Month: October 1996

How do you calculate total speaker impedance? For example, 4 speakers wired in s…

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How do you calculate total speaker impedance? For example, 4 speakers wired in series or parallel. Is there a formula? — PV, Atlanta, GA

You can calculate the impedance of a collection of speakers the same way you would calculate the resistance of a collection of resistors. Each time two speakers are connected in series, so that the electric current must pass through one and then the other to get to its destination, their impedances add. Thus two 4-ohm speakers in series are equivalent to one 8-ohm speaker (4 ohm + 4 ohm = 8 ohm). Each time two speakers are connected in parallel, so that the electric current can pass through one or the other to get to its destination, the reciprocals of their impedances add to give the reciprocal of their overall impedance. Thus two 4-ohm speakers in parallel are equivalent to one 2-ohm speaker (1/4 ohm + 1/4 ohm = 1/2 ohm). Once you have figured out the impedance of a pair of speakers, you can treat it as though it were one speaker and proceed to figure out the impedance of a larger group of speakers. For example, four 4-ohm speakers in series have an overall impedance of 16 ohms and four 4-ohm speakers in parallel have an overall impedance of 1 ohm.

Is there any substance that can stop magnetic fields

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Is there any substance that can stop magnetic fields — K, Mendenhall, MS

Magnetic fields are related to what are call magnetic flux lines. These magnetic flux lines extend unbroken from north magnetic poles to south magnetic poles. Where the flux lines are close together, the magnetic field is strong. Thus to avoid magnetic fields, you need to keep magnetic flux lines away. Because magnetic flux lines can’t be broken, they can’t simply be made to disappear. To “stop” a magnetic field in a particular region of space, you have to either terminate the flux lines at a magnetic pole or you have to divert the flux lines away the region that you’re interested in. The first strategy has a problem: no isolated magnetic poles (so-called “magnetic monopoles”) have ever been found. That means that every north pole you find has a south pole attached to it. Thus you can’t simply end the flux lines with magnetic poles because for each flux line you end with a south pole, you’ll start a new one with the attached north pole. But the second strategy is reasonable. There are many materials that divert magnetic flux lines. One of the most important of these is a metal called “mu metal,” an alloy that’s made from nickel, iron, chromium, and copper. Mu metal attracts flux lines. It draws flux lines through itself so that if you were to wrap yourself in a layer of mu metal, any magnetic flux lines that would have gone through you (and thus exposed you to magnetic fields) will go through the mu metal instead. Mu metal and similar alloys are used routinely to shield objects that can’t tolerate magnetic fields.

How do I figure out how much energy is used to heat the water in our gas hot wat…

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How do I figure out how much energy is used to heat the water in our gas hot water heater? I know that 1 BTU is the energy to heat 1 lb. of water 1°. Do I figure out how many gallons in 1 lb. of water; and then multiply that by the difference in room temperature and 140°? — JH, Maple Grove, MN

Yes. A gallon of water weighs about 8.3 pounds, so a typical 40-gallon hot water heater tank holds 332 pounds of water. To raise that water from its delivery temperature (about 60° F) to its final temperature (about 140° F) takes about 26,560 BTUs.

Are there any risks, other than a case of implosion, with regards to exposure to…

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Are there any risks, other than a case of implosion, with regards to exposure to normal fluorescent lighting? – RR

While the phosphors in fluorescent lamps are not considered to be toxic, they do contain a tiny amount of mercury. This mercury is an essential part of the operation of the lamp (it is what creates the initial light during the electric discharge). While most fluorescent lamps are simply discarded into landfill, some facilities (including the University of Virginia) dispose of them more carefully. The University of Virginia breaks the lamps to collect the phosphors and then distills the mercury out of the phosphors. The phosphors are then entirely non-hazardous and the mercury is recycled.

Can we add a section to a microwave oven that gets the food or drinks cold? – MH

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Can we add a section to a microwave oven that gets the food or drinks cold? – MH

Not without adding a full-blown refrigerator. While it’s relatively easy to add thermal energy to food or drink, it’s much harder to remove that thermal energy. Since energy is conserved, the thermal energy that you remove from the food must be transferred elsewhere. Since heat (moving thermal energy) normally flows from a hotter object to a colder object, you must make something colder than the food before the heat will leave the food. While it’s possible to cool an object to a temperature lower than its surroundings, this cooling process requires a heat pump, a device that actively pumps heat from a cold object to a hot object (against its natural direction of flow). A refrigerator is such a heat pump.

How do you boil ice water? (I think it has something to do with a vacuum.) – MW

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How do you boil ice water? (I think it has something to do with a vacuum.) – MW

You’re right, it does have to do with a vacuum. While water molecules can evaporate from the surface of liquid water at almost any temperature, boiling can only occur when the evaporation rate is high enough to support the appearance of evaporation bubbles inside the body of the liquid water. Normally, atmospheric pressure pushes inward on cold water so hard that any evaporation bubble that appears inside the water is immediately crushed out of existence. But in water that’s at 100° C, evaporation is so rapid that the evaporation bubbles in the liquid water can survive and grow, despite the crushing inward forces of atmospheric pressure. The hot water boils.

Water boils not because it’s hot but because any evaporation bubble that forms inside it is able to survive and grow despite the surrounding atmospheric pressure. At normal atmospheric pressure, the water does have to be hot for this to happen. But if you remove the atmospheric pressure, the water can boil at much lower temperatures. In fact, at sufficiently low pressures, even ice water will boil! It’s funny to see ice cubes floating in a container of boiling water, but it happens when you remove the air from around the ice water.

How does a light bulb work?

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How does a light bulb work? — DH, Casselberry, FL (and also KH)

In a common incandescent light bulb, an electric current flows through a double-spiral coil of very thin tungsten wire. As the electric charges in the current flow through this tungsten filament, they collide periodically with the tungsten atoms and transfer energy to those tungsten atoms. The current gives up its energy to the tungsten filament and the filament’s temperature rises to about 2500° C. While all objects emit thermal radiation, very hot objects emit some of the thermal radiation as visible light. A 2500° C object emits about 12% of its heat as visible light and this is the light that you see coming from the bulb. Most of the remaining heat emerges from the bulb as invisible infrared light or “heat” light. The glass enclosure shields the filament from oxygen because tungsten burns in air.

Can a compound have triple bonds? If so, please give an example.

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Can a compound have triple bonds? If so, please give an example. — BA, IL

Yes, some compounds contain triple bonds. Acetylene is the simplest such molecule, with two carbon atoms connected by a triple bond. Each carbon atom has one hydrogen atom attached to it, so the entire molecule is a four-atom chain: hydrogen-carbon-carbon-hydrogen. The triple bond between carbon atoms is extremely strong—the atoms are sharing 6 electrons between them.

How do light sticks work? – AE

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How do light sticks work? – AE

When you bend a plastic light stick, you break a small glass ampoule and allow two chemicals that are contained inside the stick to mix. One of these chemicals is a powerful oxidizing agent and the other is a chemical that when oxidized (“burned”) is left in an electronically excited state. In other words, the chemical reaction between the molecules of the two chemicals creates a new molecule that has excess energy in it. The molecule releases this energy as a particle of light, a photon. Although I am not certain exactly which chemicals are used in a modern light stick, I believe that one is hydrogen peroxide (the oxidizer) and the other is luminol (the chemical that is oxidized). Upon oxidization, luminol emits a photon of blue or ultraviolet light. The green light that you see emerging from a typical light stick is actually a second photon that is emitted by a fluorescent dye contained in the light stick. This dye absorbs the blue or ultraviolet photon emitted by the luminol and then reemits a new photon with somewhat less energy and a green color.