How does a Bourdon tube pressure gauge work? – AM

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How does a Bourdon tube pressure gauge work? – AM

A Bourdon tube pressure gauge works on much the same principle as a party favor that inflates and unrolls when you blow in its tube. The hollow Bourdon tube of the pressure gauge isn’t circular in cross-section—it’s somewhat oval. When the pressure inside the tube increases, the tube’s oval walls are distorted and the tube’s cross-section becomes slightly more circular. However, the tube is wrapped in a coil and as its walls become more circular, the tube uncoils slightly. The amount of uncoiling that occurs is almost exactly proportional to the pressure inside the Bourdon tube. As the tube uncoils, its motion activates a rack-and-pinion gear system that turns the needle on the pressure dial of the gauge. While all that you see when you look at the gauge is this needle pointing at the current pressure, you should understand that there is a small, bent tube that’s coiling and uncoiling with each change in the pressure inside that tube.

What is gravity and how do you define it?

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What is gravity and how do you define it?

There are two levels at which to work. First, there is Newtonian gravity—an attraction that exists between any two objects and that pulls each object toward the center of mass of the other object with a force that’s equal to the gravitational constant times the product of the two masses, divided by the square of the distance separating the two objects. For example, you are attracted toward the earth’s center of mass with a force equal to the gravitational constant times the product of the earth’s mass and your mass, divided by the square of the distance between the earth’s center of mass and your own center of mass. This force is usually called “your weight.” The earth is attracted toward your center of mass with exactly the same amount of force.

Second, there is the gravity of Einstein’s general relativity—a distortion of space/time that’s caused by the local presence of mass/energy. Space is curved around objects in such a way that two freely moving objects tend to accelerate toward one another. As long as those objects aren’t too large or too dense, this new description of gravity is equivalent to the Newtonian version—they both predict exactly the same effects. But when one or both of the objects is extremely massive or very dense, general relativity provides a more accurate prediction of what will happen. In reality, mass/energy really does warp space/time and general relativity does provide the correct view of gravity in our universe. The next level of theory, quantum gravity (which will reconcile the theory of general relativity with the theory of quantum physics), is still in the works.

Is magic a real possibility?

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Is magic a real possibility? — LM, Dartmouth, Nova Scotia

I would define magic as any phenomenon that can’t be explained by the normal laws of nature. In that case, I’m afraid that it isn’t a possibility. Like most physicists, I’m convinced that the laws of physics can ultimately explain everything that we observe. Violations in those laws would have such terrible complications that even a single “magic” event just can’t occur. No doubt, there are people who believe in magic and that view physicists as just another group with a different and incorrect opinion about the world. That’s just wishful thinking. Physics has been extraordinarily successful at explaining how the world works. Unlike magic, physics has an internal consistency that is astonishing and it has the ability to predict behavior with enormous accuracy.

I’ve heard from many people that you should not stand directly in front of and n…

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I’ve heard from many people that you should not stand directly in front of and no closer than 30 feet while a microwave oven is on? Why? If this is a myth how did it get started?

This idea is just a myth. There should be virtually no microwaves leaking from the oven so it shouldn’t matter where you stand. If you’re concerned about microwaves, you can buy a microwave oven tester from a local appliance store or from www.comforthouse.com (or for a more accurate and reliable measurement, take your microwave to a service shop for inspection with an FDA certified meter). I have no idea how such a myth got started, but it’s clear that microwave ovens scare people because they don’t understand them. Given how easy it is to burn yourself on a conventional oven, I’d guess that there are fewer health risks with microwave cooking than with conventional cooking.

Does cooking in a microwave oven destroy the nutritional value of foods? Are mic…

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Does cooking in a microwave oven destroy the nutritional value of foods? Are microwaves radioactive? Does radiation “leak” from the oven? – DL

Microwaves are essentially high frequency radio waves. They heat food by twisting its water molecules back and forth so that those water molecules rub against one another. Like all electromagnetic waves, microwaves are absorbed and emitted as particles or “photons,” but the photons of microwaves have so little energy that they are unable to cause chemical changes in the molecules they encounter. They simply heat food; they don’t “irradiate” it. The only way a microwave oven damages the nutritional value of foods is if it overheats. Microwaves are not radioactive—radioactivity is the spontaneous fragmentation of the nuclei of atoms and is usually associated with the emission of high-energy particles; particles that can induce chemical changes in the molecules they encounter. Finally, if a microwave oven was properly constructed and hasn’t been damaged, virtually no microwaves leak from it. A small amount of microwaves won’t hurt you anyway—they are present all around us already because of satellite transmissions, cellular telephones, and even the thermal radiation from our surroundings.

Why does a microwave oven heat organic material and not inorganic material?

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Why does a microwave oven heat organic material and not inorganic material? — JM, Columbus, OH

A microwave oven heats anything that contains liquid water. Since many organic materials contain water, they will become hot in a microwave oven. But some organic materials such as pure salad oil don’t contain water and won’t become hot in a microwave oven. There are also some inorganic materials such as damp unglazed pottery that contain water and that will become hot.

