Author: Lou Bloomfield

How does an electric motor work? – BR

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How does an electric motor work? – BR

An electric motor uses the attractive and repulsive forces between magnetic poles to twist a rotating object (the rotor) around in a circle. Both the rotor and the stationary structure (the stator) are magnetic and their magnetic poles are initially arranged so that the rotor must turn in a particular direction in order to bring its north poles closer to the stator’s south poles and vice versa. The rotor thus experiences a twist (what physicists call a torque) and it undergoes an angular acceleration—it begins to rotate. But the magnets of the rotor and stator aren’t all permanent magnets. At least some of the magnets are electromagnets. In a typical motor, these electromagnets are designed so that their poles change just as the rotor’s north poles have reached the stator’s south poles. After the poles change, the rotor finds itself having to continue turning in order to bring its north poles closer to the stator’s south poles and it continues to experience a twist in the same direction. The rotor continues to spin in this fashion, always trying to bring its north poles close to the south poles of the stator and its south poles close to the north poles of the stator, but always frustrated by a reversal of the poles just as that goal is in sight.

Is it possible to sense when a person touches a car, even if the car is painted?…

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Is it possible to sense when a person touches a car, even if the car is painted? – AW

Yes. I wouldn’t try to detect mechanical contact, because you’d have trouble differentiating between forces exerted on the car by a hand and those exerted on it by sound waves. But you can tell whether a conducting object (such as a person) is near the car by looking at the car’s electric properties. If you were to send electric charge on and off the car rapidly with a source of high-frequency alternating current, you would find that the amount of charge that flowed on or off the car during each cycle would change as the person’s hand approached the car. That’s because the charges on the car would push or pull on charges in the person’s hand and the charges in the person’s hand would move. In effect, the person’s hand would make the car “larger” and it would draw more charge from your current source. Even if the person didn’t touch the car, the nearness of the hand and car would change the way current flowed on and off the car. Such a change would be easy to detect with laboratory equipment and could probably be made by cheap consumer equipment, too. The only complications would be in not detecting everything—passing cars for example—and in not damaging the device with static discharges. Still, I think all of that could be done.

What is the principle of the Trinitron Sony TV system?

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What is the principle of the Trinitron Sony TV system? — JPD, Spiennes, Belgium

To form a color image, a color television illuminates a dense pattern of tiny spots—some red, some green, and some blue. By mixing various amounts of these three primary colors of light, the color television can make us perceive any color. But the television must control the amounts of these three colors at each spot on the screen, a very difficult task. A typical color television does this by shining three separate beams of electrons through a mask with holes in it and onto a screen that’s covered with tiny phosphor spots. Because the three beams approach the mask at different angles, they illuminate different portions of the screen after passing through the holes. Thus the “blue” beam only illuminates spots of blue phosphor, the “red” beam illuminates red spots, and the “green” beam illuminates green spots.

However, the Sony Trinitron system uses a line mask rather than one containing holes and the phosphors are coated onto the screen in stripes rather than spots. Again, three separate electron beams are used but they now illuminate specific stripes of phosphor rather than spots of phosphor. The advantage of the stripe approach is that there is more active phosphor on the screen (fewer dark places between spots) so the image is brighter.

What is the black holey stuff on the doors of microwave ovens? Is it for looks, …

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What is the black holey stuff on the doors of microwave ovens? Is it for looks, protection, or what? – K

The black holey stuff on the window of a microwave oven is a metal shield that keeps the microwaves inside the cooking chamber. Because the holes in this metal sheeting are so much smaller than the wavelengths of the microwaves (about 12 cm), the microwaves respond to the sheeting as though it were solid metal and they reflect almost perfectly. By keeping the microwaves inside the oven, this sheeting speeds cooking and protects you from the microwaves.

Why is it bad to put metal in a microwave oven? – OR

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Why is it bad to put metal in a microwave oven? – OR

It isn’t necessarily bad to put metal in a microwave oven, but it can cause cooking problems or other trouble. Microwaves cause currents to flow in metals. In a thick piece of metal, these currents won’t cause problems for the metal. However, in thin pieces of metal, the currents may heat the metal hot enough to cause a fire. Metallic decorations on fine porcelain tend to become hot enough to damage the porcelain. But even thick pieces of metal can cause problems because they tend to reflect the microwaves. That may cause cooking problems for the food nearby. For example, a potato wrapped in aluminum foil won’t cook at all in a microwave oven because the foil will reflect the microwaves. The currents flowing in the metal can also produce sparks, particularly at sharp points, and these sparks can cause fires. In general, smooth and thick metallic objects such as spoons aren’t a problem, but sharp or thin metallic objects such as pins or metal twist-ties are.

