Author: Lou Bloomfield

I am interested in experimenting with colored flames, maybe by adding a substanc…

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I am interested in experimenting with colored flames, maybe by adding a substance to the flame. Please tell me how to do it and with what kind of substances. — M

You can produce colored flames by adding various metal salts to the burning materials. That’s what’s done in fireworks. These metal salts decompose when heated so that individual metal atoms are present in the hot flame. Thermal energy in the flame then excites those atoms so that their electrons shift among the allowed orbits or “orbitals” and this shifting can lead to the emission of particles of light or “photons”. Since the orbitals themselves vary according to which chemical element is involved, the emitted photons have specific wavelengths and colors that are characteristic of that element.

To obtain a wide variety of colors, you’ll need a wide variety of metal salts. Sodium salts, including common table salt, will give you yellow light—the same light that’s produced by sodium vapor lamps. Potassium salts yield purple, copper and barium salts yield green, strontium salts yield red, and so on. The classic way to produce a colored flame is to dip a platinum wire into a metal salt solution and to hold the wire in the flame. Since platinum is expensive, you can do the same trick with a piece of steel wire. The only problem is that the steel wire will burn eventually.

I want to support a group of bird feeders on a horizontal cable, one end of whic…

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I want to support a group of bird feeders on a horizontal cable, one end of which will be fastened to my house and the other end of which will run over an 8 inch pulley attached to a large tree. That end of the cable will be attached to some concrete blocks which must be heavy enough to keep the horizontal cable taut at all times. The idea is to prevent the cable from snapping when the tree moves in high winds. It’s already done so twice, even though I left what I thought was adequate slack in the line. I guess this sounds like a Rube Goldberg solution, but I can’t think of any other solution. How much should the concrete blocks weigh? — HS, Burk’s Falls, Ontario

Your solution should work nicely—the pulley and weight system should protect your cable from breaking because the weights should maintain a constant tension in the line. As the tree swings back and forth, the weights should rise and fall while the tension in the cord remains almost steady. Obviously, if the rising weights reach the pulley the cord will pull taut and break, so you must leave enough hanging slack.

However, if the tree’s motion is too violent, even this weight and pulley system may not save the cable. As long as everything moves slowly, the tension in the cord should be equal to the weight of the weights. But if the tree moves away from the house very suddenly, then the tension in the cord will increase suddenly because the cord must not only support the weights, it must accelerate them upward as well. Part of the cord’s tension acts to overcome the weights’ inertia. Just as a sudden yank on a paper towel will rip it free from the roll, so a sudden yank on your cable will rip it free from the weights. If sudden yanks of this type cause trouble for you, you can fix the problem by coupling the cord to the weights via a strong spring. On long timescales, the spring will have no effect on the tension in the cord—it will still be equal to the weight of the weights. But the spring will stretch or contract during sudden yanks on the cord and will prevent the tension in the cord from changing abruptly either up or down. The spring shouldn’t be too stiff—the less stiff and the more it stretches while supporting the weights, the more effectively it will smooth out changes in tension.

As far as the weight of the weights, that depends on how much curvature you want in the cable supporting the feeders. The more weight you use, the less the cable will sag but the more stress it will experience. You can determine how much weight you need by pulling on the far end of the cable with your hands and judging how hard you must pull to get a satisfactory amount of sag.

Why do only certain orbitals exist in an atom?

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Why do only certain orbitals exist in an atom?

Because the electrons in an atom move about as waves, they can follow only certain allowed orbits that we call orbitals. This limitation is equivalent to the case of a violin string—it can only vibrate at certain frequencies. If you try to make a violin string vibrate at the wrong frequency, it won’t do it. That’s because the string vibrates in a wave-like manner and only certain waves fit properly along the strong. Similarly, the electron in an atom “vibrates” in a wave-like manner and only certain waves fit properly around the nucleus.

When an electron hits a neon atom, does it transfer its energy to the atom and l…

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When an electron hits a neon atom, does it transfer its energy to the atom and lose its own forever?

