. How does a catalytic converter help emissions in a car? — JAM, Littleton, Colorado
While the burned gases that emerge from an ideal car engine would consist only of water vapor, carbon dioxide, and nitrogen gas, a real car engine is far from ideal. In addition to these gases, a real engine emits nitrogen oxides, carbon monoxide, and various unburned hydrocarbons left over from the gasoline. Because these gases are major contributors to urban smog, car manufacturers have been forced to reduce them in various ways.
One of the most effective tools for eliminating the unburned hydrocarbons and carbon monoxide is a catalytic converter. It is essentially a pipe containing a ceramic honeycomb on which there are countless tiny particles of platinum and palladium. As the unwanted molecules pass through the honeycomb, they land on the metal particles briefly and are combined with oxygen atoms to form water vapor and carbon dioxide. The catalytic converter is burning these molecules in a controlled way, with the precious metal particles acting as catalysts to assist the burning process.
Like all catalysts, these particles are not consumed in the process of burning the gases, but they can easily be contaminated. That's why it's so important not to put leaded gasoline in a car with a catalytic converter—one tank of leaded gas is all it takes to lead-coat the tiny platinum and palladium particles and to render them useless. Another interesting note is that the catalytic converter is usually located on the underside of the car, protected only by a thin metal shield. The converter becomes very hot in operation, both because hot exhaust gas is passing through it and because the controlled combustion taking place inside it heats it up. Don't park a car with a catalytic converter over a pile of leaves! Many an autumn car fire has started when a hot catalytic converter ignited the pile of leaves beneath it.
. Why can we see through glass and some minerals? — KH, Newport Beach, CA
Light consists of electromagnetic waves—fluctuating electric and magnetic fields that travel through space at enormous speeds. As light passes through an insulator such as glass, diamond, quartz, or salt, the light's fluctuating electric and magnetic fields cause electric charges in the insulator to vibrate back and forth. This interaction between light and the charged particles in a material is the first step in absorption—the material is "trying" to absorb the light. But light carries energy with it and any material that absorbs light must be prepared to accept the light's energy. The charged particles in insulators generally have no quantum states that allow them to accept that light energy. As a result, the insulator's charged particles respond to the light as it passes, but they can't actually absorb the light. The light simply passes through the insulator. However, the light is delayed by its interaction with the charged particles in the insulator (the speed of light in a material is less than the speed of light in vacuum) and the light may be redirected (reflected or scattered) by encounters with inhomogeneities. So glass and the other insulators don't absorb light and are often transparent. Those that aren't transparent are usually white—they scatter light in all directions.
. I understand how computer information travels as bits from source to destination, but how does each bit travel through the wire at a molecular level? — JCC, Atlanta, Georgia
At the simplest level, a bit travels as a packet of positive or negative charge through a wire. To start this movement, the source injects a small amount of charge onto the end of the wire. Since like charges repel and opposite charges attract, this new charge pushes on charges further down the wire, and those charges push on charges still farther down the wire, and so on. Overall, a wave of forces and responses rushes along the wire until it reaches the destination end of the wire. There charges flow off the wire and into the destination device. While these charges aren't really the same ones that were put on the wire by the source, they have the same charge and one can imagine that charge has simply moved from the source device to the destination device by way of the wire. The destination device can examine this charge to determine whether the source was sending a 0 or a 1.
. It is difficult for me to understand current flowing from a battery through a circuit. A battery has both a positive end and a negative end. Which direction does the current flow? — SK
When you connect a battery in a circuit, negatively charged electrons flow away from the battery's negative end and they return toward the battery's positive end. The battery then pumps the electrons back to its negatively charged end and they begin the journey all over again (hence the name "circuit"). But because the electrons have a negative charge, current does not flow in their direction. Instead, current is defined as flowing in the direction of positive charge flow. In the present case, current flows from the battery's positive end, through the circuit, and back to the battery's negative end. Current is thus flowing in the direction opposite to the direction of electron movement! If you want to know which way current is flowing, you can normally find the direction in which electrons are flowing and then reverse it. Life for physicists and electrical engineers would be so much simpler if Benjamin Franklin hadn't made an unfortunate choice that gave electrons—the principal carriers of electricity—a negative electric charge. We have been living with the consequences of that choice ever since.
. Why is a rainbow in an arch? Does it have something to do with an equal distance from me to the raindrops and if so, is the arc really a parabola? — MM, Seattle, WA
A rainbow is truly circular, not parabolic. Passing through the exact center of that circle is the line that runs between the sun and your head. Each colored arc of the rainbow is located at a particular angle away from this line—the red arc is farther from the line than the violet arc is.
