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Page 58 of 160 (1595 Questions and Answers)

571. Could you slow down the molecules to cool food quickly instead of heating it up?
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Heat naturally flows from hotter objects to colder objects. As a result, you can heat food by putting it in hotter surroundings and cool food by putting it in colder surroundings. However, you can also heat food by converting an ordered form of energy into thermal energy, right inside the food. For example, microwaves can penetrate the food and their energy can become thermal energy inside the food, speeding up the cooking process.

However, there is no analogous way to reach inside the food and extract its thermal energy. You must wait for the thermal energy inside the food to drift to its surface and to be transferred to the colder surroundings. This requirement is the result of the laws of thermodynamics, which govern the interconversions of work and heat. While it's easy to turn mechanical work into heat (just rub your hands together), it's very difficult to turn heat into work. Because of this difficulty, thermal energy must usually be transferred elsewhere. You can't build a "microwave refrigerator" that turns thermal energy into microwaves inside the food.


572. How can glass be shattered with sound? — JI, Rapid City, SD
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When sound shatters glass, it breaks the glass in the usual way: by distorting the glass to its breaking point. Whenever glass is bent too far, a crack propagates into the glass from its surface (usually at a defect) and the glass tears. For sound to cause this tearing process, the sound must distort the glass substantially. An extremely loud sound can distort the glass to its breaking point in a single motion. For example, an explosion shatters windows when a surge in air pressure (which you hear as a very loud "pop" sound) exerts so much force on those windows that they bend and break.

However, a moderately loud tone can also break certain glass objects by pushing on those objects rhythmically until they distort beyond their breaking points. To understand how that's possible, recall that you can get a child swinging strongly on a playground swing either by giving the child one hard push or by giving the child many carefully timed gentle pushes. The gentle pushes transfer energy to the child via a mechanism called resonant energy transfer—the child is exhibiting a natural resonance and you are using that resonance to transfer energy to the child a little bit at a time.

While most glass objects exhibit only very weak natural resonances and are therefore extremely difficult to break via resonant energy transfer, a good crystal wineglass is resonant enough to be broken by a loud tone. You can hear the appropriate tone by flicking the wineglass with your finger. If the wineglass emits a clear bell-like tone, you will be able to break that wineglass by exposing the wineglass to a loud version of that same tone. When the wineglass is exposed to this tone, it begins to vibrate in its natural resonance. Each rise and fall in air pressure associated with the tone adds energy to the vibrating wineglass until its surface is distorting wildly. If the tone is loud enough and its pitch is exactly right, the wineglass will distort a remarkable amount and it may shatter. I know from experience with this effect that the distortion a crystal wineglass can undergo without shattering is amazing—it usually won't break until it's upper lip is almost as oval-shaped as an egg. Finding the right tone and holding that tone accurately enough and loudly enough requires sophisticated equipment. Few humans have any chance of breaking a wineglass because the pitch accuracy and volume needed are beyond the abilities of all but the most remarkable opera singers. However, Enrico Caruso was apparently able to do this trick with a wineglass held directly in front of his mouth. Note also that normal window glass and normal drinking glasses are made from soft forms of glass that exhibit no strong resonances—if you tap them, you hear only a dull "thunk" sound, not a bell-like tone. As a result, you can't break them with tones.


573. How are light and sound the same? How are they different? — JS, Binghamton, NY
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There are so many answers to these questions that I'll have to pick and choose. For their similarities, I'll note that they're both disturbances that travel through space and that both have wavelengths and frequencies. Sound is a pressure disturbance in the air (or in another material) and consists of compressions and rarefactions that travel outward from their origin. The distance between adjacent regions of compression (or rarefaction) is the sound's wavelength and the number of compressed regions that pass by a particular point each second is the sound's frequency (or pitch). Light is an electromagnetic disturbance in space itself, although materials that are present in that space can alter its characteristics somewhat. It consists of electric and magnetic fields that travel outward as waves from their origin. The distance between adjacent regions of maximum electric field (or magnetic field) in one direction is the light's wavelength and the number of regions in which the electric field points maximally in a particular direction that pass by a particular point each second is the light's frequency (or color). I hope that you can see some of the similarities in these descriptions.

As for differences, sound is a longitudinal wave—meaning that the air involved in the pressure fluctuations moves back and forth in the direction of the wave's travel. Thus if sound is moving from left to right, the air is also fluctuating back and forth from left to right. In contrast, light is a transverse wave—meaning, that the electric and magnetic fields involved in the wave fluctuate back and forth at right angles to the direction of the wave's travel. Thus if light is moving from left to right, the electric and magnetic fields associated with it are fluctuating either up and down or toward you and away from you (or both). Another difference is that sound travels about 300 meters per second and its speed depends on the speed of the air through which it travels. Light, on the other hand, travels about 300,000 kilometers per second and its speed in vacuum (empty space) is absolutely constant. The speed of light is one of the fundamental constants of the universe.


