Author: Lou Bloomfield

Why do we have time?

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Why do we have time? — KD, McMinnville, Oregon

Time is the fourth dimension, similar to but not equivalent to the three spatial dimensions. With four dimensions in our universe, we need four values to specify the exact location of each event—three values that specify that event’s location in space and one value that specifies its location in time. Space and time are intimately related so that we perceive time in terms of space and space in terms of time. For example, you sense the distance of a remote city by how long it would take you to get there. Similarly, you sense the large separation between two moments in time by how far you could travel between those two moments. But as to “why we have time,” I can only answer that it’s part of the nature of our universe.

How do neon lights work?

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How do neon lights work? — MT, Cement City, MI

A neon light uses a very high voltage to propel an electric current through a low-density gas of neon atoms. These neon atoms are trapped inside a glass tube and the current passes between two metal electrodes at opposite ends of that tube. A high voltage power supply—typically a neon sign transformer—pumps a large number of negative charges onto one electrode and a large number of positive charges onto the other electrode. Because like charges repel while opposite charges attract, there are strong forces pushing the charges from one electrode toward those on the other electrode. Eventually, charges at the two ends of the tube begin to leap off the electrodes and into the neon gas so that they can flow toward one another. Current begins to flow through the tube. As the charges move through the gas, they frequently collide with neon atoms and occasionally transfer some of their energies to those neon atoms. During such an energy transfer, an electron in the neon atom shifts from its normal orbital to a higher energy orbital in which the electron doesn’t normally travel. The electron soon returns to its normal orbital and releases a particle of light—a photon—in the process. Since the most common orbital shift in an excited neon atom releases a particle of red light, a neon light emits a bright, reddish glow.

What are the frequency characteristics of transformers? Are they related to the …

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What are the frequency characteristics of transformers? Are they related to the circuit components and the ratio of primary to secondary turns around the iron core? — JM, Lakewood, Colorado

The frequency characteristics of a transformer are determined principally by the materials in the transformer’s core. Power flows from the primary circuit to the secondary circuit by way of the magnetization of the transformer’s core. With each half-cycle of the alternating current in the primary circuit, the transformer’s core must magnetize and demagnetize. A transformer core’s ability to magnetize and demagnetize properly depends on the frequency of the alternating current in the transformer’s coils. If that frequency is too low, the core may saturate—reach its maximum possible magnetization—during the half-cycle. In that case, the core will not be able to transfer the requisite amount of energy to the secondary coil and the power transferred between the two coils will be inadequate. That’s why low frequency transformers often contain huge iron cores—cores that avoid saturation by spreading out the magnetization and stored energy over large volumes of iron.

On the other hand, if the frequency of current in the primary is too high, the core may be unable to magnetize and demagnetize fast enough to keep up with it and the power transfer will again be inadequate. The core may also become hot due to friction-like losses in the core material. That’s why high frequency transformers use special core materials such as ferrite powders or even air. Although air (or really empty space) can’t store large amounts of energy in small volumes when it magnetizes, it can respond extremely quickly. Air-core transformers operate well at extremely high frequencies.

How does an integrated circuit perform computations? I know that it has transist…

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How does an integrated circuit perform computations? I know that it has transistors embedded into it, but how can a circuit of semiconductors be used for multiplication? — DF, Marina Del Rey, California

The transistors used in digital integrated circuits, including microprocessors, act primarily as electronically controlled switches. These transistor switches permit the electric charge on or electric current in one wire to control the electric charge on or current in another wire. In digital electronics, a wire’s charge or current state is used to represent a single binary digit—either a 1 or a 0. By combining transistors in modestly complicated arrangements, the states of several wires together can control the states of several other wires. This increased complexity allows for simple functions such as binary addition to be performed—for example, the charges on two wires can be used to control the charges on two other wires so that the charges on the second pair of wires represent the single binary sum of the two individual numbers represented by charges on the first pair of wires. More complicated adders can be assembled from more transistors and finally multipliers can be assembled from a collection of adders. Overall, it only takes a few arrangements of electrically controlled switches to form the primitive elements from which incredibly complicated digital processors can be built.

When two identical items are cooked, one with a microwave oven and the other on …

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When two identical items are cooked, one with a microwave oven and the other on the stove, which will cool faster? — CR

If the distributions of temperatures inside the items were the same after cooking, they would cool at the same rate. However, a microwave oven tends to cook relatively evenly throughout the food while the stove tends to cook from the outside of the food inward. That means that food cooked in a microwave oven tends to have more thermal energy near its center than food cooked on a stove, even when those foods contain the same total amount of thermal energy. Since foods lose heat through their surfaces, the extra thermal energy in the food cook by microwave will take longer to flow out to the surface of the food and from there to its surroundings. All else being equal, I would expect the food cooked in the microwave oven to cool slightly slower than the food cooked on the stovetop.

Why are there two tides per day?

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Why are there two tides per day? — JF

The tide is caused primarily by the moon’s gravity. Gravity is what keeps the moon and earth together as a pair—the moon and earth orbit one another because each is exerting an attractive force on the other. While they are effectively falling toward one another as the result of this gravitational attraction, their sideways motion keeps them from smashing together and they instead travel in elliptical paths around a common center of mass. But the moon’s gravity is slightly stronger on the near side of the earth than it is on the far side of the earth. As a result, the water on the near side of the earth bulges outward toward the moon. The water on the far side of the earth also bulges outward because the earth itself is falling toward the moon slightly faster than that more distant water is. The distant water is being left behind as a bulge.

