Author: Lou Bloomfield

You claim that the metal walls of the cooking chamber in a microwave oven protec…

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You claim that the metal walls of the cooking chamber in a microwave oven protect us from the microwaves. How can they protect us from microwaves when they aren’t even able to keep sound contained? You can hear popcorn popping through the walls. — RB, Beltsville, MD

The fact that sound waves can pass through the cooking chamber’s metal walls doesn’t mean that microwaves can. These two types of waves are very different and the chamber’s walls handle them very differently.

Any type of wave will partially reflect from a surface if passing through that surface causes the wave’s speed to change or, more generally, introduces a change in the “impedance” the wave experiences. Impedance is a quantity that relates various parts of a wave to one another—it relates pressure to velocity in sound and it relates the electric field to the magnetic field in a microwave. Since both sound waves and microwaves change speeds and impedances when they encounter the cooking chamber’s metal walls, they both partially reflect. The sound that you hear when popcorn pops inside the oven is slightly muffled because the sound is having some trouble escaping from the cooking chamber. However, the impedance change for the microwaves is so enormous that the reflection is complete. No microwaves at all escape from the cooking chamber! The same effect occurs when you hold a large mirror up in front of your face. You can hear what’s happening on the other side of the mirror because some sound can pass through the mirror. But light is completely reflected and you can’t see through the mirror at all.

If there was a hole drilled directly through the center of the earth and a ball …

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If there was a hole drilled directly through the center of the earth and a ball was dropped into it, what would happen to the ball? Would it oscillate up and down in the hole until it remained suspended in the center? — JC, Dallas, TX

Yes, if the hole were drilled from the north pole to the south pole, the ball would behave just as you say. Assuming that there were no air resistance, the ball would drop through the center of the earth and rise to the surface on the other side. It would then return via the same path and travel all the way back to your hand. This motion would repeat over and over again, with the ball taking 84 minutes to go from your hand to your hand. That time is the same as it would take a satellite to orbit the earth once at sea level. In effect, the ball is orbiting through the earth rather than around it!

However, because there would be air resistance unless you maintained a vacuum inside the hole, the ball wouldn’t rise to its original height after each passage through the earth. It would gradually loss energy and speed, and would eventually settle down at the very center of the earth.

Finally, the reason for drilling the hole from the north pole to south pole is to avoid complications due to the earth’s rotation. If you were to drill the hole anywhere but through the earth’s rotational axis, the ball would hit the sides of the hole as it fell and its behavior would be altered.

Could the traffic flow on freeways be modeled as a one-dimensional gas? You can …

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Could the traffic flow on freeways be modeled as a one-dimensional gas? You can see waves of motion among the cars and these waves travel faster as the cars pack more tightly. — RH, Escondido, California

There are many similarities between the cars traveling on a freeway and the molecules in a gas. As you point out, disturbances at one point in the traffic cause ripples of motion to spread backward through the cars—similar to what happens in a gas. However, normal gas molecules only interact with one another when they actually touch, while cars interact at much larger distances—unlike gas molecules, cars don’t do so well when they collide with one another. To avoid collisions, the drivers watch what’s happening far ahead of them and react accordingly. In that sense, traffic’s behavior resembles that of a non-neutral plasma—a gas of charged particles that all have the same electric charge and therefore repel one another even at large distances. If you were to send such a plasma through a narrow pipe, its particles would jostle back and forth as they tried to stay as far as possible from one another. Ripples of motion would pass through the plasma and this motion would be very similar to that of cars on a freeway.

How does the tachometer in a new car work? It looks like a magnet wrapped with w…

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How does the tachometer in a new car work? It looks like a magnet wrapped with wire that’s located very near a saw-toothed wheel that spins as the engine turns. — TR, Provo, UT

The device you describe is essentially an electric generator. The toothed wheel is made of pure iron so that its teeth can become temporarily magnetized while they are close to the permanent magnet. When a tooth becomes magnetized as it approaches the permanent magnet, or demagnetized as it moves away from the permanent magnet, it changes the shape and strength of the magnetic field around the permanent magnet. Since changing magnetic fields produce electric fields, the tooth’s movement causes an electric field to appear around the magnet. This electric field pushes on mobile electric charges in the wire coil wrapped around the magnet and generates electricity. The current in the coil flows one way as a tooth approaches the magnet and reverses when that tooth moves away from the magnet. Also, the faster the tooth moves, the stronger the change in the magnetic field and the higher the voltage generated in the coil. The tachometer can tell how fast the engine is turning by how frequently the current in the coil reverses directions or by how much voltage the coil generates.

