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Electronic Air Cleaners
Page 1 of 2 (12 Questions and Answers)

216. How does sliding your feet on carpet give you a static charge?
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Sliding friction tends to wipe electric charge from one surface to another. Which surface acquires positive charge and which negative charge depends on the chemical properties of those surfaces. You end up with one charge and the carpet with the opposite charge.

217. If one were to use an electrostatic precipitator in a house full of smokers, would the smell from the cigarettes disappear as well? Why or why not? Isn't the smell/odor contained in the molecules and the molecules are contained in the smoke particles, thus removing the odor from the room?
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I'm not sure what fraction of the odor of cigarette smoke is associated with the particles of smoke. An electrostatic precipitator can certainly remove most of the particles and with them, at least a good fraction of the smell. But I suspect that some of the odor is in individual molecules that are less likely to be removed from the air. They are best removed by adsorbing them (sticking them) to a surface, such as the vast surface on granules of activated charcoal. Such granules have pores that allow the molecules to touch lots of internal surface and stick there.

218. If you rub a comb through your hair and hold it near a thin stream of water flowing from a faucet, the stream of water will deflect toward the comb. Why?
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A stream of water can become charged when another charge comes near it. The negatively charged comb attracts positive charge onto the water stream and pushed negative charge off of it. As a result, the stream acquired a positive charge and the rest of the world, a negative charge. The stream deflects toward the oppositely charged comb.

718. How does an electric lighter work? - AW
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In a piezoelectric lighter, a spring-driven mass strikes a piezoelectric crystal and exposes that crystal to a sudden enormous strain. This strain changes the shapes of the electronic levels in the crystal and produces an imbalance in the electric charges on the crystal's surfaces. One side of the crystal acquires a large positive charge, the other a large negative charge. The potential energies of these imbalanced charges are large enough that they have enormous voltages—typically 10,000 to 50,000 volts. With that much voltage (energy per charge), the charges can leap through the air for about a centimeter or more. If you allow these charges to pass through your hand, they will give you a mild shock—there aren't enough charges moving to give you a dangerous shock.

733. What is an electron and what keeps its mass and charge together so that when the mass moves, the charge moves with it? — WG, Calgary, Canada
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An electron is a fundamental particle that has as two of its attributes, a mass and an electric charge. Because the electron appears to be structureless, it has no size and it wouldn't make sense for its mass to be located at a distance from its charge. With a less fundamental particle such as a proton, the charge and mass can be somewhat spread out and displaced so that the charge and mass can move slightly independently. Still, even in the case of a proton there are effects that keep the mass from getting far away from the charge.

861. How does a smoke alarm work? — GL, San Leandro, CA
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The most common type of smoke detector uses a tiny amount of a radioactive element called americium to inject electric charges into the air. In the absence of smoke, these electric charges attach themselves to individual air molecules and the resulting ions (electrically charged molecules) move rather easily through the air. However, if there is smoke present in the air, the electric charges attached themselves to the smoke particles and the resulting charged smoke particles don't move easily through the air. The smoke detector measures how easily the charged items it produces move through the air. If these items move easily, then the air is clean. However if they don't move easily, the air contains smoke and the detector signals danger.

862. What is induced polarization and what are its applications? — PSD, Rio de Janeiro, Brazil
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An electrically neutral object contains both positive and negative electric charges, however, those opposite charges are equal in amount and therefore cancel one another. But this cancellation doesn't mean that the charges are unaffected by another nearby charge. If you hold an electrically neutral object near an electrically charged object, the charged object will cause a slight rearrangement of the charges in the neutral object. Charges opposite to that of the charged object will shift toward that object while charges like that of the charged object will shift away from that object. The neutral object will acquire an "induced polarization", meaning that it its positive and negative charges are displaced relative to one another and that this displacement is "induced" by the presence of nearby charge. Induced polarization is a common effect and is present whenever lightning is about to strike the ground. As an electrically charged cloud drifts overhead, the objects on the ground acquire induced polarization. Their tops become covered with charge opposite that of the cloud and a lightning strike may occur between the cloud and the oppositely charged top of a tree or building.

1011. What causes different types of lightning? — CR, Tokyo, Japan
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Cloud to ground lightning is caused by difference in electric charges between the cloud and the ground. Cloud to cloud lightning is caused by difference in electric charges among the clouds themselves. While I'm not at all expert on the subject, I would guess that the various different types of lightning discharges are caused by differences in the distances between charged objects, by variations in the local electric conductivity of the air and clouds during a discharge, and by the sizes and shapes of the clouds and ground.

1147. Does a device that has radio waves and uses ozone and negative ions have the ability to clean the air in my home? — KTR, Halifax, Nova Scotia
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There are many simple electronic devices that claim to clean the air in your home by making negative ions and ozone (if they involve any radio waves, it's a minor side effect of their internal electronics). The claim is accurate—they do make both ozone and negative ions, and they do clean the air in your home. However, that's not the whole story. First, ozone may have the "fresh" smell that occurs after a thunderstorm (a potent producer of ozone), but ozone is a powerful oxidizing agent and chemical irritant that's considered an environmental pollutant rather than a charming scent. The manufacturers are taking a nuisance effect and touting it as a "valuable feature." Second, the negative charges emitted by these electronic devices attach themselves to dust, ash, pollen, and smoke particles and cause those particles to bind themselves to your walls and furniture. The air really does become cleaner, but every surface in your home becomes dirtier as a result.

If you're seriously interested in cleaning the air in your home, you are probably better off with a full electrostatic air cleaner. Small home versions of this common industrial workhorse are easy to obtain at a local heating and air conditioning store. Properly designed machines use positive ions to avoid producing ozone and provide a negatively charged surface for the positively charged dirt to stick to so that it doesn't deposit itself on your walls.


1275. Can lightning strike a flying airplane? — DC, Denver, CO
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An object doesn't have to be on the ground to be a target for lightning. In fact, most lightning strikes don't reach the ground at all—they occur between different clouds. All that's needed for a lightning strike between two objects is for them to have very different voltages, because that difference in voltages means that energy will be released when electricity flows between the objects.

If an airplane's voltage begins to differ significantly from that of its surroundings, it's going to have trouble. Sooner or later, it will encounter something that will exchange electric charge with it and the results may be disastrous. To avoid a lightning strike, the airplane must keep its voltage near that of its surroundings. That's why it has static dissipaters on the tips of its wings. These sharp metal spikes use a phenomenon known as a corona discharge to spray unwanted electric charges into the air behind the plane. Any stray charges that the plane picks up by rubbing against the air or by passing through electrically charged clouds are quickly released to the air so that the plane's voltage never differs significantly from that of its surroundings and it never sticks out as a target for lightning. While an unlucky plane may still get caught in an exchange of lightning between two other objects, the use of static dissipaters significantly reduces its chances of being hit directly.


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