Month: December 1996

What does the inside of a microwave oven look like? Please show illustrations.

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What does the inside of a microwave oven look like? Please show illustrations. — Dade County, FL

A microwave oven contains (1) a magnetron that produces the microwaves, (2) a high voltage direct current power supply (a high voltage transformer, a set of rectifiers, and a capacitor) that provides power to the magnetron, and (3) a computerized control system that turns the power supply and magnetron on and off. A metal pipe connects the magnetron to the cooking chamber of the oven. While there are photographs and drawings of the insides of a microwave oven in my book, I can’t reproduce them here because of copyright issues.

How does a magnetron work?

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How does a magnetron work? — MM, Czech Republic

A magnetron has a ring of resonant electromagnetic cavities around a hot central filament. Each resonant cavity acts like an electromagnetic “tuning fork”—electric charges and electromagnetic waves swing back and forth inside a resonant cavity at a particular frequency; the cavity’s resonant frequency. As electrons are “boiled” off the hot filament, a high voltage attracts them toward the walls of the resonant cavities. The resonant cavities tend to have at least small amounts of electric charge “sloshing” back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities’ positively charged walls than to their negatively charged walls.

However, there is also a magnetic field present in the magnetron and this field deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge sloshing in the walls of the resonant cavities, the newly arrived electrons add to it. As electrons flow to the resonant cavities, more and more charge sloshes in the resonant cavities and these cavities accumulate huge amounts of energy. Some of this energy is tapped by a small wire loop and a microwave antenna. This antenna radiates some of the energy from the cavities into a metal channel that leads away from the magnetron. In a microwave oven, this channel leads to the cooking chamber so that energy from the resonant cavities is delivered to the food in the oven. Energy is extracted from the magnetron slowly enough that the filament and high voltage power supply can replace it and the operation continues indefinitely.

How does electricity work and is it possible to design a light bulb that will le…

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How does electricity work and is it possible to design a light bulb that will let you know when it is about to stop working? — LS, Chicago, IL

Electricity involves electric charges. While static electricity involves stationary electric charges, the electricity you are probably referring to is dynamic: electricity in which the electric charges move. Most (dynamic) electricity is the movement of electrons—tiny negatively charged particles that form the outer part of atoms. The electricity in the wires leading to and from a lamp is the flow of electrons through those wires. A lamp has two wires attached to it because the electrons flow into the lamp through one wire and out through the other wire. However, because the electricity we normally use is alternating current, the direction in which the electrons flow through those two wires reverses 120 times a second (60 full cycles of reversal, over and back, each second).

As the electrons flow through the lamp’s filament, they leave behind much of their energy. This energy is deposited in the tiny filament and the filament becomes extremely hot. It begins to emit much of its thermal energy as thermal radiation, part of which is visible light. So you can think of the electricity as a steady stream of tiny delivery trucks (the electrons), carrying energy to the lamp’s filament, and then returning to the power company to pick up some more energy. The filament sends this energy into the room as heat and light.

When a light bulb burns out, it’s because the filament has became so thin that a section of it has overheated and melted. This thinning process is caused by the slow evaporation (or actually sublimation) of tungsten atoms from the filament. A thinned filament usually fails as you turn the bulb on because that’s the time of maximum power delivery to the filament and thus maximum stress. Unfortunately, it’s very hard to tell in advance whether the filament will be able to tolerate the next attempt to turn it on. Probably the best predictor is the number of hours the bulb has been on. If you always replace a bulb after it has operated for 750 hours at full power, you’ll probably avoid most outages.

How does food cook?

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How does food cook? — KJ, Irving, TX

There are two parts to this question: how does thermal energy (or heat) reach the food and what does that thermal energy do when it arrives. I’ll start with the first part, but first let me define thermal energy as a form of energy associated with the random jittering about of the atoms and molecules in a material. The hotter a material is, the more average thermal kinetic energy (energy of motion) each atom has—in effect, the more vigorously the atoms and molecules jiggle. Thermal energy naturally tends to flow from hotter objects to colder objects, so that when you put cold food on a hot stove or in a hot oven, thermal energy will flow toward the food. This moving thermal energy is called heat.

