Would it be possible for a spacecraft to use electrically powered propulsion? Co…

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Would it be possible for a spacecraft to use electrically powered propulsion? Could it gather atoms and molecules from space and then use an electromagnetic field to push them through a nozzle? — JC, Burnaby, British Columbia

Not only is it possible to use electrically powered propulsion, such systems are already in use on several spacecraft. While they don’t scavenge atoms and molecules from space, these ion propulsion engines uses electric forces to accelerate ionized atoms to enormous speeds. As the engine pushes on the ions it accelerates, those ions push back on the engine. The ions rush out into space in one direction and the engine experiences a modest thrust in the opposite direction. While the overall thrust from an ion engine is small, it uses its stored-atom “fuel” very efficiently and can be sustained for a very long time in a solar- or nuclear-powered satellite. Ion engines are used in spacecraft that need small but steady thrust for a long time. Scavenging atoms from space would allow these engines to run for an even longer time, but it’s probably not realistic. The atoms in space are typically so rare and so fast-moving that they would be more trouble than they’re worth.

How do thermals affect the atmosphere and air currents?

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How do thermals affect the atmosphere and air currents? — RM, Praire Farm Schools, Wisconsin

Thermals are air currents in the atmosphere. When sunlight and exposure to warm ground raises the temperature of surface air, that air expands—its molecules travel faster and bounce against one another more vigorously, so they push themselves farther apart. This expanded air weighs less per cubic foot or meter than cooler air, so the cooler air around it lifts it upward in a rising current of warm air—a “thermal.” The air can’t simply accumulate way up overhead forever, so cooler air descends to take its place. The overall result is rising warm air and descending cool air. These air currents are part of giant circulation loops or “convection cells” that also include surface winds and high altitude winds.

If you lived on the moon, would it be easier to adapt to living with the moon’s …

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If you lived on the moon, would it be easier to adapt to living with the moon’s gravity, or to create an artificial environment with the gravity of earth? — MK, Orlando, FL

Building an environment that made you feel what appeared to be the earth’s gravity would be a substantial undertaking. The only way to simulate gravity is through acceleration and the only way to make a person experience acceleration continuously is to swing them around in a circle. So this environment will have to swing its occupants around in a circle. However, we are extremely sensitive to changes in orientation, so that we can tell the difference between true gravity and the experience of being swung around in a small circle. To avoid the dizzying feeling of having our orientations changed rapidly, the turning environment would have to be extremely large. It would have to be a huge rotating wheel, looking like a heavily banked circular racetrack that spun at a steady pace and completed something like one full turn per minute. The occupants would have to live on the long, thin surface of this turning racetrack. Building such a device on earth wouldn’t be easy. Building it on the moon would be much harder. I wouldn’t plan on trying to simulate the earth’s gravity on the moon. So I vote for just putting up with the moon’s weaker gravity.

How can people lay on a bed of nails and still survive?

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How can people lay on a bed of nails and still survive? — LW, Marion, OH

If you push gently on the tip of one nail, it won’t pierce your finger. When you push on the nail, it pushes back on you, but the force pushing the nail against your finger isn’t strong enough to break your skin. If you push twice as hard on two nails at once, using two different fingers, then the force you exert on each nail will be the same as before and each nail will push back against one of your fingers with the same force as before. Once again, the nails won’t break your skin. If you now push 100 times as hard against 100 nails, each nail won’t push hard enough against you to break your skin. In fact, a few hundred nails will be able to push on you with an overall force equal to your weight without piercing you. That’s the idea behind a bed of nails—by lying on many nails at once, you allow so many nails to push upward on you that, while the overall force they exert on you is enough to balance your weight, the force exerted by each individual nail isn’t enough to draw blood. These nails have to be spread out around your body so that no individual nails bear more than their fair share of your weight. If one of the nails took too much of your weight, you’d be hurt by it.