What makes an airplane fly?

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What makes an airplane fly? — SDH, Vicksburg, MS

While there are several ways to understand how air supports a plane’s weight, I will look at it first in terms of the deflection of the air flowing past the plane’s wings. As the plane moves forward, air flows both over and under the plane’s wings. It flows across the wing from its leading edge to its trailing edge. The air that strikes the inclined lower surface of the wing is deflected downward and leaves the wing’s trailing edge with a slight downward component to its motion. The air that flows over the arced and inclined upper surface of the wing travels a more complicated route, curving up, over, and down before leaving the wing’s trailing edge with a slight downward component to its motion. In both cases, the wing has made the air accelerate downward by pushing the air downward and it is the nature of our universe that the air must push upward on the plane in response. It’s a case of action and reaction: if one object pushes on another, the second object must push back on the first object with an equal but oppositely directed force. So the plane’s wing pushes down on the air and the air pushes up on the plane. When the plane is moving fast enough and the wings are properly shaped and/or tilted, the upward force that the air exerts on the wings can support the weight of the plane and suspend it in the air.

Another important view of flight involves air pressure in the streams of air flowing over and under the plane. When the air passing under the wing curves downward, it actually does so because the pressure just under the wing is higher than the pressure far from the wing—the air stream is experiencing an overall downward force due to this pressure imbalance and this downward force is deflecting the air stream downward. When the air passing over the wing arcs up, over, and down, it is also doing so because the pressure just above the wing is different from that far from the wing. In this case, the pressure just over the wing’s leading edge is quite high—enough to deflect the air stream upward initially. But the pressure over the rest of the wing’s upper surface is very low and the air stream curves inward toward the wing; arcing downward so that it leaves the wing’s trailing edge with a small downward component to its motion. Overall, there is a low average pressure above the wing and a high average pressure below it. This pressure imbalance produces an overall upward force on the wing and supports the plane’s weight.

These two views of flight—one involving deflection of the air stream and the other involving pressure imbalances—are intimately related to one another and really only two descriptions of the same process. Incidentally, the low pressure just over most the wing causes the air flowing over that wing to speed up. That’s Bernoulli’s equation in action—when air following a streamline experiences a drop in pressure, it accelerates in the forward direction.

What is the speed with which electric power is transmitted through the power gri…

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What is the speed with which electric power is transmitted through the power grid? Believe it or not, the education center at an important nuclear power plant claims that “electrons travel at the speed of light,” an obvious impossibility for current in a copper wire. What is the maximum speed of an electron in a commercial electric power grid? in a superconductor? — AW, Alexandria, VA

Amazingly enough, the speed at which electric power travels through a wire is very different from the speed at which electrons move through that wire. In most wires, electric power travels at very nearly the speed of light while the electrons themselves travel only millimeters per second! This statement is true whether the electricity is traveling in a copper wire or a superconductor!

To understand how this difference in speeds is possible, think about what happens when you turn on the water to a long hose. If that hose is already filled with water, water will immediately begin pouring out of the hose’s end even though the water is flowing quite slowly through the hose. While the water itself moves slowly, the water’s effects travel through the hose at the speed of sound in water—several miles per second! Water at the end of the hose “knows” that you have opened the faucet long before new water from the faucet arrives.

Similarly, when you turn on a flashlight, electrons begin to flow out of the battery’s negative terminal at speeds of only a few millimeters per second. But these electrons don’t have to travel all the way to the light bulb for the bulb to light up. When these electrons leave the battery, they push on the electrons in front of them, which push on the electrons in front of them, and so on. They produce an electromagnetic wave that rushes through the wire at an incredible speed. As a result, electrons begin flowing through the light bulb only a few billionths of a second after the first electron left the battery. So while the electrons that carry electricity through the power grid flow rather slowly, the power they deliver moves remarkably fast.