574. How do some paints and stickers glow in the dark? — DD, Sandy, UT
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Glow in the dark paints and materials contain molecules that are able to store energy for long periods of time and then release that energy as light. To understand how this delayed emission works, let's examine the interactions of molecules and light. The electrons in any molecule are normally arranged in what is called the "electronic ground state," an arrangement that gives those electrons the least possible energy. However, the electrons in a molecule can also be arranged in one of many "electronically excited state," in which they have more than the minimum energy. Whenever a molecule is exposed to light, its electrons may rearrange and the molecule may find itself in one of the electronically excited states. If that occurs, the molecule will have absorbed a particle of the light, a "photon," and used the photon's energy to rearrange its electrons.

In a typical molecule, the extra energy is released almost immediately, either as light or as the vibrational energy that we associate with heat. But in a few special molecules, this extra energy can become trapped in the molecule. When an electron shifts from one arrangement to another and the total energy of that molecule decreases, the missing energy may leave as a photon of light. But electrons behave as though they were spinning objects and in shifting between arrangements, the electron normally can't change the direction of its spin. In most rearrangements that lead to the emission of light, the electron spins remain unchanged.

However, a glow in the dark molecule is one in which there is an electronically excited state that can only shift to the ground state if one of the electrons changes its direction of spin as the photon of light is being produced. In some molecules, this process is almost totally forbidden by the laws of physics and proceeds so slowly that the molecule may wait for minutes, hours, or even days before it emits the photon and returns to its ground state. When you expose a material containing these molecules to light, its molecules become trapped in these special electronically excited states and they then glow in the dark for a long while afterward.


575. How does a paper airplane fly? — CL, South Bend, IN
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A paper airplane flies for roughly the same reason that a normal airplane flies: the air pressure below its wings is somewhat higher than the air pressure above its wings. As a result of this pressure difference, the paper airplane experiences an overall upward force due to air pressure and this upward force is strong enough to balance the airplane's downward weight.

In a paper airplane, the most important effect is a rise in pressure below the wings. To understand why this pressure rise occurs, think about the movement of air from the perspective of a bug that's riding on the airplane. To the bug, the air is flowing toward the front of the airplane. As this stream of air encounters the undersurface of the wing, the air slows down. You can think of this air as hitting a slanted wall. Whenever a moving stream of air slows down, its pressure rises. You experience this pressure rise when you hold your hand out of the window of a moving car and feel the slowing air push your hand toward the back of the car.

The dynamics of the air above the wing is more complicated and depends on the design of the wing. But in any case, the air above the wing doesn't slow down and its pressure never rises above atmospheric pressure. In a well-designed wing, it actually drops below atmospheric pressure! Since the air pressure rises under the paper airplane's wing and doesn't rise above the airplane's wing, the wing experiences an upward force due to pressure. It's this upward force that supports the airplane.


576. Why is it harder to balance on a stationary motorbike compared to a moving one? Is it a gyroscopic effect on the wheels? — DF, Morley Perth, Australia
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As you suspect, gyroscopic effects do play a role. Because it has only two wheels, a motorbike is inherently unstable. When it's stationary, it is only in equilibrium—that is it experiences no net force or torque—when it's perfectly upright. The slightest tip causes it to fall over. You must be very careful and agile to keep it balanced. A physicist would say that the motorbike is statically unstable or that it has an unstable static equilibrium.

For the motorbike to remain upright, you must keep the overall center of gravity (yours and the motorbike's) directly above the wheels (actually the line formed by their contact points on the ground). That's very hard to do when the motorbike is stationary. But when the motorbike is heading forward, it naturally steers itself under the center of gravity. If the motorbike begins to tip to one side, its front wheel automatically steers in the direction of the tip and the forward moving motorbike soon drives its wheels back under the center of gravity. This automatic steering is due to both gyroscopic precession in its spinning front wheel and to the shape and angle of the front wheel fork. If you hold the motorbike (or a bicycle) off the ground, spin its front wheel the right direction, and then tip the motorbike, you'll see its wheel turn toward the direction of the tip because of gyroscopic precession. If you return the motorbike to the ground and then tip it to one side, you'll see that its wheel will automatically turn toward that side because of the fork shape.

With both effects helping the motorbike steer under the center of gravity, the moving motorbike is very stable. A physicist would say that it is dynamically stable. Everything I've said also applies to bicycles and was pointed out by British physicist David Jones in 1970. Bicycles are so dynamically stable that almost anyone can ride them without hands and not tip over!


