. Einstein's famous equation E=mc2
says that mass is directly proportional to energy. Does this mean that an object that is suspended overhead has more mass than an object located at ground level? — ST, Denver, CO
Yes, the mass/energy of a suspended object is greater than the mass/energy of that same object at ground level. The extreme example of this result comes with lowering an object slowly toward the surface of a black hole—as the object descends, its mass/energy diminishes until it reaches zero at the surface of the black hole.
. What is a vacuum? Is it filled with charges with no mass? — AW, Karachi, Pakistan
In principle, a vacuum is a region of space containing no real particles (no atoms, molecules, electrons, or other subatomic particles). Because the universe is filled with particles that pass easily through lots of matter (neutrinos, for example), it's very hard to obtain a true vacuum. But let's suppose that you could actually obtain a region of space with no real particles in it. That region of space would still contain large numbers of virtual particles at any given moment. These virtual particles are temporary quantum fluctuations of the vacuum; brief excursions of the quantum fields associate with various subatomic particles. These excursions are permitted by the Heisenberg uncertainty principle, which allows temporary violations of the conservation of mass/energy as long as those violations are extremely brief. While the presence of these virtual particles can only be detected indirectly, they are not massless. Except for their short lifetimes, these particles have characteristics similar to those of normal particles. In fact, if enough energy is used in the process of looking for a virtual particle, that virtual particle can be converted from virtual to real so that it can be detected directly. The energy of detection serves to "pay" for the mass of the particle so that it can leave the virtual realm and become a real, permanent particle.
. What makes a paper airplane fly when its wings are not shaped like real airplane wings? — JC, Idaho Falls, ID
Even though a paper airplane's wings are flat, they experience all of the aerodynamic forces found in more sophisticated wings. For example, when the air flowing past the paper airplane encounters the lower surfaces of its wings, this air slows down and its pressure rises above atmospheric pressure. However, while the air flowing over a sophisticated airplane wing experiences a substantial increase in speed and consequently a drop in pressure, this effect is very small in a paper airplane's wing. Depending on how the air flows over or around the wing's leading edge and whether or not it breaks away from the wing's upper surface, the air pressure above the wing will be at or slightly below atmospheric pressure. Nonetheless, the air pressure below the wing is always slightly higher than that above the wing and the wing experiences a net upward aerodynamic force—a lift force. If you examine the airflow around a well-designed paper airplane wing, all of the flow features that occur around a sophisticated wing will be present but weak. Bowing the wing outward, as is done in a sophisticated wing, simply enhances those features so that the wing can lift a larger load.
. 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.
. 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.
. 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.
. Is it safe to live near high-power lines?
It's probably fine. While the high-tension wires do create modest alternating electric and magnetic fields, there is no credible evidence that these fields cause any injury and no one has proposed a convincing mechanism whereby those fields could affect biological tissue.
. 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.
. 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°.
. 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.