Month: September 1997

Why are swept wings preferred for transonic/supersonic flight, but not for lower…

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Why are swept wings preferred for transonic/supersonic flight, but not for lower speeds? — CL

While the designers of low speed planes focus primarily on lift and drag, designers of high speed planes must also consider shock waves—pressure disturbances that fan out in cones from regions where the plane’s surface encounters supersonic airflow. The faster a plane goes, the easier it is for the plane’s wings to generate enough lift to support it, but the more likelihood there is that some portions of the airflow around the plane will exceed the speed of sound and produce shock waves. Since a transonic or supersonic plane needs only relatively small wings to support itself, the designers concentrate on shock wave control. Sweeping the wings back allows them to avoid some of their own shock waves, increasing their energy efficiencies and avoiding shock wave-induced surface damage to the wings. Slower planes can’t use swept wings easily because they don’t generate enough lift at low speeds.

How does one find out the speed of a quark? Is it 7000 times the speed of light?…

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How does one find out the speed of a quark? Is it 7000 times the speed of light? — D

It seems that quarks are forever trapped inside the particles they comprise—no one has ever seen an isolated quark. But inside one of those particles, the quarks move at tremendous speeds. Their high speeds are a consequence of quantum mechanics and the uncertainty principle—whenever a particle (such as a quark) is confined to a small region of space (i.e. its location is relatively well defined), then its momentum must be extremely uncertain and its speed can be enormous. In fact, a substantial portion of the mass/energy of quark-based particles such as protons and neutrons comes from the kinetic energy of the fast-moving quarks inside them.

But despite these high speeds, the quarks never exceed the speed of light. As a massive particle such as a quark approaches the speed of light, its momentum and kinetic energy grow without bounds. For that reason, even if you gave all the energy in the world to a single quark, its speed would still remain just a hair less than the speed of light.

What happens to a permanent magnet’s magnetic field if its temperature is lowere…

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What happens to a permanent magnet’s magnetic field if its temperature is lowered? What happens to a magnetic field at absolute zero?

Thermal energy is actually bad for permanent magnets, reducing or even destroying their magnetizations. That’s because thermal energy is related to randomness and permanent magnetization is related to order. Not surprisingly, cooling a permanent magnet improves its ordering and makes its magnetization stronger (or at least less likely to become weaker with time). At absolute zero, a permanent magnet’s magnetic field will be in great shape—assuming that the magnet itself doesn’t suffer any mechanical damage during the cooling process.

What are the effects of water pressure on fish, submarines and divers?

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What are the effects of water pressure on fish, submarines and divers?

All three of these objects contain solids, liquids, and gases, so I’ll begin by describing how pressure affects those three states of matter. Solids and liquids are essentially incompressible, meaning that as the pressure on a solid or a liquid increases, its volume doesn’t change very much. Without extraordinary tools, you simply can’t squeeze a liter of water or liter-sized block of copper into a half-liter container. Gases, on the other hand, are relatively compressible. With increasing pressure on it, a certain quantity of gas (as measured by weight) will occupy less and less volume. For example, you can squeeze a closet full of air into a scuba tank.

Applying these observations to the three objects, it’s clear that the solid and liquid portions of these objects aren’t affected very much by the pressure, but the gaseous portions are. In a fish or diver, the gas-filled parts (the swim bladder in a fish and the lungs in a diver) become smaller as the fish or diver go deeper in the water and are exposed to more pressure. In a submarine, the hull of the submarine must support the pressure outside so that the pressure of the air inside the submarine doesn’t increase. If the pressure did reach the air inside the submarine, that air would occupy less and less volume and the submarine would crush. That’s why the hull of a submarine must be so strong—it must hide the tremendous water pressure outside the hull from the air inside the hull.

Apart from these mechanical effects on the three objects, there is one other interesting effect to consider. Increasing pressure makes gases more soluble in liquids. Thus at greater depths and pressures, the fish and diver can have more gases dissolved in their blood and tissues. Decompression illness, commonly called “the bends”, occurs when the pressure on a diver is suddenly reduced by a rapid ascent from great depth. Gases that were soluble in that diver’s tissue at the initial high pressure suddenly become less soluble in that diver’s tissue at the final low pressure. If the gas comes out of solution inside the diver’s tissue, it causes damage and pain.

I recently read in a sales brochure for a major international energy services co…

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I recently read in a sales brochure for a major international energy services company that the speed of light had been exceeded in 1995. Is this true? If so, could you explain how this was accomplished? — TS

For very fundamental reasons, the speed of light in vacuum cannot be exceeded. Calling it the “speed of light” is something of a misnomer—it is the fundamental speed at which all massless particles travel. Since light was the first massless particle to be studied in detail, it was the first particle seen to travel at this special speed.

