Category: wood stoves

Is it better to use warm or cold air to defrost your windshield?

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.

How does a heat pipe work? — SG, Sugar Land TX

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.

What is heat? What actually flows from a hot body to a cold body? — AW, Pakistan

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.

Since cold water is drawn into a hot water heater at the same time that hot water is being drawn out, why doesn’t the water turn cold soon after you start taking a hot shower? — NG, Golden, Colorado

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.

If I have two glass containers with equal amounts of water both at the same temperature (say 80° F), and put one in the refrigerator and one in the freezer, which container will cool to 40° F first? Because the freezer is colder, I would guess the freezer. — JL, Eagan, MN

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.

Why don’t batteries work as well in cold environments? — KS

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.

If you take a compressed metal spring and place it in a container of metal dissolving acid, what happens to the energy in the spring assuming the entire spring dissolves at one time? — BR, Mount Pleasant, SC

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.

What is convection? — DB, Corona, CA

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.

Can an object be heated no hotter than the temperature of the flame beneath it? For example, if the temperature of a candle flame is 1770° C and the melting point of the solid being heated above it is 1800° C, would the solid ever melt if the flame were held under it long enough? — MR, Ohio

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.

At what temperature does paper burn? — KR

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.