If you can't alter the air's humidity, warm air will definitely heat up your window faster and defrost it faster than cold air. The only problem with using hot air is that rapid heating can cause stresses on the window and its frame because the temperature will rise somewhat unevenly and lead to uneven thermal expansion. Such thermal stress can actually break the window, as a reader informed me recently: "On one of the coldest days of this Boston winter, I turned up the heat full blast to defrost the windshield. The outside of the window was still covered with ice, which I figured would melt from the heat. After about 10 minutes of heating, the windshield "popped" and a fracture about 8 inches long developed. The windshield replacement company said I would have to wait a day for service, since this happened to so many people over the cold evening that they were completely booked." If you're nervous about breaking the windshield, use cooler air.

About the humidity caveat: if you can blow dry air across your windshield, that will defrost it faster than just about anything else, even if that air is cold. The water molecules on your windshield are constantly shifting back and forth between the solid phase (ice) and the gaseous phase (steam or water vapor). Heating the ice will help more water molecules leave the ice for the water vapor, but dropping the density of the water vapor will reduce the number of water molecules leaving the water vapor for the ice. Either way, the ice decreases and the water vapor increases. Since you car's air condition begins drying the air much soon after you start the car than its heater begins warming the air, many modern cars concentrate first on drying the air rather than on heating it.

Heat pipes use evaporation and condensation to move heat quickly from one place to another. A typical heat pipe is a sealed tube containing a liquid and a wick. The wick extends from one end of the tube to the other and is made of a material that attracts the liquid--the liquid "wets" the wick. The liquid is called the "working fluid" and is chosen so that it tends to be a liquid the temperature of the colder end of the pipe and tends to be a gas at the temperature of the hotter end of the pipe. Air is removed from the pipe so the only gas it contains is the gaseous form of the working fluid.

The pipe functions by evaporating the liquid working fluid into gas at its hotter end and allowing that gaseous working fluid to condense back into a liquid at its colder end. Since it takes thermal energy to convert a liquid to a gas, heat is absorbed at the hotter end. And because a gas gives up thermal energy when it converts from a gas to a liquid, heat is released at the colder end.

After a brief start-up period, the heat pipe functions smoothly as a rapid conveyor of heat. The working fluid cycles around the pipe, evaporating from the wick at the hot end of the pipe, traveling as a gas to the cold end of the pipe, condensing on the wick, and then traveling as a liquid to the hot end of the pipe.

Near room temperature, heat pipes use working fluids such as HFCs (hydrofluorocarbons, the replacements for Freons), ammonia, or even water. At elevated temperatures, heat pipes often use liquid metals such as sodium.

Heat is thermal energy that is flowing from one object to another. While several centuries ago, people thought heat was a fluid, which they named "caloric," we now know that it is simply energy that is being transferred. Heat moves via several mechanisms, including conduction, convection, and radiation. Conduction is the easiest to visualize--the more rapidly jittering atoms and molecules in a hotter object will transfer some of their energy to the more slowly jittering atoms in molecules in a colder object when you touch the two objects together. Even though no atoms or molecules are exchanged, their energy is. In convection, moving fluid carries thermal energy along with it from one object to another. In this case, there is material exchanged although usually only temporarily. In radiation, the atoms and molecules exchange energy by sending thermal radiation back and forth. Thermal radiation is electromagnetic waves and includes infrared light. A hotter object sends more infrared light toward a colder object than vice versa, so the hotter object gives up thermal energy to the colder object.

A hot water heater is built so that hot water is drawn out of its top and cold water enters it at its bottom. Since hot water is less dense than cold water, the hot water floats on the cold water and they don't mix significantly. As you take your shower, you slowly deplete the hot water at the top of the tank and the level of cold water rises upward. But the shower doesn't turn cold until almost all the hot water has left the tank and the cold water level has risen to its top.

You're right. The greater the temperature difference between two objects, the faster heat flows between them. This effect is useful whenever you forget to chill drinks for a party. Just don't leave a glass bottle in the freezer too long; if the water inside freezes, it may expand enough to break the bottle.

A battery uses electrochemical processes to provide power to a current passing it. This statement means that if you send an electric charge through the battery in the normal direction, that charge will emerge from the battery with more energy than it had when it entered the battery. But while it might seem that the number of electric charges passing through the battery each second doesn't matter--that each charge will pick up the usual amount of extra energy during its passage--that's not always the case. To understand this fact, let's look at how charges "pass through" the battery and how they pick up energy.

What's really happening is that electrochemical processes are spontaneously separating charges from one another inside the battery and placing those separated charges on the battery's terminals--the battery's negative terminal becomes negatively charged and its positive terminal becomes positively charged. This charge separating process proceeds in a random, statistical manner until enough charges accumulate on the terminals to prevent any further charge separation. Because like charges repel one another, sufficiently large accumulations of positive charges on the positive terminal and negative charges on the negative terminal stop further arrivals of those charges.