While nothing can travel faster than this special speed, it’s easy to go slower. In fact, light itself travels more slowly than this when it passes through a material. Whenever light encounters matter, its interactions with the charged particles in that matter delay its movement. For example, light travels only about 2/3 of its vacuum speed while traveling in glass. Because of this slowing of light, it is possible for massive objects to exceed the speed at which light travels through a material. For example, if you send very, very energetic charged particles (such as those from a research accelerator) into matter, those particles may move faster than light can move in that matter. When this happens, the charged particles emit electromagnetic shock waves known as Cherenkov radiation—there is light emitted from each particle as it moves.

I suppose that the brochure could have been talking about this light/matter interaction. But since that effect has been observed for decades, there is nothing special about 1995. More likely, the brochure is talking about nonsense.

How do bipolar transistors work?

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How do bipolar transistors work? — BR

A bipolar transistor is a sandwich consisting of three layers of doped semiconductor. A pure semiconductor such as silicon or germanium has no mobile electric charges and is effectively an insulator (at least at low temperatures). Dope semiconductor has impurities in it that give the semiconductor some mobile electric charges, either positive or negative. Because it contains mobile charges, doped semiconductor conducts electricity. Doped semiconductor containing mobile negative charges is called “n-type” and that with mobile positive charges is called “p-type.” In a bipolar transistor, the two outer layers of the sandwich are of the same type and the middle layer is of the opposite type. Thus a typical bipolar transistor is an npn sandwich—the two end layers are n-type and the middle layer is p-type.

When an npn sandwich is constructed, the two junctions between layers experience a natural charge migration—mobile negative charges spill out of the n-type material on either end and into the p-type material in the middle. This flow of charge creates special “depletion regions” around the physical p-n junctions. In this depletion regions, there are no mobile electric charges any more—the mobile negative and positive charges have cancelled one another out!

Because of the two depletion regions, current cannot flow from one end of the sandwich to the other. But if you wire up the npn sandwich—actually an npn bipolar transistor—so that negative charges are injected into one end layer (the “emitter”) and positive charges are injected into the middle layer (the “base”), the depletion region between those two layers shrinks and effectively goes away. Current begins to flow through that end of the sandwich, from the base to the emitter. But because the middle layer of the sandwich is very thin, the depletion region between the base and the second end of the sandwich (the “collector”) also shrinks. If you wire the collector so that positive charges are injected into it, current will begin to flow through the entire sandwich, from the collector to the emitter. The amount of current flowing from the collector to the emitter is proportional to the amount of current flowing from the base to the emitter. Since a small amount of current flowing from the base to the emitter controls a much larger current flowing from the collector to the emitter, the transistor allows a small current to control a large current. This effect is the basis of electronic amplification—the synthesis of a larger copy of an electrical signal.

I cannot understand a step-up transformer. Why is the voltage doubled when we do…

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I cannot understand a step-up transformer. Why is the voltage doubled when we double the secondary turns? What isn’t it possible to have a dc transformer; since the law of induction says that when a current passes through a conductor it provides a magnetic field, isn’t it the same as ac? — C

A transformer only works with ac current because it relies on changes in a magnetic field. It is the changing magnetic field around the transformer’s primary coil of wire that produces the electric field that actually propels current through the transformer’s secondary coil of wire.

When dc current passes through the primary coil of wire, the coil does have a magnetic field around it, but it doesn’t have an electric field around it. The electric field is what pushes electric charges through the secondary coil to transfer power from the primary coil to the secondary coil. In contrast, when ac current passes through that primary coil of wire, the magnetic field around the coil flips back and forth in direction and this changing magnetic field gives rise to an electric field around the coil. It is this electric field that pushes on electrically charged particles—typically electrons—in the secondary coil of wire. These electrons pick up speed and energy as they move around the secondary coil’s turns. The more turns these charged particles go through, the more energy they pick up. That’s why doubling the turns in a transformer’s secondary coil doubles the voltage of the current leaving the secondary coil.

How do slot machines work?

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How do slot machines work? — DD, Thunder Bay, Ontario, Canada

A slot machine is a classic demonstration of rotational inertia. When you pull on the lever, you are exerting a torque (a twist) on the three disks contained inside the machine. These disks undergo angular acceleration—they begin turning toward you faster and faster as you complete the pull. When you stop pulling on the lever, the lever decouples itself from the disks and they continue to spin because of their rotational inertia alone—they are coasting. However, their bearings aren’t very good and they experience frictional torques that gradually slow them down. They eventually stop turning altogether and then an electromechanical system determines whether you have won. Each disk is actually part of a complicated rotary switch and the positions of the three disks determine whether current can flow to various places on an electromechanical counter. That counter controls the release of coins—coins that are dropped one by one into a tray if you win. Sadly, computerized gambling machines are slowly replacing the beautifully engineered electromechanical ones. These new machines are just video games that handle money—they have little of the elegant mechanical and electromechanical physics that makes the real slot machines so interesting.