Author: Lou Bloomfield

Would it be possible to put a thermometer inside a microwave oven? Would the mic…

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Would it be possible to put a thermometer inside a microwave oven? Would the microwaves have an effect on an electronic thermometer? Would they have an effect on a mercury thermometer? — R

This is an interesting question because it brings up the tricky issue of what is the temperature in a microwave oven. In fact, there is no specific temperature in the oven because the microwaves that do the cooking are not thermal. Rather than emerging from a hot object with a well-defined temperature, these microwaves are produced in a coherent fashion by a vacuum tube. Like the light emerging from a laser, these microwaves can heat objects they encounter as hot as you like, or at least until heat begins to escape from those objects as fast as it’s being added.

So instead of measuring the “temperature of the microwave oven,” people normally put thermometers in the food to measure the food’s temperature. This works well as long as the thermometers don’t interact with the microwaves in ways that make them either hotter or inaccurate. Electronic thermometers are common in high-end microwaves. There is nothing special about these electronic thermometers except that they are carefully shielded so that the microwaves don’t heat them or affect their readings. By “shielded,” I mean that each of these thermometers has a continuous metallic sheath that reflects the microwaves. This sheath extends from the wall of the oven’s cooking chamber all the way to the thermometer probe’s tip so that the microwaves themselves can’t enter the measurement electronics. Since the sheath reflects microwaves, the thermometer isn’t heated by the microwaves and only measures the temperature of the food it contacts.

On the other hand, putting a mercury thermometer in a microwave oven isn’t a good idea. While mercury is a metal and will reflect most of the microwaves that strike it, the microwaves will push a great many electric charges up and down the narrow column of mercury. This current flow will cause heating of the mercury because the column is too thin to tolerate the substantial current without becoming warm. The mercury can easily overheat, turn to gas, and explode the thermometer. (A reader of this web site reported having blown up a mercury thermometer just this way as a child.) Moreover, as charges slosh up and down the mercury column, they will periodically accumulate at the upper end. Since there is only a thin vapor of mercury gas above this upper surface, the accumulated charges will probably ionize this vapor and create a luminous mercury discharge. The thermometer would then turn into a mercury lamp, emitting ultraviolet light. I used microwave-powered mercury lamps similar to this in my thesis research fifteen years ago and they work very nicely.

I wear glasses for distance vision, but my near vision is good. Why is it that w…

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I wear glasses for distance vision, but my near vision is good. Why is it that when I use a nearby mirror to view distant objects, I must wear my glasses to see them clearly? I should be able to see the nearby mirror well without glasses. — JFJ

When you view something in a flat mirror, you are looking at a virtual image of the object and this virtual image isn’t located on the surface of the mirror. Instead, it’s located on the far side of the mirror at a distance exactly equal to the distance from the mirror to the actual object. In effect, you are looking through a window into a “looking glass world” and seeing a distant object on the other side of that window. The reflected light reaching your eyes has all the optical characteristics of having come the full distance from that virtual image, through the mirror, to your eyes. The total distance between what you are seeing and your eyes is the sum of the distance from your eyes to the mirror plus the distance from the mirror to the object. That’s why you must use your distance glasses to see most reflected objects clearly. Even when you observe your own face, you are seeing it as though it were located twice as far from you as the distance from your face to the mirror.

I understand that to calculate the heat released or absorbed during a nuclear re…

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I understand that to calculate the heat released or absorbed during a nuclear reaction you find the difference between the product mass and reactant mass and use the formula (E=mc2). But what about heat released or absorbed during a chemical reaction? The book I have says that mass is conserved during a chemical reaction, so where does the heat energy come from? — TC

While your book’s claim is well intended, it’s actually incorrect. The author is trying to point out that atoms aren’t created or destroyed during the reaction and that all the reactant atoms are still present in the products. But equating the conservation of atoms with the conservation of mass overlooks any mass loss associated with changes in the chemical bonds between atoms. While bond masses are extremely small compared to the masses of atoms, they do change as the results of chemical reactions. However even the most energy-releasing or “exothermic” reactions only produce overall mass losses of about one part in a billion and no one has yet succeeded in weighing matter precisely enough to detect such tiny changes.

How do propane or kerosene refrigerators work

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How do propane or kerosene refrigerators work—ones that require no electricity at all and are called “ice from fire” units? — KN

Heater-based refrigerators make use of an absorption cycle in which a refrigerant is driven out of solution as a gas in a boiler, condenses into a liquid in a condenser, evaporates back into a gas in an evaporator, and finally goes back into solution in an absorption unit. The cooling effect comes during the evaporation in the evaporator because converting a liquid to a gas requires energy and thus extracts heat from everything around the evaporating liquid.

The most effective modern absorption cycle refrigerators use a solution of lithium bromide (LiBr) in water. What enters the boiler is a relatively dilute solution of LiBr (57.5%) and what leaves is dense, pure water vapor and a relatively concentrated solution of LiBr (64%). The pure water vapor enters a condenser, where it gives up heat to its surroundings and turns into liquid water. To convert this liquid water back into gas, all that has to happen is for its pressure to drop. That pressure drop occurs when the water enters a low-pressure evaporator through a narrow orifice. As the water evaporates, it draws heat from its surroundings and refrigerates them.

Finally, something must collect this low pressure water vapor and carry it back to the boiler. That “something” is the concentrated LiBr solution. When the low-pressure water vapor encounters the concentrated LiBr solution in the absorption unit, it quickly goes back into solution. The solution becomes less concentrated as it draws water vapor out of the gas above it. This diluted solution then returns to the boiler to begin the process all over again.

Overall, the pure water follows one path and the LiBr solution follows another. The pure water first appears as a high-pressure gas in the boiler (out of the boiling LiBr solution), converts to a liquid in the condenser, evaporates back into a low-pressure gas in the evaporator, and finally disappears in the absorption unit (into the cool LiBr solution). Meanwhile, the LiBr solution shuttles back and forth between the boiler (where it gives up water vapor) and the absorption unit (where it picks up water vapor). The remarkable thing about this whole cycle is that its only moving parts are in the pump that moves LiBr solution from the absorption unit to the boiler. Its only significant power source is the heater that operates the boiler. That heater can use propane, kerosene, electricity, waste heat from a conventional power plant, and so on.

If one metric ton of antimatter comes into contact with one metric ton of matter…

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If one metric ton of antimatter comes into contact with one metric ton of matter, how much energy would be released? — TC

Since the discovery of relativity, people have recognized that there is energy associated with rest mass and that the amount of that energy is given by Einstein’s famous equation: E=mc2. However, the energy associated with rest mass is hard to release and only tiny fractions of it can be obtained through conventional means. Chemical reactions free only parts per billion of a material’s rest mass as energy and even nuclear fission and fusion can release only about 1% of it. But when equal quantities of matter and antimatter collide, it’s possible for 100% of their combined rest mass to become energy. Since two metric tons is 2000 kilograms and the speed of light is 300,000,000 meters/second, the energy in Einstein’s formula is 1.8×1020 kilogram-meters2/second2 or 1.8×1020 joules. To give you an idea of how much energy that is, it could keep a 100-watt light bulb lit for 57 billion years.

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 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.

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.