Month: July 1998

You said that microwaves heat food by twisting water molecules back and forth an…

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You said that microwaves heat food by twisting water molecules back and forth and having those water molecules rub against one another to experience a molecular form of “friction.” Since vibrating molecules are the fundamental manifestation of heat, why is the friction necessary at all? — GS, Kanata, Canada

While it’s true that microwaves twist water molecules back and forth, this twisting alone doesn’t make the water molecules hot. To understand why, consider the water molecules in gaseous steam: microwaves twist those water molecules back and forth but they don’t get hot. That’s because the water molecules beginning twisting back and forth as the microwaves arrive and then stop twisting back and forth as the microwaves leave. In effect, the microwaves are only absorbed temporarily and are reemitted without doing anything permanent to the water molecules. Only by having the water molecules rub against something while they’re twisting, as occurs in liquid water, can they be prevented from remitting the microwaves. That way the microwaves are absorbed and never remitted—the microwave energy becomes thermal energy and remains behind in the water.

Visualize a boat riding on a passing wave—the boat begins bobbing up and down as the wave arrives but it stops bobbing as the wave departs. Overall, the boat doesn’t absorb any energy from the wave. However, if the boat rubs against a dock as it bobs up and down, it will converts some of the wave’s energy into thermal energy and the wave will have permanently transferred some of its energy to the boat and dock.

Do VCR’s work on the same principle as audio tape players? If so, how does a VCR…

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Do VCR’s work on the same principle as audio tape players? If so, how does a VCR generate a signal while it’s on pause?

Yes, VCR’s work on the same principle as an audio tape player: as a magnetized tape moves past the playback head, that tape’s changing magnetic field produces a fluctuating electric field. This electric field pushes current back and forth through a coil of wire and this current is used to generate audio signals (in a tape player) or video and audio signals (in a VCR).

However, there is one big difference between an audio player and a VCR. In an audio player, the tape moves past a stationary playback head. In a VCR, the tape moves past a spinning playback head. When you pause an audio tape player, the tape stops moving and there is no audio signal. But when you pause a VCR, the playback head continues to spin. As the playback head (actually 2 or even 4 heads that trade off from one another) sweeps across a few inches of the tape, it experiences the changing magnetic fields and fluctuating electric fields needed to produce the video and audio signals. That’s why you can still see the image from a paused VCR. To prevent the spinning playback heads from wearing away the tape, most VCRs limit the pause time to about 5 minutes.

What does a transformer do?

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What does a transformer do?

A transformer transfers power between two or more electrical circuits when each of those circuits is carrying an alternating electric current. Transfers of this sort are important because many electric power systems have incompatible circuits—one circuit may use large currents of low voltage electricity while another circuit may use small currents of high voltage electricity. A transformer can move power from one circuit of the electric power system to another without any direct connections between those circuits.

Now for the technical details: a transformer is able to make such transfers of power because (1) electric currents are magnetic, (2) the magnetic fields from an alternating electric current changes with time, (3) a time-varying magnetic field creates an electric field, and (4) an electric fields pushes on electric charges and electric currents. Overall, one of the alternating currents flowing through a transformer creates a time-varying magnetic field and thus an electric field in the transformer. This electric field does work on (transfers power to) another alternating current flowing through the transformer. At the same time, this electric field does negative work on (saps power from) the original alternating current. When all is said and done, the first current has lost some of its power and the second current has gained that missing power.

In your discussion of event horizons, you stated that light falls just like ever…

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In your discussion of event horizons, you stated that light falls just like everything else. I thought that light does not speed up when falling but just gains energy—that it is blue-shifted. Conversely, when it rises in a gravitational field, it does not slow down but just loses energy—that it is red-shifted. Is that correct? — B

Yes. For very fundamental reasons, light can’t change its speed in vacuum; it always travels at the so-called “speed of light.” So light that is traveling straight downward toward a celestial object doesn’t speed up; only its frequency and energy increase. But light that is traveling horizontally past a celestial object will bend in flight, just as a satellite will bend in flight as it passes the celestial object. This trajectory bending is a consequence of free fall. While the falling of light as it passes through a gravitational field is a little more complicated than for a normal satellite—the light’s trajectory must be studied with fully relativistic equations of motion—both objects fall nonetheless.

