Author: Lou Bloomfield

Is there an effective shield for the EMF generated from mercury vapor ballasts?

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Is there an effective shield for the EMF generated from mercury vapor ballasts? — CS, Washington, DC

An electric field can always been shielded by encasing its source in a grounded conducting shell. Electrically charged particles in the shell will naturally rearrange themselves in such a way as to cancel the electric fields outside the shell. But magnetic fields are harder to shield, particularly if they don’t change very rapidly with time. The difficulty with shielding magnetic fields comes from the apparent absence of isolated magnetic poles in our universe—there is no equivalent of electrically charged particles in the case of magnetism. As a result, the only way to shield magnetic fields is to take advantage of the connections between electric and magnetic fields.

Because changing magnetic fields are always accompanied by electric fields, the two can be reflected as a pair by highly conducting surfaces or absorbed by poorly conducting surfaces. In these cases, the electric fields push and pull on electric charges in the surfaces and it is through these electric fields that the magnetic fields are reflected or absorbed. However, this effect works much better at high frequencies than at low frequencies, where very thick materials are required. Appliances that operate from the AC power line have magnetic fields that change rather slowly with time (only 120 reversals per second or 60 full cycles of reversal each second) and that are extremely hard to shield with conducting material. Instead, their magnetic fields have to be trapped in special magnetic materials that draw in magnetic flux lines and keep them from emerging into the surrounding space. One of the most effective magnetic shield materials is called “mu metal”, a nickel alloy that’s like a sponge for magnetic flux lines. Since it also conducts electricity pretty well, it is an effective shield for electric fields. So if you wrap your mercury vapor ballasts in mu metal, there would be almost no electric or magnetic fields detectable outside of the mu metal surface.

There is a debate amongst the teachers in our school as to what are the three pr…

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There is a debate amongst the teachers in our school as to what are the three primary colors. Some say Red, Green, and Blue, others say Red, Yellow, and Blue. Do you have an explanation? — RS, Farmington Hills, MI

The true primary colors of light are Red, Green, and Blue. This empirical result is determined by physiological characteristics of the three types of color sensitive cells in our eyes. These cells are known as cone cells and are most sensitive to red light, green light, and blue light respectively. Light that falls in between those wavelength ranges stimulate the three groups of cells to various extents and our brains use their relative stimulations to assign a color to the light we’re seeing. For example, when you look at yellow light, the red sensitive and green sensitive cone cells are stimulated about equally and your brain interprets this result as yellow. When you look at an equal mixture of red light and green light, the red sensitive and green sensitive cells are again stimulated about equally and your brain again interprets this result as yellow. Thus you can’t tell the difference between true yellow light and an equal mixture of red light and green light. That’s how a television tricks your eyes into seeing all colors. If you look closely at a color television screen, you’ll see tiny dots of red, green, and blue light. But when you back up, you begin to see a broad range of colors. The television is mixing the three primary colors of light to make you see all the other colors.

Incidentally, the three primary colors of pigment are yellow, cyan, and magenta. Yellow pigment absorbs blue light, cyan pigment absorbs red light, and magenta pigment absorbs green light. When exposed to white light, a mixture of these three pigments controls the mixture of the reflected lights (red, green, and blue) and thus can make you see any possible color.

When making an electromagnet, why does a hard core stay permanently magnetized w…

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When making an electromagnet, why does a hard core stay permanently magnetized while a soft core does not? — CD, Houston, TX

Iron and steel are intrinsically magnetic materials, meaning that at the atomic scale they exhibit magnetic order and have magnetic poles present. Most materials, including copper and aluminum, have no such magnetic order—they are nonmagnetic all the way to the atomic scale. But while it is composed of magnetic atoms, a large piece of iron or steel normally doesn’t appear magnetic. That’s because a large piece of iron or steel contains many tiny magnetic domains. Although each of these magnetic domains is highly magnetic, with a north pole at one end and a south pole at the other end, the metal appears nonmagnetic at first because these domains point equally in all directions and their magnetizations cancel one another. Before the magnetic character of a piece of iron or steel will become visible, something must align its magnetic domains.

