. How does a fax machine send written words over telephone wires? — AM, Halifax, CA
The fax machine uses a row of optical sensors to detect dark and light spots on the original document. It scans the document one line at a time and enters the pattern of dark and light spots into a digital controller or simple computer. The controller or computer than encodes this pattern, together with enough information to correct minor transmission errors if they occur, as a series of numbers. The numbers are then sent through the telephone system in much the same way that computer information is sent through the telephone wires by a modem. The numbers becomes specific patterns of tones and volumes. While the electric currents flowing through the telephone system are meant to represent voice sounds, they can do a moderately good job of representing numbers instead. Because of various limitations on the currents that the phone wires can carry well, the fax system can only so much information each second. The receiving fax machine analyzes the tones and volumes it receives over the telephone wires and recreates the pattern of dark and light spots. It then uses one of several printing techniques to reproduce that pattern on a piece of paper. It recreates the document one line at a time.
. You mentioned that time perception is different for different locations in the universe. Were could we find a place where one day is equal to one thousand years of time on earth? — AWG, Karachi, Pakistan
The perception of time is different for observers who are in motion relative to one another. The issue is not how far away they, it's how fast they are moving relative to one another. If you were to observe a person who is traveling past the earth at almost the speed of light, you would notice that their watch is running extremely slowly. It might be as though you'd have to wait one thousand years for their watch to show that a day has passed for them. Yet paradoxically, they would make the same observation about you! You would see them aging slowly and they would see you aging slowly! The resolution to this apparent paradox lies in the differences in the perceptions of space that these differences in the perceptions of time. In this short answer, I can hardly begin to resolve the paradox. I'll simply point out that the mixing of space and time associated relativity are caused by relative motion not by relative position.
. How do balloon pilots navigate around countries that forbid overflights? — LS, Ashland, OR
The speeds and directions of the winds vary considerably with altitude. For example, while surface winds near the sea blow toward shore on a hot summer day, high-altitude winds blow away from the shore at the same time, completing a huge circulation loop. Unlike a sailboat, which is at the mercy of the surface winds, a balloonist can adjust the balloon's altitude to search for winds heading in the desired direction. The balloonist makes these altitude adjustments by changing the balloon's weight and volume so that it sinks or rises.
. How does desiccant absorb and hold water? — JP, Houston, TX
Water molecules from the air are continuously colliding with surfaces and sometimes one of those water molecules will stay attached to a surface for some amount of time. That water molecule forms a weak chemical bond with the surface and remains there until thermal energy knocks it back into the air. As a result of this occasional sticking, most surfaces have a thin layer of water molecules on them. Desiccants are materials that tend to keep those water molecules for a relatively long time and that have lots of surface area on which those water molecules can stick. However, the strongest desiccants react chemically with water molecules so that those water molecules essentially never leave.
. What effect does ice have on potholes? - AH
Water and ice are major contributors to potholes. When water flows into cracks in the road and then freezes, it tears the roadway apart. That's because ice takes up more room than the water from which it's formed—ice is less dense than water. Since the water expands as it freezes, it enlarges the cracks that contain it and gradually breaks up the roadway.
. Do regular magnets lose their magnetism or do they stay magnetized always? What about electric magnets, like the ones used in wrecking yards? — KM, Delta, British Columbia
Permanent magnets are made from materials with two important magnetic characteristics. First, these materials are intrinsically magnetic, meaning that some of the electrons in these materials retain their natural magnetism. While electrons are always magnetic, that magnetism is lost in most materials because of complete cancellations—each magnetic electron is paired with another magnetic electron so that they cancel one another perfectly. However, there are some materials in which the cancellation is imperfect and these materials (including iron, cobalt, nickel, and many steels) are the basis for most permanent magnets.
Second, the materials used in permanent magnets have internal structures that make the magnetic electrons align along particular directions. Once the electrons are aligned along one of those directions, they stay aligned and the material exhibits strong magnetic characteristics. It becomes a "permanent magnet."
A permanent magnet remains its magnetization as long as nothing spoils the alignments of its magnetic electrons. These electrons can be knocked out of alignment by vibrations, heat, or other magnets. If you hit a permanent magnet with a hammer or heat it in the oven, you will change and perhaps destroy its magnetization. This magnetization can be recovered by exposing the permanent magnet to the magnetic influences of an electric current. In fact, permanent magnets are originally magnetized by placing them near electric currents that align their magnetic electrons. Moreover, even a material that doesn't have the internal structures needed to keep its electrons aligned along a particular direction will become magnetized temporarily by placing it near an electric current. That's how a wrecking yard magnet works-an electric current temporarily turns a large piece of iron into a strong magnet.
. Magnets stick to metal, but can you make a magnet repel metal? - M
Yes, but not in the way you're thinking of. When you bring a magnet near a piece of steel, the intrinsic magnetic character of that steel causes it to become magnetic in such a way that it attracts the magnet. There is no way for the steel, or another similar metal, to become magnetic in such a way that it would repel the magnet.
However, if the metal is already magnetized it can repel an approaching magnet. A more interesting case is when a magnet approaches a normally non-magnetic metal at high speeds; in which case electric currents begin to flow through the metal and these currents do repel the approaching magnet.
. How does a crystal radio work?
A crystal radio uses a crystal diode to detect tiny fluctuating currents in its antenna system. When a radio wave passes across an antenna, the wave's electric field pushes electric charges up and down the antenna. The crystal diode acts as a one-way gate that allows some of this moving charge to flow onto another wire and then prevents it from returning to the antenna. Since the charge can't return to the antenna, it flows elsewhere—passing through a sensitive earphone and creating sound. An AM radio station encodes sound as changes in the intensity (or amplitude) of the radio wave. As the radio wave's intensity fluctuates, the amount of electric charge flowing through the earpiece of the crystal radio also fluctuates and you hear sound.
. In cooking, what are some examples of absorbing microwaves, transferring microwaves, and reflecting microwaves? - K
In a microwave oven, water-containing foods absorbs microwaves. The microwaves disappear as they pass through the food and the food becomes hotter. Microwaves are transferred from the small antenna near the magnetron to the cooking chamber by sending those microwaves through a metal pipe. This rectangular pipe is typically a few inches wide and an inch or so tall, and is called a "wave guide." Finally, the walls of the cooking chamber reflect the microwaves. When a microwave encounters a metal surface, it pushes electric charges back and forth in the metal and this moving charge causes the microwave to reflect.
. In your explanation of why microwaves don't penetrate the oven door, you said it is because the holes in the screen are smaller than the wavelength of a microwave. Wouldn't it be the amplitude of the wave and not its wavelength? - P
When a microwave tries to pass through the holes in the metal screen, electric charges in that screen begin to move. The microwave's electric field fluctuates back and forth rapidly and the charges reverse directions rapidly as a result. If the electric current made up of these charges has enough time to travel all the way around each hole before it reverses directions, it will be as though the screen were made of solid metal and the screen will be able to completely reflect the microwave.
Like any electromagnetic wave, a microwave has a wavelength (the spatial distance between adjacent wave crests) and a period (the temporal spacing between adjacent wave crests). The electric current that a microwave propels through a metal travels about one microwave wavelength during one microwave period. Therefore, the current can work its way around a hole in the metal only if the hole is significantly smaller than the microwave wavelength. The amplitude of the microwave doesn't matter—increasing the amplitude of the microwave just makes more current flow.