA astronaut who has actually performed combustion experiments in space and she described those experiments to me. In them, a drop of fuel was supported on a fiber and ignited. The flame front radiated outward from the fuel drop at ignition to form a spherical shell around the drop, which shrank slowly as it was consumed. Because convection requires gravity, there was no rising current of air to bring in new oxygen and to sweep away the burned gases. Instead, oxygen had to diffuse into the burning sphere and it did so quite slowly—the burns lasted for as much as 30 seconds on only a few cc’s of fuel. Water vapor that formed during the combustion also had a tendency to diffuse into the fuel and dilute it so that it eventually stopped burning.
A home steam heating system consists of a boiler, pipes, and radiators. The boiler is located in the basement and uses a burning fuel or electricity to heat water until it boils. Steam forms as the water boils and this steam accumulates above the liquid water. Steam isn’t the mist that forms above a teapot—that’s really just droplets of water. Steam itself is the clear gaseous form of water and it travels upward through the pipes to radiators in the rooms. Steam is actually a lighter-than-air gas and it’s lifted upward by the same buoyant force that makes helium float. When the steam arrives inside the radiators, it begins to condense back into liquid water. As it does so, it releases an enormous amount of heat—the water molecules begin to stick to one another and they release chemical potential energy. After a short time, the temperature of the radiator rises until a balance is reached where the steam and the water are in equilibrium—typically about 100° Celsius, but dependent on the gas pressure inside the radiator. The hot radiator then heats the room. The water that forms as the steam condenses is carried by gravity back down the same pipe through which the steam arrived and returns to the boiler to be reheated.
In an electric convection oven, a fan circulates air rapidly through the cooking chamber. This rapid force circulation of air has two principal effects. First, it ensures that the temperatures throughout the oven are almost exactly equal. In a normal electric oven, the differences in temperatures that often occur lead to uneven cooking and require that you put the food in specific areas of those ovens to make sure that the food cooks properly. Since a convection oven has no temperature differences, you can put the food anywhere and you can fill the cooking chamber more completely with food. Second, a convection oven transfers heat more evenly to the food. By blowing hot air past the food, the oven prevents regions of colder air from building up near the surfaces of cool foods. Since the food in a convection oven is always in contact with hot air, it picks up heat faster and cooks faster. In a normal oven, heat is transferred to the food through normal convection (rising hot air and sinking cold air) and by radiation (particularly when the broiler is used). Both of these process are relatively slow and can be interrupted by over-filling the oven or blocking the line of sight between the hot filament and the colder food. In a convection oven, heat is transferred to the food mostly by forced convection (fan-driven hot air that circulates rapidly through the oven). This process is relatively fast and can’t be interrupted by over-filling the oven (within reason) or blocking any line of sight between the hot filament and the food.
For a piece of fuel to burn, it needs a source of oxygen. In open space, there is no oxygen and thus no way for fuel alone to burn. However, materials such as gunpowder that contain both a fuel and an oxidizing agent can burn in open space. In fact, because they don’t rely on convection to bring new oxygen into the flame and to carry the burned gases away from the flame, such materials burn almost exactly the same way in space as they do on earth.
When you pull on the string with a 10 N force, you create 10 N of tension in that string. If there is less tension anywhere in the string, then that portion of the string will accelerate toward the side with more tension. That’s why the tension in each string of a multiple pulley is 10 N when you pull on its loose end with a force of 10 N. The 5 strings are really just parts of the same string and that string has to have 10 N of tension in it.
As you have noticed, buses, trucks, and trains often use air as the hydraulic fluid in their braking systems. That’s because air is cheap and non-toxic, so that spilling it isn’t a problem. While air’s compressibility makes it a bit more complicated to work with than a liquid hydraulic fluid, it still works well in power braking systems.