In an electromagnet, an iron or steel core is surrounded by a coil of wire. When you run current through that coil of wire, the magnetic field of the current causes the core’s magnetic domains to change sizes—the domains that are aligned with the field grow at the expense of the domains misaligned with the field and the whole piece of iron or steel becomes highly magnetic. When you stop current from flowing through the coil of wire, the domains may return to their original sizes and shapes and the iron or steel may become nonmagnetic again.

The abilities for magnetic domains to change sizes depends on the chemical and physical properties of the metal, particularly its crystalline structure. In some magnetic materials, the domains change size extremely easily. These materials are considered to be “soft”—they magnetize easily in the presence of a magnetic field and demagnetize easily when that field is removed. Most electromagnets are made from such soft magnetic materials because it takes only a small current in a wire coil to magnetize the electromagnet’s soft core and that core quickly becomes nonmagnetic when you stop the current from flowing.

But in other magnetic materials, the domains don’t change size easily. These materials are considered to be “hard”—they are both difficult to magnetize and difficult to demagnetize. You must put lots of current through the coil of wire around a hard magnetic material in order to magnetize that material. But once you turn off the current, the material will retain its magnetization and it will be a permanent magnet.

How do rockets work?

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How do rockets work?

Rockets push stored materials in one direction and experience a thrust force in the opposite direction. They make use of the observation that whenever one object pushes on a second object, the second object exerts an equal but oppositely directed force back on the first object. This statement is the famous “action-reaction” concept that is generally known as Newton’s third law. While it seems sensible that when you push on a wall it pushes back on you, this situation is extraordinarily general. For example, if you push a passing car forward, that car will still push backward on you with an equal but oppositely directed force. If you push on your neighbor, your neighbor will push back on you with an equal but oppositely directed force even if your neighbor is asleep! In the case of a rocket, the rocket pushes burning fuel downward and the burning fuel pushes upward on the rocket with an equal but oppositely direct force. If the rocket pushes its fuel downward hard enough, the fuel will push up on the rocket hard enough to overcome the rocket’s weight and accelerate it upward into the sky and beyond.

How does one calculate the pressure of air flowing in a tube? My specific applic…

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How does one calculate the pressure of air flowing in a tube? My specific application is air traveling in a 1/2-inch tube at a velocity of 14 inches/second. I know that Bernoulli would have the answer, but I cannot find it myself. — NT, Cambridge, MA

Without more information about the air in your tube, it’s not possible to determine its pressure. Bernoulli’s equation is frequently misunderstood to say that high-speed air is low-pressure air and that low speed air is high-pressure air—two observations that aren’t necessarily true. Just because air is moving rapidly doesn’t mean that its pressure is low. For example, the air in an airplane cabin is moving quickly but its pressure is higher than that of the air outside the cabin. Similarly, if you were to throw a tank of compressed air across the room, its pressure would remain high despite its increase in speed.

What Bernoulli’s equation really says is that air has three forms for its energy and that as long as that air flows smoothly and without significant friction through a system of stationary obstacles, the sum of those three energies can’t change. The three energies are kinetic energy (the energy of motion), gravitational potential energy, and an energy associated with pressure that I call pressure potential energy. The obstacles must remain stationary so that they can’t do work on the air and thus change its total energy. Since the sum of those three energies doesn’t change as air flows through a stationary environment, its pressure typically falls whenever its speed rises and vice versa. If the air also changes altitude significantly, then gravitational potential energy must be included in these energy exchanges.

So the reason why I can’t answer your question about air in a pipe is that I don’t know what the air’s total energy was before it flowed through the pipe. While I can calculate the air’s kinetic energy from its speed and we can neglect gravitational potential energy because the air isn’t changing altitudes much in the pipe, I need to know what the air’s total energy is in order to determine its pressure potential energy and thus its pressure.

What is the difference in distance that a soccer ball will travel if the air pre…

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What is the difference in distance that a soccer ball will travel if the air pressure in the ball changes? — AB

A properly inflated soccer ball bounces well when you drop it on a hard floor because the ball stores energy by compressing the air during the bounce and the air returns this energy quite efficiently during the rebound. An under inflated soccer ball doesn’t bounce so well because it stores energy by bending its leather surface during the bounce and the leather doesn’t return energy very efficiently during the rebound. The same result holds true when you kick a ball rather than dropping it on the floor. Whether a moving ball hits a stationary surface or a stationary ball hits a moving surface, the ball is still bouncing from a surface. When you kick a ball with your foot, the ball is bouncing from your foot and a properly inflated ball will bounce more efficiently from your foot than an under inflated ball. The properly inflated ball will rebound at a higher speed and will travel farther.

