How Everything Works   Page 18 of 160 (1595 Questions and Answers) Printer Friendly Version

171. If a flame always burns up, if you are in a weightless environment, how will the flame burn?
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.

172. If lighter colors reflect more light, then why is it easier for a pale person to sunburn than someone with a darker skin tint?
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.

173. In fast food restaurants, when they keep your hamburger warm under lights, is that heat from convection or radiation?
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.

174. What is fire?
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.

175. Why doesn't glass have electrons to carry heat. What is glass made of?
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.

176. How does wearing a hat keep you warm (or cool)?

177. If metal is a conductor of heat, why is it that aluminum foil will insulate food and reflect heat?
Aluminum may be a good conductor of heat, but its a terrible emitter or absorber of thermal radiation. When you wrap food in aluminum foil, you dramatically reduce that food's ability to lose heat via radiation if it's hotter than its surroundings or its ability to gain heat via radiation if it's colder than its surroundings. Aluminum foil doesn't have much effect on heat transferred to or from the food via conduction or convection because aluminum itself is a good conductor of heat.

178. At what point is it more efficient to leave a light on when leaving and the returning to a room?
Since turning an incandescent bulb on and off doesn't shorten the life of its filament significantly, you do well to turn it off whenever possible. The same isn't true of a fluorescent tube—turning it on ages its filaments significantly (due to sputtering processes) so you shouldn't turn a fluorescent lamp off if you plan to restart it in less than about 1 minute.

179. Can I produce light without using electric power?
Since light carries energy with it, something must provide that energy. However, the energy doesn't have to come from electric power. Since objects emit visible thermal radiation when they have temperatures above about 500 C, anything that heats an object to high temperatures will make light. But light can also be made without heat. There are many ways to convert electric energy into light without making anything hot (for example, a neon sign or a light-emitting diode). But you ask about making light with electricity. The next best choice is light-emitting chemical reactions, such as those used in light sticks (liquid-filled plastic sticks that you bend to activate and which then glow bright green for about 12 hours). However, such reactions don't produce all that much light and they consume the chemicals fairly quickly. If you are trying to produce large amounts of light without electric power, I'm afraid that you'll have to burn sometime. That's what people did before 1879 and the electric lamp.

180. How does a heat-seeking missile and a radar-homing missile work?
A heat-seeking missile studies the infrared light coming toward it from the sky in front of it. It uses a lens to form a real image of that light on an array of infrared sensors. If there is a hot object in front of the missile, that object will emit more infrared light than its surroundings and the missile's lens will form a bright image of the hot object on one of the infrared sensors. If the bright image falls on the central sensor, the missile will do nothing—it will flight straight ahead. But if the bright image falls on one of the side sensors, the missile will turn. It will turn by deflecting its rocket exhaust so that the missile begins to rotate in flight. As the missile rotates, the image of the hot object will move from one sensor to the next and it will eventually fall on the central sensor. At that point, the missile will stop turning and will flight straight ahead. Since the missile automatically turns to head toward the hot object, it will eventually fly right into the hot object and explode. A radar-seeking missile will do that same things, except that it will look for an object that is emitting lots of microwaves (radar), rather than lots of infrared light. A radar-guided missile is much more complicated, since it must first emit a burst of microwaves and then analyze the reflected microwaves to look for something to fly toward. Many laser-guided missiles are just like heat-seeking missiles except that they look for an object that is reflecting a laser beam. The people who fire the missile simply illuminate the target with a bright laser beam and the missile flies directly toward the laser spot on the target.

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