Most of the collisions between an electron and a neon atom are completely elastic—the electron bounces perfectly from the neon atom and retains essentially all of its kinetic energy. But occasionally the electron induces a structural change in the neon atom and transfers some of its energy to the neon atom. In such a case, the electron rebounds weakly and retains only a fraction of its original kinetic energy. The missing energy is left in the neon atom, which usually releases that energy as light.

You said that some rooms in the physics building are made with metal to specific…

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You said that some rooms in the physics building are made with metal to specifically keep electromagnetic waves out. How does that work?

Some experiments are so sensitive to electromagnetic waves that they must be performed inside “Faraday cages”. A Faraday cage is a metal or metal screen box. Its walls conduct electricity and act as mirrors for electromagnetic waves. As long as a wave has a wavelength significantly longer than the largest hole in the walls, that wave will be reflected and will not enter the box. This reflection occurs because the wave’s electric field pushes charges inside the metal walls and causes those charges to accelerate. These accelerating charges redirect (absorb and reemit) the wave in a new direction—a mirror reflection. Just as a box made of metal mirrors will keep light out, a box made with metal walls will keep electromagnetic waves out.

Can microwave ovens leak microwaves? Is my mother’s warning not to stand in fron…

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Can microwave ovens leak microwaves? Is my mother’s warning not to stand in front of the microwave while it’s on valid?

A properly built and maintained microwave oven leaks so little microwave power that you needn’t worry about it. There are also inexpensive leakage testers available that you can use at home for a basic check, or for a more reliable and accurate check—as recommended by both the International Microwave Power Institute (IMPI) and the FDA—you can take your microwave oven to a service shop and have it checked with an FDA certified meter. It’s only if you have dropped the oven or injured its door in some way that you might have cause to worry about standing near it. If it were to leak microwaves, their main effect would be to heat your tissue, so you would feel the leakage.

Is a CB radio also an AM radio?

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Is a CB radio also an AM radio?

CB or citizens band radio refers to some parts of the electromagnetic spectrum that have been set aside for public use. You can operate a CB radio without training and without serious legal constraints, although the power of your transmitted wave is strictly limited. The principal band for CB radio is around 27 MHz and I think that the transmissions use the AM audio encoding scheme. As you talk, the power of your transmission increases and decreases to represent the pressure fluctuations in your voice. The receiving CB radio detects the power fluctuations in the radio wave and moves its speaker accordingly.

What kinds of things get stored in read-only memory, as opposed to storing them …

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What kinds of things get stored in read-only memory, as opposed to storing them on the hard drive?

When you first turn on a typical computer, it must run an initial program that sets up the operating system. This initial program has to run even before the computer is able to interact with its hard drive, so the program must be available at the very instant the computer’s power becomes available. Read-only memory is used for this initial bootup operation. Unlike normal random access memory, which is usually “volatile” and loses its stored information when power is removed, read-only memory retains its information without power. When you turn on the computer, this read-only memory provides the instructions the computer uses to begin loading the operating system from the hard drive.

Is it true that you shouldn’t put a speaker near a microwave oven?

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Is it true that you shouldn’t put a speaker near a microwave oven?

A microwave oven that’s built properly and not damaged emits so little electromagnetic radiation that the speaker should never notice. The speaker might have some magnetic field leakage outside its cabinet, and that might have some effect on a microwave oven. However, most microwaves have steel cases and the steel will shield the inner workings of the microwave oven from any magnetic fields leaking from the speaker. The two devices should be independent.

Why can you force the current from the n-type semiconductor to the p-type after …

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Why can you force the current from the n-type semiconductor to the p-type after a p-n junction has been created but you can’t force current from the p-type to the n-type?

Actually, you are asking about a current of electrons, which carry a negative charge. It’s true that electrons can’t be sent across the p-n junction from the p-type side to the n-type side. There are several things that prevent this reverse flow of electrons. First, there is an accumulation of negative charge on the p-type side of the p-n junction and this negative charge repels any electrons that approach the junction from the p-type end. Second, any electron you add to the p-type material will enter an empty valence level. As it approaches the p-n junction, it will find itself with no empty valence levels in which to travel the last distance to the junction. It will end up widening the depletion region—the region of effectively pure semiconductor around the p-n junction; a region that doesn’t conduct electricity.