During a recent ice storm, I was standing in my front doorway before dawn and th…

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During a recent ice storm, I was standing in my front doorway before dawn and the entire southern sky turned brilliant blue-green for about five seconds or more. What caused this effect? People who missed it tell me it was just a transformer “blowing up” but I’ve seen one blow up on our street and there is no comparison. The light I saw virtually filled the entire horizon.

You probably saw a sustained high-voltage arc between high-tension wires and/or the ground. I would guess that the ice pulled down one of the wires or caused a tree to fall across them. While transformer explosions often involve hundreds of kilowatts of electric power being turned into light and heat, most of that light is hidden from view inside the transformer. Such an explosion can be dramatic, with some nice sparks and flashes, but it’s usually not very bright. However, when a high-tension wire arcs, a significant fraction of the many megawatts of power flowing through the arc is converted directly into light. In effect, a high-pressure arc lamp forms right in the air and it looks like a camera flash that just keeps going until something stops the arc or the power is shut off. The blue-green color you saw comes from characteristics of the air and metal wires involved in the arc. As you saw, a couple of million watts of light are enough to light up the predawn sky quite effectively!

There is, however, an alternative explanation: you may have seen the “green flash” that occasionally appears just as the sun reaches the horizon at sunrise or sunset. This flash is a refraction effect in the atmosphere in which only blue-green light from the sun reaches the viewer’s eyes for a second or two while the sun is just below the horizon. However, this green flash should appear in the eastern sky just before dawn, not the southern sky.

How does an ultrasonic bath work?

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How does an ultrasonic bath work? — PT

An ultrasonic cleaner exposes a bath of liquid to very intense, very high frequency sound. Sound itself consists of regions of high and low pressure that move through a material as waves. As these waves pass through the liquid in the bath, each tiny portion of liquid vibrates back and forth in response to these pressure fluctuations. Near the surface of an object immersed in the bath, the liquid is pushed first toward the object and then away from it. The pressures involved are large and the changes in velocity within the liquid are so intense that occasionally the liquid will actually pull away completely from the object so that a tiny empty cavity forms. In effect, the liquid is jumping up and down on the object’s surface and it occasionally jumps so hard that it leaves the surface altogether. Cavities of this sort are unstable and the liquid soon returns to the object. When it does return, the liquid collides violently with the surface and the liquid’s pressure skyrockets as it transfers all of its momentum to the object in millionths of a second. This “cavitation” process is what cleans objects immersed in the ultrasonic bath—the dirt and grime are pounded free by the liquid when it returns to fill cavities that have formed during the vibrations.

I have heard that there is a substantial cost to starting a fluorescent light fi…

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I have heard that there is a substantial cost to starting a fluorescent light fixture. When entering and exiting a room frequently, is it better to leave a fluorescent light turned on, or to turn it off when leaving each time? — GEW

Whenever you turn on a fluorescent lamp, a small amount of metal is sputtered away from the electrodes at each end of the tube. These electrodes are what provide electric power to the gas discharge inside the lamp and sputtering is a process in which fast moving ions (electrically charged atoms) crash into a surface and knock atoms out of that surface. Because sputtering is most severe during start up, a typical fluorescent tube can only start a few thousand times before its electrodes begin to fail. To avoid the expense and hassle of having to replace the tube frequently, you shouldn’t cycle the lamp more than once every ten minutes. If you will only be away for a minute or two, leave the lamp on. But if you will be away for more than about ten minutes, turn it off. Incidentally, the claim that a fluorescent lamp uses a fantastic amount of electric power during start-up is nonsense. It’s just a myth.

is a photon a specific unit of measurement of light? Has it been decided if ligh…

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is a photon a specific unit of measurement of light? Has it been decided if light is a particle or a wave? Why? — J, Australia

There is no doubt about it: light is both a particle and a wave. While it is traveling, light behaves as a wave—for example, it has a wavelength. But when it is being emitted or absorbed, light behaves as a particle—for example, it may transfer momentum, angular momentum, and energy to whatever it hits. A photon is a quantum of light, the smallest packet of light that can exist. You can’t have half a photon of light—it’s all or nothing. The amount of energy in a particular photon of light depends on the frequency (or wavelength) of that light.