There are thus two separate tidal bulges in the earth’s oceans: one on the side nearest the moon and one on the side farthest from the moon. But the earth rotates once a day, so these bulges move across the earth’s surface. Since there are two bulges, a typical seashore passes through two bulges a day. At those times, the tide is high. During the times when the seashore is between bulges, the tide is low. Because the moon moves as the earth turns, high tides occur about 12 hours and 26 minutes apart, rather than every 12 hours. Since local water must flow to form the bulges as the earth rotates, there are cases where the tides are delayed as the water struggles to move through a channel. However, even in those cases, the high tides occur every 12 hours and 26 minutes. The sun’s gravity also contributes to the tides, but its effects are smaller and serve mostly to vary the heights of high and low tide.

How does a relay work?

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How does a relay work? — CS, Fairfax, Virginia

A relay is an electromagnetically operated switch. It contains a coil of wire that acts as an electromagnet. Since electric currents are magnetic, this coil of wire develops north and south magnetic poles whenever current passes through it. A metal core is often placed inside the coil of wire to enhance its magnetism. Adjacent to the coil of wire is a moveable piece of iron. While iron normally appears nonmagnetic when it’s by itself, it becomes highly magnetic whenever it’s exposed to a nearby magnetic pole. The iron piece becomes magnetic as current flows through the coil and the two are attracted toward one another. As the iron piece shifts toward the coil, it moves various electric contacts that are attached to it. These contacts close some circuits while opening others. The coil remains magnetic and continues to hold the iron piece near it until current stops flowing through the coil. When the current does stop, the coil loses its magnetism and so does the iron piece. A spring in the relay then pulls the two apart and the electric contacts return to their original positions.

How does the telephone work?

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How does the telephone work? — JB, Sydney, Nova Scotia

A telephone uses an electric current to convey sound information from your home to that of a friend. When the two of you are talking on the telephone, the telephone company is sending a steady electric current through your telephones. The two telephones, yours and that of your friend, are sharing this steady current. But as you talk into your telephone’s microphone, the current that your telephone draws from the telephone company fluctuates up and down. These fluctuations are directly related to the air pressure fluctuations that are the sound of your voice at the microphone.

Because the telephones are sharing the total current, any change in the current through your telephone causes a change in the current through your friend’s telephone. Thus as you talk, the current through your friend’s telephone fluctuates. A speaker in that telephone responds to these current fluctuations by compressing and rarefying the air. The resulting air pressure fluctuations reproduce the sound of your voice. Although the nature of telephones and the circuits connecting them have changed radically in the past few decades, the telephone system still functions in a manner that at least simulates this behavior.

How can you demonstrate that sounds are waves produced by the vibration of mater…

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How can you demonstrate that sounds are waves produced by the vibration of material objects? — TP, Huntington Park, California

I can’t think of an easy way to make sound waves visible while they travel through air, but it’s relatively easy to make sound waves visible as they travel through materials. If you choose a system in which the sound waves bounce back and forth many times through a material, you can sometimes see the sound waves as they move. For example, partially fill a crystal wine glass with water and then rub your wet finger gently around the rim of the glass. With some practice, you’ll be able to get the wine glass to emit a pure tone as your finger alternately sticks and slips its way around the glass rim. As this tone appears—the vibration of the crystal glass itself—the water will begin to exhibit beautiful ripple patterns. You should be able to see these ripples by looking at a bright light reflected from the water’s surface. The ripples are sound waves that are travel through the water, back and forth, as the glass vibrates.

Another system that makes the movement of waves visible is a stiff, thin aluminum plate that’s supported rigidly and horizontally at only one point. If you sprinkle fine sand lightly over the surface of this plate and then bow its edge with a violin bow, it will begin vibrating with a clear tone. As it vibrates, the sand will drift into places where there is very little surface motion—the nodes of the vibrating surface. Once again, sound waves are traveling back and forth across this surface and the up-down motions squeeze the sand into certain parts of the plate. In this case, the surface’s vibrations and the sound waves in that surface are the same thing—in example of the fact that vibrations and sound waves are intimately related and are in many respects exactly the same thing.

I’m helping on a lesson plan for grades 3-12 where students make ice cream. Addi…

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I’m helping on a lesson plan for grades 3-12 where students make ice cream. Adding salt to the ice makes the ice colder. I’m having trouble explaining why we put salt on the roads to melt ice, but in making ice cream the salt actually lowers the temperature of the ice. — N

These two observations—that salt melts ice and that salt makes ice colder—are actually consistent with one another. When you add salt to ice, you make a relatively ordered mixture—pure crystalline ice and pure crystalline salt. This orderly arrangement is looked on unfavorably by nature; given a chance, nature tends to maximize randomness. There is a much more disorderly arrangement available—salt water—and nature tends toward disorderly arrangements. When you put the salt and ice together, nature’s tendency toward randomness begins to drive the system to rearrange. The ice begins to melt so that the salt can dissolve in it. Although the melting of ice requires energy, the randomness this melting and dissolving produces makes this process take place. The energy needed to melt the ice is extracted from the remaining ice and that ice gets colder. When you’re making ice cream, some of the energy needed to melt the ice also comes from the ice cream mix, so that it gets colder, too. If there is enough salt around, the ice will melt completely to form very cold salt water—the desired result with salt on a slippery sidewalk. The salt water remains liquid well below the normal freezing temperature of water because forming ice crystals would require the salt and water to separate from one another—an orderly and therefore unlikely event. In short, nature’s trend toward disorder causes salt to melt ice, even though that melting lowers the temperatures of everything involved well below the freezing temperature of pure water.