How does a fan compare to a propeller? Why does a fan blow air while a propeller…

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How does a fan compare to a propeller? Why does a fan blow air while a propeller has “lift”? — DB, Austin, TX

A fan and a propeller are actually the same thing. Both are rotating wings that push the air in one direction and experience a reaction force in the opposite direction as a result. Each experiences a “lift” force, typically called “thrust,” in the direction opposite the airflow. If you put a strong fan on a low-friction cart or a good skateboard, it will accelerate forward as it pushes the air backward. Similarly, if you prevent a propeller plane from moving, its spinning blades will act as powerful fans.

How can I clean a dirty CD which has a very difficult to remove stain? Which mat…

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How can I clean a dirty CD which has a very difficult to remove stain? Which materials are best for cleaning? — AM, Mexico

Most CD’s are made from polycarbonate plastic (though other plastics with the same index of refraction are occasionally used). Polycarbonate is a pretty tough material, so it should survive most common stain or gum removing solvents. Try your favorite solvent on an unimportant CD first; such as one of the free discs that come occasionally in the mail. However, if the stain molecules have diffused into the plastic and have become trapped within the tangle of plastic molecules, you’re probably out of luck. Removing such a stain will require wearing away some of the plastic. Since the disc’s surface finish must remain smooth and the thickness of the disc shouldn’t change much, serious resurfacing is likely to make the disc unplayable. Also, stay away from the printed side of the disc—it has only a thin layer of varnish protecting the delicate aluminum layer from injury. Solvents can wreck this side of the disc. Finally, if the stain is a white mark (or a scratch), you may be able to render the disc clear again by filling the tiny air gaps that make it white with another plastic. I’ll bet that a clear furniture polish or liquid wax will soak into the white spot, replace the air, and render the disc clear and playable.

I once saw a green sunrise. Can you explain this?

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I once saw a green sunrise. Can you explain this?

Apparently there are conditions in which green light from the sun is bent by the atmosphere so that it is visible first as the sun begins to rise above the horizon. Instead of seeing the yellow edge of the sun peaking up from behind the water or land, you see a green edge that lasts a second or two before being replaced by the usual yellow. This green flash is the result of refraction (bending of light) and dispersion (color-dependent light-speed) in air and is discussed in considerable detail at http://www.isc.tamu.edu/~astro/research/sandiego.html. According to the author of that site, Andrew Young, given a low enough horizon, which is the primary consideration, and clear air, which is also important, and a little optical aid, which helps a lot, one can certainly see green flashes at most sunsets.

How does the carbon in an organic material affect the flow of light through it?

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How does the carbon in an organic material affect the flow of light through it? — TM

When light passes into a material, it interacts primarily with the negatively charged electrons in that material. Since light consists in part of electric fields and electric fields push on charged particles, light pushes on electrons. If the electrons in a material can’t move long distances and can’t shift from one quantum state to another as the result of the light forces, then all that will happen to the light as it passes through the material is that it will be delayed and possibly redirected. But if the electrons in the material can move long distance or shift between states, then there is the chance that the light will be absorbed by the material and that the light energy will become some other type of energy inside the material.

Which of these possibilities occurs in a particular organic material depends on the precise structure of that material. Carbon atoms can be part of transparent organic materials, such as sugar, or of opaque organic materials, such as asphalt. The carbon atoms and their neighbors determine the behaviors of their electrons and these electrons in turn determine the optical properties of the materials.

The frequency at which microwave ovens operate is about 2.45 GHz, which is about…

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The frequency at which microwave ovens operate is about 2.45 GHz, which is about the resonant frequency of the free water molecule. Can you calculate this resonant frequency or was it determined experimentally? — GW

While most microwave ovens operate at 2.45 GHz, that frequency is not a resonant frequency for the water molecule. In fact, using a frequency that water molecules responded to strongly (as in a resonance) would be a serious mistake—the microwaves would all be absorbed by water molecules at the surface of the food and the center of the food would remain raw. Instead, the 2.45 GHz frequency was chosen because it is absorbed weakly enough in liquid water (not free water molecules) that the waves maintain good strength even deep inside a typical piece of food. Higher frequencies would penetrate less well and cook less evenly. Lower frequencies would penetrate better, but would be absorbed so weakly that they wouldn’t cook well. The 2.45 GHz frequency is a reasonable compromise between the two extremes.

Why does a single phase 220 volt motor run off two legs of a three-phase circuit…

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Why does a single phase 220 volt motor run off two legs of a three-phase circuit?

In three-phase power, the voltages of the three power wires fluctuate up and down cyclically so that they are “120 degrees” apart. By “120 degrees” apart, I mean that each wire reaches its peak voltage at a separate time—first the X wire, then the Y wire, and then the Z wire—with the Y wire reaching its peak 1/3 of the 360 degree cycle (or 120 degrees) after the X wire and the Z wire reaching its peak 1/3 of the 360 degree cycle (or 120 degrees) after the Y wire.