577. What are the different types of light bulbs and how do they work? - BS
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An incandescent light bulb works by heating a solid filament so hot that the filament's thermal radiation spectrum includes large amounts of visible light. A fluorescent tube uses an electric discharge in mercury vapor to produce ultraviolet light, which is then transformed into visible light by fluorescent phosphors on the inner surface of the tube. A gas discharge lamp uses an electric discharge in a gas inside that lamp (often high pressure mercury, or sodium vapor, or even neon) to produce visible light directly.

578. How does a steam engine work? — MP, New Fairfield, CT
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Like the internal combustion engines used in automobiles, a steam engine is a type of heat engine—a device that diverts some of the heat flowing from a hotter object to a colder object and that turns that heat into useful work. The fraction of heat that can be converted to work is governed by the laws of thermodynamics and increases with the temperature difference between the hotter and colder objects. In the case of the steam engine, the hotter the steam and the colder the outside air, the more efficient the engine is at converting heat into work.

A typical steam engine has a piston that moves back and forth inside a cylinder. Hot, high-pressure steam is produced in a boiler and this steam enters the cylinder through a valve. Once inside the cylinder, the steam pushes outward on every surface, including the piston. The steam pushes the piston out of the cylinder, doing mechanical work on the piston and allowing that piston to do mechanical work on machinery attached to it. The expanding steam transfers some of its thermal energy to this machinery, so the steam becomes cooler as the machinery operates.

But before the piston actually leaves the steam engine's cylinder, the valve stops the flow of steam and opens the cylinder to the outside air. The piston can then reenter the cylinder easily. In many cases, steam is allowed to enter the other end of the cylinder so that the steam pushes the piston back to its original position. Once the piston is back at its starting point, the valve again admits high-pressure steam to the cylinder and the whole cycle repeats. Overall, heat is flowing from the hot boiler to the cool outside air and some of that heat is being converted into mechanical work by the moving piston.


579. Can you suspend an object in midair with magnetism? — JA, Holmen, WI
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Yes. However, you can't suspend a stationary object in midair with permanent magnets. Instead, you must either use a moving object or you must use electromagnets that can be adjusted in strength in order to balance the object. Such magnetic suspension is an important issue because people are trying to suspend trains above tracks using magnetic forces. Magnetic levitation is useful because it eliminates the friction and wear that occur between wheels and track. Some of these schemes are based on electronic feedback that turns electromagnets on or off in order to keep the train floating properly. Other schemes use electromagnetic induction to turn the metal track into a magnet so that the moving magnetic train automatically hovers above the track. I should also note that there is a wonderful toy called a Levitron that's a spinning permanent magnet that hovers above a permanent magnet in its base. The spinning behavior of the magnetic top keeps it stably suspended about an inch above the base. It's a fantastic invention.

580. How does a battery work? How many different kinds of batteries are there? - BW
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Batteries use chemical reactions to move electric charges from one terminal to another. A chemical reaction is a process that rearranges molecules—you begin with a certain collection of molecules and end up with a different collection of molecules. As the atoms in those molecules rearrange, they stick to one another more tightly than before and they release some of their chemical potential energy. This released energy then takes another form. While some chemical reactions such as burning will turn this released energy into thermal energy, a battery uses this released energy to move electric charges from one place to another. The battery moves extra positive charges onto its positive terminal and extra negative charges onto its negative terminal. While you can't see those charges, you can tell that they're there. If you use wires to connect the terminals to the two sides of a light bulb, the charges will rush through the wires and the light bulb will glow.

There are many types of batteries, but two of the most important modern batteries are alkaline batteries (used in flashlights and toys) and lead-acid batteries (used in automobiles). An alkaline battery uses a reaction between zinc metal and manganese dioxide to move electric charges between its two terminals. The battery's negative terminal is made of powdered zinc and its positive terminal is surrounded by manganese dioxide. Between the two terminals is an alkaline paste of potassium hydroxide. As the chemical reaction proceeds, negative charges are transferred to the battery's negative terminal and positive charges are transferred to the battery's positive terminal. As these charges are used by the flashlight or toy, the battery replaces them with new charges. Since each transfer of charges consumes some of the battery's original chemicals, the more the battery's charges are used, the more its chemicals are consumed. Eventually the powdered zinc is gone and the battery stops working. Once the powdered zinc has been used up, it can't be replaced.

A lead-acid battery uses a reaction between lead metal, lead oxide, and sulfuric acid to move electric charges. It, too, consumes its original chemicals while transferring charges. However, a lead-acid battery can be recharged easily by pushing charges through it backward. When a car is running, its generator pushes charges backward through the lead-acid battery and converts the consumed chemicals back into their original forms. This recharged battery is almost as good as new, so it can be used over and over again and lasts for several years.


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