Author: Lou Bloomfield

What is the purpose of the grid on the glass door of the microwave oven?

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What is the purpose of the grid on the glass door of the microwave oven?

The metal grid reflects microwaves and keeps them inside the oven. Electromagnetic waves are unable to pass through holes in conducting materials if those holes are significantly smaller than their wavelengths. The wavelengths of visible light are very short, so light has no trouble passing through the holes in this grid. But the microwaves used in the oven have wavelengths of about 12.4 cm and are unable to propagate through the grid. Thus you can see the food cook while the microwaves are trapped inside the oven.

At what angles do light rays reflect out of a prism?

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At what angles do light rays reflect out of a prism? — BC, Farmersville, TX

It depends on the shape of the prism and the angle at which the light arrived at the prism. Whenever light’s speed changes as it passes through a surface at an angle, the light bends. Since light travels faster in air than in glass (or plastic), it bends when it goes from air to glass or from glass to air. When light enters glass, it slows down and it bends toward the normal to the surface (toward the line that’s at right angles to the surface). When light leaves glass, it speeds up and it bends away from the normal to the surface. To know exactly how far the light bends, you need to know how much the glass slows light (the glass’s refractive index) and the angle at which the light encountered the glass surface (the angle of incidence). You can then use one of the basic laws of optics, Snell’s law, to determine the angle at which the light continues through the glass. You can then do the same for the light’s emergence from the glass and determine the angle at which it leaves.

In a CD player, how is the digital optical signal transformed into an electrical…

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In a CD player, how is the digital optical signal transformed into an electrical signal? — IM, Oxford, UK

The ridges and flat regions on a compact disc’s aluminum layer determine how laser light is reflected from that layer. As the disc turns and the player’s laser scans across ridges and flat regions, the intensity of the reflected light fluctuates up and down. This reflected light is directed onto an array of silicon photodiodes that provide both the signals needed to keep the laser focused tightly on the aluminum layer and the signal that the player uses to recreate sound. The sound is encoded in the lengths of the ridges. A computer monitors the amount of light returning from the disc to determine how long each ridge is and how much spacing there is between it and the next ridge. The computer uses this information to obtain a series of 16 bit binary numbers for each of the two sound channels that are represented by an audio CD. A digital-to-analog converter uses these 16 bit numbers to produce currents that are eventually amplified and used to produce sound.

How does hair spray work?

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How does hair spray work? — KC, IL

While I don’t know exactly what chemicals are used in hairspray, the main constituents are almost certainly polymer molecules—otherwise known as plastics. In the container, these polymer molecules are dissolved in a volatile solvent such as an alcohol or water, and pressurized with a chemical such as propane or a hydrofluorocarbon. When you spray the mixture onto your hair, the solvent evaporates and leaves the polymer molecules clinging to the hairs. These molecules are very long chains of atoms that form a stiff web around each hair and stiffen it. In general, the characteristics of polymers change with temperature and chemical environment. The polymer used in hairspray should be in the “glassy” regime, meaning that its atoms and molecules are essentially immobile at room temperature. Once the solvent is gone, the web of polymer molecules on the hairs is stiff and keeps the hairs from changing shape. Before you panic at the idea of spraying plastic onto your hair, consider that starch is also a polymer, as is hair itself. So putting hairspray on your hair is no different from putting starch on clothes.

What are some everyday examples of friction? (For example, we couldn’t walk with…

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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…

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.

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 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.

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.

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.