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Page 145 of 160 (1595 Questions and Answers)

1441. The force of gravity decreases as we go down toward the center of the earth and becomes equalized at the center. So why does pressure increase with depth, for example in the ocean? — HN, Vancouver, British Columbia
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It's true that the force of gravity decreases with depth, so that if you were to find yourself in a cave at the center of the earth, you would be completely weightless. However, pressure depends on more than local gravity: it depends on the weight of everything being supported overhead. So while you might be weightless, you would still be under enormous pressure. Your body would be pushing outward on everything around you, trying to prevent those things from squeezing inward and filling the space you occupy. In fact, your body would not succeed in keeping those things away and you would be crushed by their inward pressure.

More manageable pressures surround us everyday. Our bodies do their part in supporting the weight of the atmosphere overhead when we're on land or the weight of the atmosphere and a small part of the ocean when we're swimming at sea. The deeper you go in the ocean, the more weight there is overhead and the harder your body must push upward. Thus the pressure you exert on the water above you and the pressure that that water exerts back on you increases with depth. Even though gravity is decreasing as you go deeper and deeper, the pressure continues to increase. However, it increases a little less rapidly as a result of the decrease in local gravity.


1442. If I mix water and crushed ice, and allow them to sit in an insulated container for about 3 minutes, will their temperature be 32 degrees Fahrenheit? — MP, San Francisco
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When he established his temperature scale, Daniel Gabriel Fahrenheit defined 32 degrees "Fahrenheit" (32 F) as the melting temperature of ice—the temperature at which ice and water can coexist. When you assemble a mixture of ice and water and allow them to reach equilibrium (by waiting, say, 3 minutes) in a reasonably insulated container (something that does not allow much heat to flow either into or out of the ice bath), the mixture will reach and maintain a temperature of 32 F. At that temperature and at atmospheric pressure, ice and water are both stable and can coexist indefinitely.

To see why this arrangement is stable, consider what would happen if something tried to upset it. For example, what would happen if this mixture were to begin losing heat to its surroundings? Its temperature would begin to drop but then the water would begin to freeze and release thermal energy: when water molecules stick together, they release chemical potential energy as thermal energy. This thermal energy release would raise the temperature back to 32 F. The bath thus resists attempts at lowering its temperature.

Similarly, what would happen if the mixture were to begin gaining heat from its surroundings? Its temperature would begin to rise but then the ice would begin to melt and absorb thermal energy: separating water molecules increases their chemical potential energy and requires an input of thermal energy. This lost thermal energy would lower the temperature back to 32 F. The bath thus resists attempts at raising its temperature.

So an ice/water bath self-regulates its temperature at 32 F. The only other quantities affecting this temperature are the air pressure (the bath temperature could shift upward by about 0.003 degrees F during the low pressure of a hurricane) and dissolved chemicals (half an ounce of table salt per liter of bath water will shift the bath temperature downward by about 1 degree F).


1443. Is it possible to construct a capacitor capable of storing the energy in lightning, then allowing that energy to flow gradually into the power grid?
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Actually, the system of cloud and ground that produces lightning is itself a giant capacitor and the lightning is a failure of that capacitor. Like all capacitors, the system consists of two charged surfaces separated by an insulating material. In this case, the charged surfaces are the cloud bottom and the ground, and the insulating material is the air. During charging, vast amounts of separated electric charge accumulate on the two surfaces—the cloud bottom usually becomes negatively charged and the ground below it becomes positively charge. These opposite charges produce an intense electric field in the region between the cloud and the ground, and eventually the rising field causes charge to begin flowing through the air: a stroke of lightning.

In principle, you could tap into a cloud and the ground beneath and extract the capacitor's charge directly with wires. But this would be a heroic engineering project and unlikely to be worth the trouble. And catching a lightning strike in order to charge a second capacitor is not likely to be very efficient: most of the energy released during the strike would have to dissipate in the air and relatively little of it could be allowed to enter the capacitor. That's because no realistic capacitor can handle the voltage in lightning.

Here's the detailed analysis. The power released during the strike is equal to the strike's voltage times its current: the voltage between clouds and ground and the current flowing between the two during the strike. Voltage is the measure of how much energy each unit of electric charge has and current is the measure of how many units of electric charge are flowing each second. Their product is energy per second, which is power. Added up over time, this power gives you the total energy in the strike. If you want to capture all this energy in your equipment, it must handle all the current and all the voltage. If it can only handle 1% of the voltage, it can only capture 1% of the strike's total energy.

While the current flowing in a lightning strike is pretty large, the voltage involved is astonishing: millions and millions of volts. Devices that can handle the currents associated with lightning are common in the electric power industry but there's nothing reasonable that can handle lightning's voltage. Your equipment would have to let the air handle most of that voltage. The air would extract power from the flowing current in the lightning bolt and turning it into light, heat, and sound. Your equipment would then extract only a token fraction of the stroke's total energy. Finally, your equipment would have to prepare the energy properly for delivery on the AC power grid—its voltage would have to be lowered dramatically and a switching system would have to convert the static charge on the capacitors to an alternating flow of current in the power lines.


