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Page 56 of 160 (1595 Questions and Answers)

551. 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.

552. 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.

553. 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.

554. 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.

555. 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.

556. 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.

557. What are some everyday examples of friction? (For example, we couldn't walk without friction.)
Before giving some examples, I'll note that there are two different types of friction. First, there's the static friction between two surfaces that are pressed together but are not sliding across one another. Second, there's the sliding or dynamic friction between two surfaces that are moving across one another. Static friction allows objects to push one another sideways but doesn't create thermal energy. Sliding friction also creates thermal energy (or heat).

Your example of walking is a case of static friction: your feet push backward on the sidewalk and the sidewalk reacts by pushing your feet (and you) forward. As further examples of static friction: holding a pencil, screwing in a light bulb, pulling a rope toward you hand over hand, pedaling a bicycle so that the ground pushes the wheel forward, keeping the dishes and silverware from blowing off a level picnic table on a windy day...

As examples of sliding friction: skidding the wheels of a automobile during a rapid start or stop, sliding down the pole in a fire station, skiing or skating, squeezing a bicycle's caliper brakes against the wheel rims, shaping metal with a grinding wheel, sharpening a knife, sanding a wooden desktop...

558. How do the different jet engines work on aircraft—the turbojet, the turbofan, and the turboprop? — KB, Charlottesville, VA
All three engines start with a turbojet engine. In a turbojet engine, a stream of air is first compressed by a rotary compressor. The air is then mixed with fuel and the mixture is burned. Finally, the hot burned gases are allowed to expand through a rotating turbine and they flow out of the back of the engine at very high speed.

To understand how all of this works, let's follow the flow of energy through the turbojet engine. Assuming the plane is moving forward, the air is moving fast when it encounters the engine's inlet duct. This inlet duct slows the air down substantially and the change in its speed causes the air's pressure to rise—an effect observed by Bernoulli. The air's energy doesn't change, but its kinetic energy (energy of motion) is partially converted to pressure potential energy. The now pressurized air is further pressurized by its passage through the rotary compressor at the front of the turbojet. The compression process adds energy to the air by doing mechanical work on that air. Now fuel is added to the high-pressure air and the mixture is burned. This combustion adds an enormous amount of energy to the air. The exhaust gases immediately expand and their speeds increase substantially as they pour out of the combustion chamber. These gases flow through a rotating turbine on their way out of the back of the engine. Even though the gases do work on the turbine, they still have lots of energy and flow out of the jet engine at a much greater speed than the air had when it arrived. Much of the fuel's chemical potential energy has become kinetic energy in these exhaust gases. The turbine provides the mechanical work that operates the rotary compressor, or the fan of a turbofan or the propeller of a turboprop. Overall, the exhaust gases leave the turbojet engine traveling faster than the air did when it arrived. Since the gases carry backward momentum with them as they leave the engine, they have evidently pushed the engine forward to give the engine and the plane forward momentum.

That's all there is to a turbojet engine. A turbofan engine uses the mechanical work from an enlarged turbine to operate a large fan that's in front of the turbojet engine itself. This fan takes air that has slowed down on entry into the jet's inlet duct and adds energy to this air. The air then speeds up as it flows out the jet's outlet duct and the air leaves the engine traveling faster than when it arrived. Once again, the engine experiences a forward thrust force as it pushes this air backward.

A turboprop engine uses mechanical work from an enlarged turbine to operate a propeller. The propeller pushes air flowing past the engine backward and the air pushes the engine and airplane forward. Because there is no duct around the propeller blades, the air passes the blades at full speed (a turbofan engine uses its duct to slow the air down before pushing on the air with its fan blades).

559. If you were in freefall from a jet airplane, would the airplane overtake you in the fall (assuming that the plane started freefalling as you jumped out the door)?
It would overtake you immediately. Airplanes are designed to experience extremely small drag forces and are remarkably aerodynamic as a result. In contrast, you would experience severe air drag (air resistance) once you left the plane. The plane would coast past you at high speed while you would slow enormously in the first second or two of exposure to the air.

560. What is a convection oven and what are its advantages?
The main mechanism by which heat is transferred to food in a normal oven is convection. In this mechanism, air heated by the gas or electric burner at the bottom of the oven rises because of buoyant forces (i.e., hot air rises) and carries heat to the food. But natural convection is slow and imperfect—if you overfill the oven, you block convection and the food cooks unevenly. In a convection oven, a fan stirs the air rapidly. Heat flows quickly and evenly from the burner to the food. Cooking occurs more quickly and you can also put more food in the oven without danger of uneven cooking.
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