How does a discotheque laser work and how could I build one?

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How does a discotheque laser work and how could I build one? — JPD, Spiennes, Belgium

If you are referring to a system that displays illuminated line drawings on a wall that move with the music, then building one is easy. You need a small isolated speaker—just the electronic device, not a whole speaker unit—that you can connect to the music amplifier. Place an elastic membrane over that speaker—a stretched sheet of thin rubber from a latex glove should work well. Then glue a tiny, front-surface mirror to that rubber membrane, choosing a point that is about midway between the middle of the speaker and its edge. A front surface mirror is one that is shiny on its top, so that light doesn’t have to go through glass before reflecting. A broken fragment of mirror, about 3 mm on a side, should work. Finally, shine the beam of a laser pointer onto the mirror and begin to play music through the speaker. The mirror will move with the music and the reflected laser beam will form pretty patterns on the wall.

Why does breathing helium make our voices sound Mickey Mouse-ish? Is there anyth…

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Why does breathing helium make our voices sound Mickey Mouse-ish? Is there anything we can drink that will have the same effect for a longer period? – AP

The pitch of your voice is largely determined by the dimensions of your larynx. That’s why men, with their larger larynxes, generally have lower voices than women. While the sound of your voice originates in the vibrations of your vocal cords, string-shaped objects aren’t very good at emitting sound. Just as a violin employs a box to assist its strings in producing sound, you use your larynx to assist your vocal cords in producing sound. Which pitches your larynx produces well depends on its size and on the speed of sound. Both of these factors are important because the air itself vibrates and either decreasing the size of your larynx or allowing sound to move faster from one side of it to the other will raise the pitch of your voice. Because the speed of sound is much higher in helium (965 m/s) than it is in air (331 m/s), the pitch of your voice rises when you breathe in helium gas. However, as soon as the helium has left your lungs and is replaced by air, your voice returns to normal. Apart from breathing gases with high speeds of sound, there isn’t anything else that will work. You can’t live on pure helium gas, so the only way to sustain this effect would be to breath a helium/oxygen mixture instead of air. Some deep-sea divers do just that and their voices continue to sound “Mickey Mouse-ish” as long as they breathe this mixture.

Can we create an invisible wall of electromagnetic fields? – AW

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Can we create an invisible wall of electromagnetic fields? – AW

Not really. While you could make an electromagnetic “wall” of laser beams or X-ray beams, it wouldn’t really be “invisible” and it wouldn’t feel like a solid wall. It would just cause injury if you put your hand through it. For a surface to feel like a wall, it would have to push your hand backward if you tried to move your hand through it. A real wall does just that and it does so with electromagnetic forces—when you touch a wall, electromagnetic forces that the wall’s atoms exert on your atoms push your hand back and prevent it from penetrating the wall. So a clear window could be described as an “invisible wall of electromagnetic fields,” but that isn’t really what you want. A freestanding electromagnetic field, one that doesn’t involve atoms yet prevents your hand from penetrating it, just isn’t possible.

Do sparks generated by Tesla coils shock humans? If not, why not? – AW

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Do sparks generated by Tesla coils shock humans? If not, why not? – AW

A Tesla coil is radio-frequency transformer that produces small currents of very high-energy electric charges. A radio frequency alternating current passes through the primary coil of this transformer and it induces a current in the secondary coil of the transformer. The frequency of the alternating current must be extremely high because there is no iron in the core of the transformer to store energy during a cycle, so that each cycle must be very brief. Because the alternating current flowing out of the secondary coil of the transformer has a very high frequency, it travels over the surface of a conductor, rather than through its center. Thus when you allow that current to pass through you, it goes along your skin and not through your body. As a result, you barely feel its passage except perhaps as surface heating (however, it can cause what is called an “RF burn” in some cases.) Also, the current from a typical Tesla coil is very small so it would barely be noticeable even if it went through your body.

Is there a touch sensor that can sense when you touch the body of a car? – AW

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Is there a touch sensor that can sense when you touch the body of a car? – AW

The same touch sensors that are used in “touch” lamps or some elevator buttons could be used to sense when you touch a car. A car is essentially insulated from the ground by its rubber wheels, so that when you touch it there is a tendency for electric charge to be transferred between the earth and the car through you. That’s why you may receive a shock when you touch a car on a cold winter day. Many electronic devices are capable of detecting this charge transfer (in fact, many of them would be damaged by such sudden and large charge transfers). So building a car touch sensor would be easy. Whether there is a commercial product that does this is another matter, and I am not sure of the answer.