. If air in a rigid 80 cubic foot scuba tank is pressurized to 3000 psi, giving the diver a certain amount of breathing time, then why does bottom time decrease with depth? I know about external pressure, but how does the pressure affect air inside
the tank? - RJ
The deeper a scuba diver goes, the greater the water pressure and the more the water presses in on the diver's chest. To be able to breathe, the air in the diver's mouth must have roughly the same pressure as the water around the diver's chest. That way, the diver will be able to use chest muscles to breathe the air into the diver's lungs. But the pressure of the air in the diver's mouth is proportional to its density and thus to the number of air molecules contained in each liter of air. At great depths, the diver must breathe dense, high-pressure air and this air contains a great many air molecules per liter. Since the scuba tank contains only so many air molecules, these molecules are consumed more rapidly at great depths than they are at shallow depths. The scuba regulator automatically controls the density of air entering the diver's mouth so that the air pressure is equal to the surrounding water pressure. That way, the air is easy to breathe. The deeper the diver goes, the more air molecules the regulator releases into each of the diver's breaths and the faster the air in the scuba tank is consumed.
. How do you determine the volume of water passing through a weir? - R
If the speed of the water were uniform as it passes through the opening, you could measure that speed and multiply it by the cross-section of the weir to obtain the volume of water passing through the weir each second. However, since the flow is faster near the center of the flow, it's difficult to calculate the volume flowing each second. Your best bet is probably to divide the opening into a number of regions and then to measure the water's velocity at the center of each region. Multiply each velocity by the cross-sectional area of that region and then sum up all the products to obtain the overall volume flow per second.
. How do flashing lights, chasing lights, and any type of Christmas lights work? - N
Years ago, many strings of Christmas lights consisted of about 20 or 30 light bulbs in series. In this series, electric current passed from one bulb to the next and deposited a small fraction of its energy in each bulb. The result was that each bulb glowed brightly so long as every bulb was working. If a single bulb burned out, the entire string went dark because no current could flow through the open circuit. If you replaced one of the bulbs in a working string with a special blinker bulb, the whole string would blink. The blinker bulb contained a tiny bimetallic switch thermostat that turned it off whenever the temperature rose above a certain point. At first, the bulb would glow and the whole string would glow with it. Then the thermostat would overheat and turn the bulb and string off. Then the thermostat would cool off enough to turn the bulb and string back on. This pattern would repeat endlessly.
But modern electronics has replaced the blinker bulbs with computers and transistor switches. Transistorized switches determine which bulbs or groups of bulbs receive current and glow at any given time and carefully timed switching can make patterns of light that appear to move or "chase." As for the problem with one failed bulb spoiling the string, a reader has informed me that the bulbs are now designed with a fail-safe feature. If a bulb's filament breaks, the sudden surge in voltage across that bulb activates this fail-safe mechanism. Wires inside the bulb connect to allow current to bypass that bulb completely. The remaining bulbs in the string glow a little more brightly than normal and their lives are shortened slightly as a result.
. Is it true that you can get lead poisoning in your home more easily from hot water than from cold water? — WH, Erial, NJ
Yes, assuming that your home has either lead pipes or copper pipes that were joined with lead-containing solders. That's because lead compounds are more soluble in hot water than they are in cold water. The amount of lead that was permitted in pipe solders has diminished over the years until now, when pipe solders can't contain any lead at all. While very little lead actually leaches out of the solder joints and enters the water, the effect is slightly more significant in hot water pipes than in cold water pipes. That's why it's recommended that you not use water from hot water pipes in cooking.
. How does electricity get from the generating station to the outlet in my living room? — JJ, Arlington, MA
The generating station uses a large generator to transfer energy from a giant turbine to an electric current flowing through a coil of wire. Current from this generating coil then flows through the primary coil of a huge transformer, where it transfers its energy to the magnetic core of the transformer. The current then returns to the generator to obtain more energy.
The magnetic core of the transformer transfers its energy to a second current—one that is passing through the secondary coil of the transformer. Because this current consists of far fewer electric charges per second, each charge receives a very large amount of energy. This large energy per charge gives the current a high voltage and it flows very easily through a high voltage transmission line. Because the amount of power that a wire loses is proportional to the square of the current passing through it, this high-voltage, low-current electricity wastes very little power in the transmission line on its way across country to your city. When the current reaches your city, it passes through another transformer and its energy is transferred to a third current. The cross country current then returns through the transmission line to the original power station to obtain more energy from the first transformer.
This third current involves more charges per second, so each charge carries less energy and the voltage is lower. This medium voltage electricity travels to your neighborhood before passing through a final transformer. This final transformer is probably either a gray metal can on a utility pole or a green box on a nearby lawn. In passing through the final transformer, the current transfers its energy to a current which then enters your home. This last current delivers energy to your appliances and lights and then returns to the final transformer to obtain more energy.