How does a thermometer work?

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How does a thermometer work? — DL

A common liquid in glass thermometer takes advantage of the fact that liquids generally expand more than solids as their temperatures increase. The glass envelope of the thermometer contains a fine hollow capillary with a sealed reservoir at its base that’s filled with a liquid such as alcohol or mercury. If both the liquid and glass expanded equally as they became warmer, the thermometer would simply change sizes slightly as its temperature increased. But the liquid expands more than the glass and can’t simply remain in place. Some of it moves up the capillary. That’s why the level of liquid in the thermometer rises as the thermometer’s temperature rises.

What metals and other substances are used in microwave ovens? Specifically, what…

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What metals and other substances are used in microwave ovens? Specifically, what is the substance on the inside of the microwave that absorbs all the microwaves? — AD, San Anselmo, CA

The walls of a microwave oven’s cooking chamber are made of highly conductive metals so that they reflect the microwaves almost completely. Only a very small fraction of the microwaves inside the oven are absorbed by these metal walls and virtually none of the microwaves escape into the room. However, there is a substance inside the cooking chamber that absorbs the microwaves: water in the food! If you don’t put water-containing food inside the microwave oven, there will be nothing to absorb the microwaves and they will reflect back to the magnetron and may damage it. The absence of an absorber in the cooking chamber will also increase any minor leakage of microwaves from the oven because the microwave intensity inside the cooking chamber will be much higher than normal.

What is the definitions of a “Hanning window”, a “rectangular window”, and a…

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What is the definitions of a “Hanning window”, a “rectangular window”, and a “triangular window”? — CV, Cape Town, South Africa

In the days before digital signal processing, the filters that were available for audio or video systems were very simple. These filters monitored the audio or video signal and produced an output signal that was related to the present input signal and to that signals value’s in the recent past. Such simple filters could enhance or diminish certain ranges of frequencies and were able to perform basic tasks such as adjusting the balance between treble, midrange, and bass in an audio system.

But with computers and digital signal processing now commonplace, filtering has become much more sophisticated. Filters can now study an audio or video input signal over a long period of time and can even use data about future values of the input signal when producing an output signal. The filters that you ask about are all digital filters that produce an output signal that is related to the past, present, and future values of the input signal. A rectangular window filter is one that determines the output signal from a certain range of past, present, and future input signal values, all weighted evenly. A triangular or “Parzen” window filter is one that determines the output signal from a certain range of past, present, and future input signal values, with the weighting of values decreasing linearly with increasing time in the past or future. A Hanning window filter is one that determines the output signal from the complete past and future input signal values, with the weighting of values decreasing as the cosine of the time in the past or future (see for example, “Numerical Recipes” by Press, Flannery, Teukolsky, and Vetterling). All three filtering windows and filters are used to keep filters that extract certain frequency ranges from the input signal from affecting other frequency ranges. For that purpose, the Hanning window is better than the Parzen window and both are better than the rectangular window. As an example of the applications of these filters, a digital audio filter that makes good use of the Hanning window can enhance the treble of an audio signal uniformly without coloring the midrange at all. Earlier filters that only used past information always colored the midrange and didn’t affect the treble uniformly.

Is it possible to greatly increase the speed of a roller coaster, while retainin…

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Is it possible to greatly increase the speed of a roller coaster, while retaining some safety, by applying the same theory that is used in Bullet Trains? — JA, Henderson, NV

While roller coasters could be made faster if they used the high performance tracks of bullet trains, smoothing out the tracks would only make the ride less jittery and wouldn’t reduce the accelerations needed to complete the turns. The faster the train moves, the faster everything must accelerate as the track bends. Doubling the speed of the roller coaster would double the changes in velocity associated with each bend and would halve the time available to complete that change in velocity. As a result, doubling the roller coaster’s speed would quadruple the accelerations it experiences on the same track and thus will quadruple the forces involved during the ride. A roller coaster ride already involves some pretty intense forces and accelerations. If those forces and accelerations were increased by a factor of 4, they would be more than most people could handle. Thus I wouldn’t expect many riders on a double-speed bullet train roller coaster.