What is the difference between a low impedance output and a high impedance outpu…

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What is the difference between a low impedance output and a high impedance output of an audio distribution amplifier? What kind of output does consumer equipment need? — JH, Eugene, OR

Some audio amplifiers provide several different outputs, each characterized by the impedance of its expected load (e.g., the impedance of the speaker that you should attach to that output). This impedance measures the relationship between voltage and current that the load needs to function optimally. The higher the impedance, the more voltage the amplifier must provide to propel a particular electric current through the speaker. If the speaker that you attach to the amplifier has the wrong impedance, the amplifier won’t be able to deliver its maximum audio power to the speaker and you may damage the amplifier, speaker, or both.

Since a typical household speaker has an impedance of 8 ohms, you should connect it to an amplifier’s 8 ohm output. However, if you connect more than one speaker to the same output, you should be careful to determine the combined impedance. For example, two 8-ohm speakers in series have a combined impedance of 16 ohms while two 8-ohm speakers in parallel have a combined impedance of 4 ohms. Many amplifiers are designed to accommodate these arrangements.

When a distribution amplifier must send current long distances through thin wires, it will often use higher voltages and lower currents to minimize power losses in the wires. Such an amplifier expects its load to have an unusually large impedance. In this situation, the speaker that is used must either have a large impedance, so that it can use this high voltage/low current power directly, or there must be an impedance matching transformer between the amplifier and the speaker.

How can you make an electromagnet from a battery and copper wire?

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How can you make an electromagnet from a battery and copper wire?

You’ll need a large steel nail or bolt, too. Wrap about 100 turns of copper wire around the nail, keeping the turns fairly uniformly spaced. Make sure that both ends of the wire coil, start and finish, project out from the windings. When you’re done winding the coil, strip off about 1 cm of the insulation from each end of the wire. Now connect one end of the wire to the positive terminal of a AA alkaline battery and the other end of the wire to the negative terminal of that battery. The nail will become a strong magnet and will be able to pick up other nails or paper clips with ease. Electricity will also heat the wire, so be prepared for the electromagnet to become uncomfortably hot. Detach the wires from the battery when you’re no longer able to hold everything safely.

How do electromagnets work?

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How do electromagnets work? — HL, Kurtistown, HI

Whenever an electric current—a current of moving electric charges—flows through a wire, that wire becomes magnetic. This phenomenon is an example of the wonderful interconnectedness of electric and magnetic effects—electricity often produces magnetism and vice versa. Because of its magnetic character, a current carrying wire will exert magnetic forces on another current carrying wire and they are both effectively electromagnets.

A more effective electromagnet uses a coil of wire and a core of very pure iron. Wrapping the wire into a coil gives it specific north and south magnetic poles and adding the iron strengthens those magnetic poles dramatically. Iron is a ferromagnetic material, meaning that it’s intrinsically magnetic. All materials contain electrons and an electron has a spinning character that makes it magnetic. But the electron magnetism in most materials cancels completely and only a few materials such as iron retain the magnetism of their electrons. While iron’s magnetism is hidden as long as its tiny internal magnets are randomly orientated, its magnetic character becomes obvious when it’s inserted in an electromagnet or placed near one. When current flows through the wire coil of the electromagnet, the iron’s magnetic poles align with those of the electromagnet and the electromagnet becomes extremely strong.

What is the relationship between dark material and heat? Why does dark material …

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What is the relationship between dark material and heat? Why does dark material absorb more heat than light material? – AR

Thermal radiation consists of electromagnetic waves. These waves are emitted and absorbed by the movements of electrically charged particles, usually electrons. Since all materials contain electrically charged particles, any of them can interact with thermal radiation. However, these interactions differ from material to material. The electrons in some materials are extremely effective at absorbing and emitting thermal radiation and these materials appear black. When the sun’s thermal radiation strikes a black material, that material absorbs the sunlight and nothing reflects. That’s why the material appears black. When you heat a black material to high temperatures, it also emits thermal radiation extremely well—for example, a hot piece of black charcoal glows brightly with its own red thermal radiation.

Materials in which the electrons are not able to absorb or emit thermal radiation have one of several familiar characteristics. Some are clear, meaning that thermal radiation passes right through them. Others are white, meaning that thermal radiation that strikes them is scattered uniformly in all directions. Still others are mirror-like, meaning that thermal radiation that strikes them is reflected in specific directions. All of these materials are virtually unable to emit their own thermal radiation: clear glass, white sand, and mirror-like aluminum emit very little thermal radiation even when they’re “red hot.”

Since black objects are best at emitting and absorbing thermal radiation, they are best at transferring heat via radiation. A black object will receive more heat from the hotter sun than a white object of similar dimensions and temperature. A black object will also radiate more heat to its colder environment than a white object of similar dimensions and temperature, although here “black” and “white” refer to the object’s behavior regarding its own thermal radiation. Near room temperature, thermal radiation is in the infrared, and many objects that appear white to visible light are actually rather black to infrared light.