. How much does it cost to run a regular 60 to 100 watt light bulb per minute or per hour? — JM, Smithfield, ME
Electricity typically costs about 7 cents per kilowatt-hour. Over the course of an hour, a 100-watt light bulb will use 100 watt-hours or 0.1 kilowatt-hours, at a cost of about 0.7 cents. That's about 0.012 cents per minute.
. In our busy trial court we have preserved the original cassette tapes since 1989. They are kept in a relatively constant room temperature environment in our modern courthouse. Should we take any further precautions to extend the life of these tapes, considering the possibility that they may need to be replayed one day, such as in the retrial of a death penalty case that is reversed a decade after trial? I've heard of the practice of unwinding and rewinding tapes for this purpose, but haven't attempted it yet. The time involved is daunting! What is your opinion? — JD, Bryan, TX
A magnetic recording tape is usually a Mylar ribbon, coated with a thin layer of plastic that's impregnated with tiny permanent magnets. As long as it's store away from heat and moisture, the Mylar film itself shouldn't age. However, the layer of permanent magnets can change slightly with time. When a tape is left tightly wound on its reel for a long time, the magnetic layers can begin to affect one another—the magnetic fields from one layer of tape can alter the magnetization of the layers above and below it. The result is that sounds from one layer of tape can gradually transfer themselves weakly to the adjacent layers, creating faint echo effects. The solution to this problem is to unwind and rewind the tape, so that the layers shift slightly relative to one another. But while these echoes may be annoying in a recording of classical music, they probably aren't important in a recording of a noisy courtroom. Unless I hear otherwise from someone reading this note, I wouldn't worry about unwinding and rewinding your tapes. The slight imperfections that will result from transfers between layers shouldn't affect their utility in later trials. Properly stored, I'd expect the tapes to outlive everyone involved with the trials, even without any unwinding and rewinding.
. Please explain the concepts of magnetism pertaining to ferromagnetism, diamagnetism, and paramagnetism. - SC
A ferromagnetic material is one that contains intrinsic magnetic order. Iron, for example, is a ferromagnetic material—meaning that if you were to examine a microscopic region of the iron, you would find that it was highly magnetic. The magnetism in a ferromagnetic material is often hidden by a domain structure, in which microscopic magnetic regions or "domains" all point in random directions to give the material no apparent magnetism. Only when you expose the ferromagnetic material to a magnetic field does its magnetic character suddenly reveal itself. A ferromagnetic material becomes strongly magnetic when it's exposed to a magnetic field.
A diamagnetic material is one in which the electrons begin moving when it's place in a magnetic field. These moving electric charges create a second magnetic field that partially cancels the original field. A diamagnetic magnetic field partially shields itself from magnetism when it's exposed to a magnetic field.
A paramagnetic material is one in which individual magnetic electrons respond magnetically to any external magnetic field. It becomes weakly magnetic when it's exposed to a magnetic field. Unlike a ferromagnetic material, a paramagnetic material has no intrinsic magnetic order before it's exposed to an external field.
. Is heating milk by microwave advisable? - I
Microwave cooking leaves no permanent mark on the food. It causes virtually no chemical damage and absolutely no radioactivity. The only drawback with heating milk by microwave is that the heating may be uneven and may denature some protein molecules in regions of the milk that become excessively hot. Since most protein molecules are disassembled by your digestion anyway, this treatment probably has no effects worth worrying about. Even with infant formula, my only concern would be the hot spots. If you carefully shake the milk after heating, so that its temperature is uniform, it should be just fine. I suspect that companies warn you not to heat milk in a microwave because they are worried that you will either not shake the milk to distribute its temperature evenly or that you will overcook it until it boils and the bottle explodes.
. How do the 2" diagonal color LCD screens used in some of the new digital video cameras work? — M, Waynesboro, MS
Like most liquid crystal displays (LCD), these devices use liquid crystals to alter the polarization of light and determine how much of that light will emerge from each point on the display. Liquid crystals are large molecules that orient themselves spontaneously within a liquid—much the way toothpicks tend to orient themselves parallel to one another when you pour them into box. The liquid crystals used in an LCD display are sensitive to electric fields so that their orientations and their optical properties can be affected electronically. The liquid crystals in the display occupy a thin layer between transparent electrodes and two polarizing plastic sheets. Light from a fluorescent lamp passes through a polarizing sheet, an electrode, the liquid crystal layer, another electrode, and another polarizing sheet. The orientation of the liquid crystal determines whether light from the first polarizing sheet will be able to pass through the second polarizing sheet. When electric charges are placed on the two electrodes, the liquid crystal's orientation changes and so does light's ability to pass through the pair of polarizing sheets.
To create a full color image, the display has many rows of electrodes on each side of the liquid crystals and a pattern of colored filters added to the sandwich. In "active" displays, there are also thin-film transistors that aid in the placement of charges on the electrodes. Overall, the display is able to select the electric charges on each side of every spot or "pixel" on the screen and can thus control the brightness of every pixel.