There are three main mechanisms for heat transfer: conduction, convection, and radiation. Heat that flows via conduction is being passed from atom to atom inside a solid or liquid. In metals, conduction is greatly assisted by mobile electrons (the same electrons that allow metals to carry electricity) that carry heat between atoms far away from one another. Conduction is important on the stovetop, where the food touches the pot and the pot touches the hot stovetop. Heat that flows via convection is carried by a moving gas or liquid. Convection is important in an oven that’s heated from below so that hot air rises to touch the food. Heat that flows via radiation is carried by electromagnetic waves (forms of light). Radiation is important in an oven that’s heated from above (as in a broiler) so that thermal radiation travels downward to the food’s surface.

Once the heat arrives at the food, it raises the food’s temperature. As the food becomes hotter, chemical reactions begin to occur and molecules begin to change shape. Thermal energy makes it possible for chemical bonds within and between the molecules to come apart so that new bonds and new molecules can form. Water and other small molecules evaporate more and more rapidly until the water begins to boil. Sugar molecules rearrange to form caramels and carbon. Protein molecules rearrange and stiffen. These molecular changes, together with the increased temperature of the food, are what we associate with cooking.

How do you figure out the weight lifting ability of a hot air balloon?

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How do you figure out the weight lifting ability of a hot air balloon? — BK, Meraux, LA

The air surrounding an object pushes upward on it with a force equal to the weight of the air the object displaces. The observation is called Archimedes’ principle. If the object weighs less than the air it displaces, the object will experience a net upward force and will float upward. Since hot air is less dense and weighs less than cold air, a balloon filled with hot air can weigh less than the air it displaces. To determine the net upward force on the balloon, you subtract the total weight of the balloon (including the air inside it) from the weight of the air it displaces.

At room temperature, air weighs about 12.2 newtons per cubic meter (0.078 pounds per cubic foot). But air’s density and weight are proportional to its temperature on an absolute temperature scale (in which absolute zero is the zero of temperature). At 200° F, air weighs about 20% less than at room temperature, or about 9.7 newtons per cubic meter (0.062 pounds per cubic foot). Thus each cubic meter of 200° F air inside the balloon makes the balloon 2.5 newtons lighter than the air it displaces (or each cubic foot of that hot air makes it 0.016 pounds lighter). If the balloon’s envelope, basket, and occupants weigh 4000 newtons (900 pounds), then the balloon will have to contain about 1600 cubic meters (56,000 cubic feet) of hot air in order to float upward.

How can one prove to students that the earth rotates. Any instructions on how to…

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How can one prove to students that the earth rotates. Any instructions on how to build a pendulum to show rotation or some other way? – KC

There are many indirect indications that the earth rotates, including the motions of celestial objects overhead, the earth’s winds—particularly the counter-clockwise rotation of surface winds in northern hemisphere hurricanes, and the outward bulge of the earth around its equator. But for a more direct indication, a Foucault pendulum is a good choice.

Unfortunately, a Foucault pendulum isn’t easy to interpret or build. It would be easiest to interpret if it were at the north pole, where it would swing back and forth in a fixed plane as the earth turned beneath it. To a person watching the pendulum from the ground, the pendulum’s swinging arc would appear to complete one full turn each day. However, elsewhere in the northern hemisphere, the plane of the pendulum does change and the pendulum’s swinging arc will appear to complete less than one full turn each day. Nonetheless, the fact that the arc shifts at all is an indication that the ground is accelerating and that the earth is turning.

The problem with building a Foucault pendulum is that it must retain its swinging energy for hours or even days and that it must not be perturbed by activities around it. It must have a very dense, massive pendulum bob supported on a strong, thin cable and that cable must be attached to a rigid support overhead. The longer the cable is, the longer it will take the bob to complete each swing and the more slowly the pendulum will move. Slow movements are important to minimize air resistance. If I were building a Foucault pendulum, I’d find a tall empty shaft somewhere, away from any moving air, and I’d attached a lead-filled metal ball (weighing at least 100 pounds but probably more) to the top of the shaft with a thin steel cable. I’d make sure that nothing rubbed and that the top of the cable never moved. (Over the long haul, there is the issue of damage to the top of the cable because of flexure…it will eventually break here. Wrapping the cable around a drum so that there is no specific bending point helps.) Then I’d pull the pendulum away from its equilibrium position and let it start swinging slowly back and forth. Over the course of several hours, its swing would decrease, but not before we would notice that its arc had turned significantly away from the original arc because of the earth’s rotation.