1444. What is the device called in some watches that transforms the kinetic energy created by the watch's motion into energy to help power the watch's battery? And how does such a device work? — KW, Washington, DC
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As a number of readers have informed me, the watches you're referring to generate electricity that then powers a conventional electronic watch. These electromechanical watches use mechanical work done by wrist motions on small weights inside the watches to generate electricity. Seiko's watch spins a tiny generator—a coil of wire moves relative to a magnetic field and electric charges are pushed through the coil as a result. I have been told that other watches exist that use piezoelectricity—the electricity that flows when certain mechanical objects are deformed or strained—to generate their electricity. In any case, your wrist motion is providing the energy that becomes electric power.

These electromechanical watches are the modern descendants of the automatic mechanical watches. An automatic watch had a main spring that was wound by the motion of the wearer's hand. A small mass inside the watch swung back and forth on the end of a lever. Because of its inertia, this mass resisted changes in velocity and it moved relative to the watch body whenever the watch accelerated. If you like, you can picture the mass as a ball that rolls about inside a wagon as you roll the wagon around an obstacle course. When the lever turned back and forth relative to the watch body, the watch was able to extract energy from it. Gears attached to the lever allowed the watch to use the mass's energy to wind its mainspring. The energy extracted from the mass with each swing was very small, but it was enough to keep the mainspring fully wound. Ultimately, this energy came from your hand—you did work on the watch in shaking it about and some of this energy eventually wound up in the mainspring.

These same sorts of motions are what power the electromechanical watches of today. Instead of winding a spring, your wrist motions swing weights about inside the watches and these moving weights spin generators to produce electric power.


1445. How do you convert a measurement in liters per second into one in gallons per minute? — MG
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Converting units is always a matter of multiplying by 1. But you must use very fancy versions of 1, such as 60 seconds/1 minute and 1 gallon/3.7854 liters. Since 60 seconds and 1 minute are the same amount of time, 60 seconds/1 minute is 1. Similarly, since 1 gallon (U.S. liquid) and 3.7854 liters are the same amount of volume, 1 gallon/3.7854 liters is 1. So suppose that you have measured the flow of water through a pipe as 283 liters/second. You can convert to gallons/minute by multiplying 283 liters/second by 1 twice: (283 liters/second)(60 seconds/1 minute)(1 gallon/3.7854 liters). When you complete this multiplication, the liter units cancel, the second units cancel, and you're left with 4,486 gallons/minute.

1446. I will be teaching first graders how to use simple magnifiers. What are the basic safety rules for magnifiers that I should share with them with regard to sunlight, heat, etc. — JR
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The only source of common light source that presents any real danger to a child with a magnifying glass is the sun. If you let sunlight pass through an ordinary magnifying glass, the convex lens of the magnifier will cause the rays of sunlight to converge and they will form a real image of the sun a short distance after the magnifying glass. This focused image will appear as a small, circular light spot of enormous brilliance when you let it fall onto a sheet of white paper. It's truly an image—it's round because the sun is round and it has all the spatial features that the sun does. If the image weren't so bright and the sun had visible marks on its surface, you'd see those marks nicely in the real image.

The problem with this real image of the sun is simply that it's dazzlingly bright and that it delivers lots of thermal power in a small area. The real image is there in space, whether or not you put any object into that space. If you put paper or some other flammable substance in this focused region, it may catch on fire. Putting your skin in the focus would also be a bad idea. And if you put your eye there, you're in serious trouble.

So my suggestion with first graders is to stay in the shade when you're working with magnifying glasses. As soon as you go out in direct sunlight, that brilliant real image will begin hovering in space just beyond the magnifying glass, waiting for someone to put something into it. And many first graders just can't resist the opportunity to do just that.


1447. When you open your eyes underwater everything is blurry, but when you wear a mask, you can see clearly. Why can't the eye focus underwater unless it has an air space, provided by the mask, in front of it? — DW, Cork City, Ireland
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Just as most good camera lenses have more than one optical element inside them, so your eye has more than one optical element inside it. The outside surface of your eye is curved and actually acts as a lens itself. Without this surface lens, your eye can't bring the light passing through it to a focus on your retina. The component in your eye that is called "the lens" is actually the fine adjustment rather than the whole optical system.

When you put your eye in water, the eye's curved outer surface stops acting as a lens. That's because light travels at roughly the same speed in water as it does in your eye and that light no longer bends as it enters your eye. Everything looks blurry because the light doesn't focus on your retina anymore. But by inserting an air space between your eye and a flat plate of glass or plastic, you recover the bending at your eye's surface and everything appears sharp again.