How does a light-detecting diode create voltage when light hits it?

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How does a light-detecting diode create voltage when light hits it? — T

Diodes are one-way devices for electric current and are thus capable of separating positive charges from negative charges and keeping them apart. Those charges can separate by moving away from one another in the diode’s allowed direction and then can’t get back together because doing so would require them to move through the diode in the forbidden direction. Given a diode’s ability to keep separated charges apart, all that’s needed to start collecting separated charges is a source of energy. This energy is required to drive the positive and negative charges apart in the first place. One such energy source is a particle of light—a photon. When a photon with the right amount of energy is absorbed near the one-way junction of the diode, it can produce an electron-hole pair (a hole is a positively charged quasiparticle that is actually nothing more than a missing electron). The junction will allow only one of these charged particles to cross it and, having crossed, that particle cannot return. Thus when the diode is exposed to light, separated charge begins to accumulate on its two ends and a voltage difference appears between those ends.

In the movie “Back to the Future,” Doc Brown completes an electrical circuit w…

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In the movie “Back to the Future,” Doc Brown completes an electrical circuit with a bolt of lightning as the source and the “flux capacitor” as the load. In the process, he receives a shock. Would the “flux capacitor” still experience a flow of electrons if Doc Brown had provided a path to the earth? — BM, Akron, Ohio

While most of the “science” in that movie is actually nonsense, the use of lightning as a source of power has some basis in reality. The current in a lightning bolt is enormous, peaking at many thousands of amperes, and the voltages available are fantastically high. With so much current and voltage available, the flow of current during a lightning strike can be very complicated. Even though Doc Brown provided one path through which the lightning current could flow into the ground, he only conducted a fraction of the overall current. The remaining current flowed through the wire and into the “flux capacitor.” This branching of the current is common during a lightning strike and makes lightning particularly dangerous. You don’t have to be struck directly by lightning or to be in contact with the main conducting pathway between the strike and the earth for you to be injured. Current from the strike can branch out in complicated ways and follow a variety of unexpected paths to ground. You don’t want to be on any one of them. Doc Brown wasn’t seriously hurt because it was only a movie. In real life, people don’t recover so quickly.

What is the cause of the power “drop” in my house, that will intermittently (e…

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What is the cause of the power “drop” in my house, that will intermittently (every 5 to 10 minutes) cause my lights to dim? — JF

Your lights are dimming because something is reducing the voltage of the electricity in your house. The lights expect the electric current passing through them to experience a specific voltage drop—that is, they expect each electric charge to leave behind a certain amount of energy as the result of its passage through the lights. If the voltage of electricity in your house is less than the expected amount, the lights won’t receive enough energy and will glow dimly.

The most probable cause for this problem is some power-hungry device in or near your house that cycles on every 5 or 10 minutes. In all likelihood, this device contains a large motor—motors have a tendency to draw enormous currents while they are first starting to turn, particularly if they are old and in need of maintenance. The wiring and power transformer systems that deliver electricity to your neighborhood and house have limited capacities and cannot transfer infinite amounts of power without wasting some of it. In general, wires waste power in proportion to the square of the current they are carrying. While the amount of power wasted in your home’s wiring is insignificant in normal situations, it can become sizeable when the circuits are overloaded. This wasted power in the wiring appears as a loss of voltage—a loss of energy per charge—at your lights and appliances. When the heavy equipment turns on and begins to consume huge amounts of power, the wiring and other electric supply systems begin to waste much more power than normal and the voltage reaching your lights is significantly reduced. Your lights dim until the machinery stops using so much power.

To find what device that’s making your lights dim, listen carefully the next time your lights fade. You’ll probably hear an air conditioner, a fan, or even an elevator starting up somewhere, either in your house or in your neighborhood. There may be nothing you can do to fix the problem, but it’s possible that replacing a motor or its bearings will reduce the problem. Another possible culprit is an electric heating system—a hot water heater, a radiant heater, an oven, a toaster, or even a hair-dryer. These devices also consume large amounts of power and, in an older house with limited electric services, may dim the lights.