The specific voltages and their relationships with ground or a possible fourth “neutral” wire depend on the exact type of transformer arrangement that supplies your home or business. In the standard “Delta” arrangement (which you can find discussed at sites dealing with power distribution), the voltage differences between any pair of the three phases is typically 240 VAC. In the standard “Wye” arrangement, the typical voltage difference between any pair of phases is 208 VAC and the voltage difference between any single phase and ground is 120 VAC. And in the “Center-Tapped Grounded Delta” arrangement, the voltage difference between any pair of phases is 240 VAC and the voltage difference between a single phase and neutral is 120, 120, and 208 VAC respectively (yes, the three phases behave differently in this third arrangement).

If you run a single-phase 220 VAC motor from two wires of a Delta arrangement power outlet, that motor will receive a little more voltage (240 VAC) than it was designed for and if you run it from two wires of a Wye arrangement outlet, it will receive a little less voltage (208 VAC) than appropriate. Still, the motor will probably run adequately and it’s unlikely that you’ll ever notice the difference.

In a three-phase induction motor, there is a rotating magnetic field in the stat…

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In a three-phase induction motor, there is a rotating magnetic field in the stator, which induces a rotating magnetic field in the rotor. Those two magnetic fields will interact together to make the rotor turn. Is the interaction attractive or repulsive? — G

The magnetic interaction between the stator and the rotor is repulsive—the rotor is pushed around in a circle by the stator’s magnetic field; it is not pulled. To see why this is so, imagine unwrapping the curved motor so that instead of having a magnetic field that circles around a circular metal rotor you have a magnet (or magnetic field) that moves along a flat metal plate. As you move this magnet across the plate, it will induce electric currents in that plate and the plate will develop magnetic poles that are reversed from those of the moving magnet-the two will repel one another. That choice of pole orientation is the only one consistent with energy conservation and is recognized formally in “Lenz’s Law”. For reasons having to do with resistive energy loss and heating, the repulsive forces in front of and behind the moving magnet don’t cancel perfectly, leading to a magnetic drag force between the moving magnet and the stationary plate. This drag force tends to push the plate along with the moving magnet. In the induction motor, that same magnetic drag force tends to push the rotor around with the rotating magnetic field of the stator. In all of these cases, the forces involved are repulsive-pushes not pulls.

If you wrap a three-phase power cord into a coil and allow it to deliver power t…

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If you wrap a three-phase power cord into a coil and allow it to deliver power to equipment, will the coil develop magnetic fields and, as a consequence exhibit both an inductive reactance and a voltage drop? — JH

If any current reaching the equipment through the three-phase power cord returns through that same power cord, then the net current in the cord is always exactly zero. Despite the complicated voltage and current relationships between the three power wires, one simple fact remains: the equipment can’t store electric charge. As a result, any current that flows toward the equipment must be balanced by a current flowing away from the equipment, and if both flows are in the same power cord, they’ll cancel perfectly. Since there is no net current flowing through the power cord, it develops no magnetic field and exhibits no inductive reactance or voltage drop.

Does a moving magnet use up its energy when it generates electricity? Does this …

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Does a moving magnet use up its energy when it generates electricity? Does this mean that the term “permanent magnet” is a misnomer because its magnetism can be used up? — MT, San Antonio, TX

When a moving magnet generates electricity, it does transfer energy to the electric current. However, that energy comes from either the magnet’s kinetic energy (its energy of motion) or from whatever is pushing the magnet forward. The magnet’s magnetism is basically unchanged by this process.

Nonetheless, a large permanent magnet isn’t really permanent. The random fluctuations of thermal energy and the influences of passing magnetic fields gradually demagnetize large permanent magnets. However, good permanent magnets demagnetize so slowly that the changes are completely undetectable. You might have to wait a billion years to detect any significant weakening in the magnetic field around such a magnet.

I am doing a science fair project on conductors and insulators. What are some of…

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I am doing a science fair project on conductors and insulators. What are some of the best and worst conductors of electricity? — LM

The best conventional conductors are silver, copper, gold, and aluminum. What makes them good conductors is that electrons move through them for relatively long distances without colliding with anything that wastes their energy. These materials become better conductors as their purities increase and as their temperatures decrease. A cold, near-perfect crystal is ideal, because all of the atoms are then neatly arranged and nearly motionless, and the electrons can move through them with minimal disruption. However, there is a class of even better conductors: the so-called “superconductors.” These materials allow electric current to travel through them will absolutely no loss of energy. The carriers of electric current are no longer simply independent electrons; they are typically pairs of electrons. Still, superconductivity appears because the moving charged particles can no longer suffer collisions that waste their energy-they move with perfect ease. We would be using superconductors everywhere in place of copper or aluminum wires if it weren’t for the fact that superconductors only behave that way at low temperatures.

As for the best insulators, I’d vote for good crystals of salts like lithium fluoride and sodium chloride (table salt), and covalently-bound substances like aluminum oxide (sapphire) or diamond. All of these materials are pretty nearly perfect insulators.