But when you send a positive charge through a wire and onto the battery's negative terminal, you reduce the amount of negative charge there and weaken the repulsive forces. As a result, the chemicals in the battery separate another pair of charges. The battery's negative terminal returns to normal, but now there is an extra positive charge on the battery's positive terminal. This extra charge flows away through a wire. Overall, it appears that your positive charge "passed through" the battery--entering the battery's negative terminal and emerging from the positive terminal with more energy than it had when it arrived at the negative terminal. But what really happened was that the battery's chemicals separated another pair of charges.

In a warm environment, the battery's chemicals can separate charges rapidly and can keep up with reasonably large currents of arriving charges. But in a cold battery, the electrochemical processes slow down and it becomes hard for the battery to keep up. If you try to send too much current through the battery while it's cold, it is unable to replace the charges on its terminals quickly enough and it voltage sags--it doesn't have enough separated charges on its terminals to give the charges "passing through" it their full increase in energy. If you use a battery while it's very cold, you should be careful not to send too much current through it because it will become inefficient and will provide less than its usual voltage.

That energy becomes thermal energy in the metal/acid solution. Before the spring dissolves, the energy it stores is actually found in the forces between adjacent metal atoms. The crystals in the metal are slightly distorted, bringing the atoms in these crystals a little too close or a little too far from one another. Since each of these displaced atoms has a little extra potential energy, it is a little more chemically reactive than normal. When the acid attacks one of these atoms and pulls it away from the crystal, the atom comes away a little more easily than normal because it brings with it a little extra energy. This extra energy enters the solution, making the solution a little warmer than it would have become had the spring not been compressed.

Convection is the transfer of heat by a circulating fluid, such as air or water. This heat is carried from a hotter object to a colder object. The fluid first passes near the hotter object and receives heat. The fluid becomes warmer and more buoyant, and it's lifted upward by the colder fluid around it--just as a hot air balloon is lifted upward by the colder air around it. The rising fluid carries the heat with it. Eventually the rising fluid spreads outward and it pass near colder objects, giving up its heat. The fluid becomes cooler and less buoyant, and soon it begins to descend back toward the ground. Eventually it's drawn back past the hotter object and this cycle begins again.

The answer is a qualified no. Heat always flows from hotter objects to colder objects, so the solid can't get any hotter than the flame that's heating it. But this observation is stems from the laws of thermodynamics, particularly the second law of thermodynamics. Unlike Newton's laws of motion, which are rigid, inviolable laws that are never, even violated in our universe, the second law of thermodynamics is a statistical laws--it says that certain events are extremely unlikely but doesn't say that they are truly impossible. The flow of heat from hotter to colder is a statistical law, not a rigid mechanical law. So it is possible, although extraordinarily unlikely, that heat can flow from the 1770° C flame to the 1799° C solid and warm that solid all the way to 1800° C. However, for any reasonable sized solid (say, more than 10 atoms), the possibility of this occurring is going to be so unbelievably small as to be ridiculous. It's as unlikely as taken a crystal wineglass that has been crushed into dust and then dropping it on the floor and having the impact reassemble the wineglass into its original pristine form. The laws of motion don't forbid such as fantastic result, but it sure would be unlikely. I've tried it several times myself, without success. But then, you're not going to be able to melt your solid with a not-hot-enough flame, either. You'd have to wait a few ages of the universe just to have that solid climb a tiny fraction of a degree above the temperature of the flame. For 20 degrees... forget it.

If the title of the book "Fahrenheit 451" is correct, then the temperature is 451° F (233° C). Actually, I'm sure that the ignition temperature depends on the exact type of paper.

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.

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."

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.

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.

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.

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.

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.

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.

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.

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.

In a sense, water is the "ash" that forms when hydrogen burns in oxygen. Like all fully burned materials, water can't burn any further. When you put cold water on a fire, it extracts heat from the fire because the water is much colder than the fire and heat naturally flows from hotter objects to colder objects. Since heating the water doesn't cause the water to burn (it can't burn), the heat that's lost by the fire doesn't create new fire (as would be the case if you threw gasoline on the fire instead of water). So the water gradually cools down the fire until the fire no longer has enough thermal energy to sustain its own chemical reactions. The fire then goes out.

The Reaumur Scale was created in 1730 by French scientist Rene-Antoine Ferchault de Reaumur, who set 0 R as the freezing point of water and 80 R as the boiling point of water. Though in common use for a time, the Reaumur Scale had more or less disappeared by the end of the eighteenth century. Each degree R is equal to 5/4 of a degree C, so T(C)=T(R)*5/4. Similarly, T(F)=T(R)*9/4+32 and T(K)=T(R)*5/4+273.15.

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.