1448. Why do faster moving fluids have lower pressure? — JH
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Actually, faster moving fluids don't necessarily have lower pressure. For example, a bottle of compressed air in the back of a pickup truck is still high-pressure air, even though it's moving fast. The real issue here is that when fluid speeds up in passing through stationary obstacles, its pressure drops. For example, when air rushes into the open but stationary mouth of a vacuum cleaner, that air experiences not only a rise in speed, it also experiences a drop in pressure. Similarly, when water rushes out of the nozzle of a hose, its speed increases and its pressure drops. This is simply conservation of energy: as the fluid gains kinetic energy, it must lose pressure energy. However, if there are sources of energy around—fans, pumps, or moving surfaces—then these exchanges of pressure for speed may no longer be present. That's why I put in the qualifier of there being only stationary obstacles.

1449. How fast do the electrons in copper flow when that copper is carrying electricity? — LH, North Hollywood
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It turns out that the electrons in copper travel quite slowly even though "electricity" travels at almost the speed of light. That's because there are so many mobile electrons in copper (and other conductors) that even if those electrons move only an inch per second, they comprise a large electric current. Picture the electrons as water flowing through a pipe or river and now consider the Mississippi River. Even if the Mississippi is flowing only inches per second, it sure carries lots of water past St. Louis each second.

The fact that electricity itself travels at almost the speed of light just means that when you start the electrons moving at one end of a long wire, the electrons at the other end of the wire also begin moving almost immediately. But that doesn't mean that an electron from your end of the wire actually reaches the far end any time soon. Instead, the electrons behave somewhat like water in a long hose. When you start the water moving at one end, it pushes on water in front of it, which pushes on water in front of it, and so on so that water at the far end of the hose begins to leave the hose almost immediately. In the case of water, the motion proceeds forward at the speed of sound. In a wire, the motion proceeds forward at the speed of light in the wire (actually the speed at which electromagnetic waves propagate along the wire), which is only slightly less than the speed of light in vacuum.

Note for the experts: as one of my readers (KT) points out, the water-in-a-hose analogy for current-in-a-wire is far from perfect. Current in a wire flows throughout the wire, including at its surface, and the wire's resistance to steady current flow scales as the cross-sectional area of the wire. In contrast, water in a hose only flows through the open channel inside the hose and the hose's resistance to flow scales approximately as the fourth power of that channel's diameter.


1450. If I wanted to magnetize a screwdriver, what would be the best way of doing this? I know it can be done by rubbing magnets across the screwdriver's tip, but I would like to know a way of doing it with a piece of coiled wire and a battery. I have heard that this can be done with a car battery. — MS, West Virginia
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Iron and most steels are intrinsically magnetic. By that, I mean that they contain intensely magnetic microscopic domains that are randomly oriented in the unmagnetized metal but that can be aligned by exposure to an external magnetic field. In pure iron, this alignment vanishes quickly after the external field is removed, but in the medium carbon steel of a typical screwdriver, the alignment persists days, weeks, years, or even centuries after the external field is gone.

To magnetize a screwdriver permanently, you should expose it briefly to a very strong magnetic field. Touching the screwdriver's tip to one pole of a strong magnet will cause some permanent magnetization. Rubbing or tapping the screwdriver also helps to free up its domains so that they can align with this external field. But the better approach is to put the screwdriver in a coil of wire that carries a very large DC electric current.

The current only needs to flow for a fraction of a second—just long enough for the domains to align. A car battery is a possibility, but it has safety problems: it can deliver an incredible current (400 amperes or more) for a long time (minutes) and can overheat or even explode your coil of wire. Moreover, it may leak hydrogen gas, which can be ignited by the sparks that will inevitably occur while you are magnetizing your screwdriver.

A safer choice for the current source is a charged electrolytic capacitor—a device that stores large quantities of separated electric charge. A charged capacitor can deliver an even larger current than a battery can, but only for a fraction of a second—only until the capacitor's store of separated charge is exhausted. Looking at one of my hobbyist electronics catalogs, Marlin P. Jones, 800-652-6733, I'd pick a filter capacitor with a capacity of 10,000 microfarads and a maximum voltage of 35 volts (Item 12104-CR, cost: $1.50). Charging this device with three little 9V batteries clipped together in a series (27 volts overall) will leave it with about 0.25 coulombs of separated charge and just over 3.5 joules (3.5 watt-seconds or 3.5 newton-meters) of energy.

Make sure that you get the polarity right—electrolytic filter capacitors store separated electric charge nicely but you have to put the positive charges and negative charges on the proper sides. [To be safe, work with rubber gloves and, as a general rule, never touch anything electrical with more than one hand at a time. Remember that a shock across your heart is much more dangerous than a shock across you hand. And while 27 volts is not a lot and is unlikely to give you a shock under any reasonable circumstances, I can't accept responsibility for any injuries. If you're not willing to accept responsibility yourself, don't try any of this.]

If you wrap about 100 turns of reasonably thick insulated wire (at least 18 gauge, but 12 gauge solid-copper home wiring would be better) around the screwdriver and then connect one end of the coil to the positively charged side of the capacitor and the other end of the coil to the negatively charged side, you'll get a small spark (wear gloves and safety glasses) and a huge current will flow through the coil. The screwdriver should become magnetized. If the magnetization isn't enough, repeat the charging-discharging procedure a couple of times, always with the same connections so that the magnetization is in the same direction.


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