With reference to power generation and transmission, can you please explain “Vo…

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With reference to power generation and transmission, can you please explain “Volt Amp Reactance” (VAR, kVAR, MVAR). What is meant by “importing/exporting VAR’s”? What is meant when a plant is “consuming/producing VAR’s”— ID, Northern Territory, Australia

In most situations of AC electric power generation or AC electric power consumption, the current flowing through the circuit is in phase with (or, more simply, directly proportional to) the voltage across the circuit. But that isn’t always the case. In situations involving reactive components (e.g., capacitors and inductors), it’s possible for the current and voltage to be out of phase with one another. If the current and voltage are a full 90° out of phase, there is no average power flowing through the circuit. I believe that VAR is a reference to this portion electricity in the circuit—the portion for which the voltage and current are 90° out of phase. While this portion of the electricity doesn’t transfer any power, it does place demands on the power transmission system. I think that the distinctions between “importing” and “exporting” and between “consuming” and “producing” are related to the phase ordering of the current and voltage (whether a device is acting as a capacitor or an inductor). In one case, the voltage leads the current by 90° and in the other the current leads the voltage by 90°.

Why is it that the same transformers seem to always be hit by lightning?

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Why is it that the same transformers seem to always be hit by lightning?

Lightning tends to strike elevated objects that acquire large charges that are opposite to those of the clouds. Since transformers are often elevated and they are connected to wires that allow them to become highly polarized when a charged cloud passes overhead, transformers are good targets for lightning.

If voltage shocks you, why does current kill you?

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If voltage shocks you, why does current kill you?

Your skin is a very good electric insulator and it prevents any current from passing through your body as long as that current doesn’t have much voltage. A higher voltage (the electric equivalent of “pressure”) is required to push charge through your skin. But once the charge is inside you body, it moves through you quite easily—your body fluids are essentially salt solutions and are relatively good conductors of electricity.

However, a small current passing through your body won’t cause injury. It takes about 0.030 amperes or 30 milliamperes to cause a life-threatening disturbance to your “electric system.” The small currents associated with static electricity are not enough to cause trouble, even through they easily pass through your skin. So high voltages are needed to break through your protective barrier—your skin—in order to give you a shock, but large currents are needed to injury you.

What accounts for the difference between two sounds having the same frequency, l…

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What accounts for the difference between two sounds having the same frequency, loudness, etc. but generated by a guitar and a sitar? — AW, Karachi, Pakistan

Different instruments sound different, even when they play the same notes at the same volumes, primarily because they add different amounts of harmonic tones to their fundamental tones and because these various tones change in volume with time. When you play a note on a guitar, you don’t hear just one pure frequency with a constant volume. Instead, you hear the fundamental frequency and all of the integer multiples of that frequency—the harmonics of that frequency. The relative volumes of those harmonics, and how those volumes change with time, are characteristic of the guitar. If you listen to the same note on a sitar, the relative volumes of the harmonics will be different and you will hear the difference. Because both instruments are plucked, the sounds they emit both start loud and gradually grow softer. If you were to bow their strings, the sound would start soft and gradually grow louder. That’s one reason that you can distinguish a guitar or sitar from a violin.

What will be the source of energy for vehicles 50 years from now?

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What will be the source of energy for vehicles 50 years from now? — AW, Karachi, Pakistan

When the earth’s petroleum supply has been depleted to point where it becomes too precious and expensive to burn, electric vehicles will probably take over. While it’s possible to synthesize chemical fuels, I don’t think it will be worth the trouble. The bigger question is where the electricity needed to charge the batteries will come from. I’ll bet on solar power. Right now, electric cars don’t save fossil fuels or keep the air significantly cleaner because the electricity those cars use is obtained by burning fossil fuels. But the electric cars of the future will probably obtain their electric power from the sun. Nuclear fission and fusion are also possibilities, but fission power has its drawbacks and its not clear when or even if fusion power will be available.