. I can understand that the strings of bubbles from the side of a glass of champagne are due to nucleating dirt or other imperfections in the glass surface, but what causes those strings of bubbles in the center of the fluid? They are quite persistent. Are they just dust? — BM, Tehachapi, CA
If there were no impurities or imperfections in a glass of champagne, bubbles would only form through statistical fluctuations—random effects would occasionally bring enough gas molecules together to form (nucleate) a bubble and that bubble would grow and rise to the surface. But such spontaneously nucleated bubbles are extremely rare and form randomly throughout the fluid, rather than in chains of steady bubbles. In fact, bubbles would be so rare in this impurity-free liquid that you would probably not even notice them—the champagne would slowly go flat by losing gas molecules from its surface alone.
In real champagne, chains of bubbles do rise upward from the center of the fluid. These bubbles are clearly forming at suspended impurities. All it takes is a tiny piece of dust to trigger bubble formation. If you swirl the champagne slightly, you should be able to see these suspended chains of bubbles move, indicating that the impurities that are triggering them are also moving with the fluid.
. What changes occur to wood when it is permanently bent with the aid of steam? — MH, Perth, West Australia
The main structural component of wood is cellulose, a polymer (plastic) consisting of long molecular chains of sugars. While cellulose is extremely useful and is by far the most common polymer/plastic in the world, it can't be melted because the temperature at which its molecular chains begin to move relative to one another is above the temperature at which those molecular chains begin to fall apart. In short, cellulose decomposes before it melts. Shaping or reshaping cellulose is very difficult, though chemical processes have made it possible to reform cellulose into such materials as cellophane and rayon.
The process you describe, bending wood while heating the wood with steam, takes advantage of the fact that cellulose molecules bind strongly to water molecules and that the water molecules then lubricate the chains so that they can move relative to one another. Water is said to be a "plasticizer" for cellulose. Heat, water, and stress allow the cellulose chains to slide slowly across one another. With enough patience, the wood's internal structure can be changed forever. When the heat, water, and stress are then removed, the wood keeps its new shape.
. I think that the speed of light could be broken by turning a very long lever. If the lever is long enough and you have enough power to turn it, the end of the lever will travel faster than the speed of light. Is this so? — NL, Hong Kong
I'm afraid that this technique won't work—the torque you would have to exert on the lever to make its end approach the speed of light would become infinite and the energy you would have to transfer to the lever would also become infinite. The Newtonian laws of motion aren't accurate at such high speeds and the full relativistic laws are required. With this shift to relativistic motion come changes in the relationship between force and acceleration, and between torque and angular acceleration. The faster the end of the lever moves, the harder it is to increase its speed any further. As the lever tip approaches the speed of light, it becomes essentially impossible to make it move faster.
As if this problem weren't enough, there is another problem: if you aren't extremely patient, the lever will bend as you turn it, forming a spiral rather than a long arm that sweeps through space. That's because the lever is kept straight by internal forces. While you are twisting the lever to make it turn faster, you are unbalancing these internal forces and causing the lever to bend. The long lever you describe will actually curl into a spiral and its end speed will never come close to the speed of light.
. When I buy a role of undeveloped film, it has a particular weight. After I have taken a picture but before I develop the film, does it weigh more or less? Does it matter what I take a picture of? — CV, Warrenville, IL
I think that a small number of atoms leave the film when it's exposed to light, so your exposed film probably weighs less than it did when you bought it. That's because light causes charge transfers within the grains of silver salts, changing silver-halide molecules into silver atoms and halogen atoms, and the halogen atoms probably leave the film or allow other atoms to leave instead. The silver atoms remain in the film, where clusters of three or four of them form the latent image—a cluster triggers the complete conversion of a silver-halide grain into silver during the development process. But the halogen atoms don't remain in the silver-halide grains. While it's possible that these halogen atoms are stabilized in the emulsion, so that the emulsion's weight remains constant, my guess is that they either diffuse out of the film or displace other atoms in the emulsion. Those displaced atoms would then leave the emulsion. Overall, I suspect that atoms leave the film when it's exposed and that the film becomes ever-so-slightly lighter.
I should point out, however, that the energy absorbed by the film does have a weight and that if the only effect of exposing film to light were that the film absorbed this additional energy, then the film's weight would increase by a fantastically small amount. But the chemistry that results from this energy absorption certainly swamps the weight of the light energy.
. Does it make sense to raise the thermostat setting on your air conditioner when you leave your house, since when you come back, you have to lower it again and the unit has to work more? Are there any energy savings? — AN, Herndon, VA
You will save energy and money by raising the thermostat setting when you leave your home and then lower it again when you return. That's because the rate at which heat flows into your home from outside is roughly proportional to the difference between the indoor and outdoor temperatures. By letting the indoor temperature rise, you slow the heat flow into your home. With less heat flowing into your home, the air conditioner doesn't have to pump as much heat outside and that saves energy. Moreover, an air conditioner is more energy efficient when the indoor temperature is closer to the outdoor temperature, so letting the indoor air warm up saves even more energy. While the air conditioner does have to work steadily for a while when you return to your home, its efficiency is still good during that time and the energy saved while you were away more than makes up for the energy consumed when you return.