I read recently that scientists at CERN produced some form of antimatter, but th…

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I read recently that scientists at CERN produced some form of antimatter, but that it could not be stored. Why can’t it be stored and, if it could, would it be a viable method of propulsion? — BC, Ottawa, Ontario

The antimatter that was formed at CERN was an antihydrogen atom, which consisted of an antiproton and an antielectron (often called a positron). Antiprotons and positrons have been available for a long time, but it has been a challenge to bring them together gently enough for them to stick to one another and form a bound system. An antihydrogen atom is hard to store because, like a normal hydrogen atom, it moves or falls so quickly that it soon collides with its container. For a normal hydrogen atom, that collision is likely to cause a chemical reaction. But for an antihydrogen atom, that collision is likely to cause annihilation. When an antiproton touches a proton, the two can destroy one another and convert their mass into energy. The same is true for a positron and an electron. To store an antihydrogen atom, you must keep it from touching any normal matter. That’s not an easy task. Because of its ability to emit its entire mass and that of the normal matter it encounters into energy, antimatter is the most potent “fuel” imaginable. But don’t expect it to show up in a rocket ship any time soon.

How do sound proof and bulletproof glasses work? – DH

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How do sound proof and bulletproof glasses work? – DH

Sound proof glass uses several separate layers of glass to make it difficult for sound to move from one room to another. Each time sound passes through a surface and experiences a change in speed, some of the sound reflects. Sound travels much more slowly in air than in glass, so with each transition into or out of a glass pane, most of the sound is reflected backward. If two rooms are separated by 3 or 4 sheets of glass, each carefully sealed into place so that there are no holes for sound to leak through, the amount of sound that can make it through the overall window will be very small. Most of the sound will be reflected.

Bulletproof glass is actually a multi-layered sandwich of glass and plastic—it’s like the front windshield of a car, but with many more layers. When a bullet hits the surface of the sandwich, it begins to tear into the layers. But the bullet loses momentum before it manages to burrow all the way through to the final layers. The bullet’s energy and momentum are transferred harmlessly to the layers of glass and plastic.

What effects do fluorescent lamps have on household plants?

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What effects do fluorescent lamps have on household plants? — SN, Milwaukee, WI

Since plants appear green, they are absorbing mostly the red and blue portions of the visible light spectrum. Blue light is particularly important to them. Incandescent light contains relatively little blue light, so it probably doesn’t help plants very much. Because fluorescent lighting provides more blue light than incandescent lighting, fluorescent lighting is certainly better for plants.

Exactly what is light? Is it a wave or particles?

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Exactly what is light? Is it a wave or particles? — MW, Catoosa, OK

Light is an electromagnetic wave—an excitation of the electric and magnetic fields that can exist even in “empty” space. Light’s electric field creates its magnetic field and its magnetic field creates its electric field and this self-perpetuating arrangement zips off through space at a phenomenal speed—the speed of light. Light is created by moving electric charges, which first excite the electromagnetic fields. Light is also absorbed by electric charges, which obtain energy from the light’s electromagnetic fields.

Like everything else in the universe, light exhibits both wave and particle behaviors. When it is traveling through space, light behaves as a wave. That means that its location is generally not well defined and that it can simultaneously pass through more than one opening (the way a water wave can when it encounters a piece of screening). But when light is emitted or absorbed, it behaves as a particle. It’s created all at once when it’s emitted from a particular location and it disappears all at once when it’s absorbed somewhere else. This wave/particle arrangement is true of everything, including objects such as electrons or atoms: while they are traveling unobserved, they behave as waves but when you go looking for them, they behave as particles.