. Is 2.45 gigahertz the best frequency for a microwave oven? Is that frequency at or near a water molecule resonant frequency? Do water molecules have a resonant frequency?
The frequency of the microwaves used in most microwave ovens, 2.45 gigahertz or 2,450,000,000 cycles per second, isn't related to any resonance of the water molecules themselves. While the isolated water molecules in steam or moist air have clear resonances associated with various vibrational and rotational modes of oscillation, these resonances are smeared out in liquid water. The water molecules in liquid water touch one another and their resonances are disturbed in much the same way that the resonances of a bell are disturbed when you touch it.
Rather than interacting with the water molecules via a resonance, the microwaves in an oven heat the water by twisting its molecules rapidly back and forth so that they rub against one another. The molecules are heated by the molecular equivalent of sliding or dynamic friction. The choice of 2.45 gigahertz gives the water molecules about the right amount of time to twist in each direction. The precise frequency isn't important, but microwave ovens are required to operate at exactly 2.45 gigahertz so that they don't interfere with communication systems using nearby frequencies. I believe that there are 2 other frequencies allocated to microwave ovens, but only a few ovens make use of those frequencies.
. I was wondering if a pitot tube is a very efficient way to measure airflow and, if so, what would be the conversion formula to cfm? - BN
A pitot tube determines airspeed by measuring the pressure rise that occurs when the airstream is slowed to a stop. Any time moving air encounters a closed chamber head-on, the air stops and it exchanges its kinetic energy—its energy of motion—for pressure potential energy—energy stored in the form of an elevated pressure. By measuring this elevated pressure, you can determine what the air's kinetic energy was while it was moving and thus how fast it was moving.
Pitot tubes are used to measure airspeed in airplanes. They're the cigar-shaped objects that project forward from the undersurfaces of airplanes near their noses. I suppose that you could use a pitot tube to measure the speed of air flowing through an air duct, but to determine the volume of air flowing through that duct, you'd need to know the dimensions of the duct. The relationship between pressure in the pitot tube and the airspeed is complicated and so is the relationship between airspeed in a real duct and the volume of air it's carrying. Overall, this doesn't look like an easy job.
. 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.
. 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.
. 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.
. 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.
. 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.
. 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.
. 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.
. 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.