If I want to create a radio controlled device, how do I make sure it does not cr…

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If I want to create a radio controlled device, how do I make sure it does not create interference with other devices or receive interference. How does digital RF work and does it stop interference problems? — KG, New York, NY

Radio interference occurs whenever two nearby radio transmitters are simultaneously emitting radio waves that overlap in space and frequency. The receivers for these two waves can’t tell them apart and end up receiving both at once. This interference is familiar with AM radio, where you can sometime hear two broadcasts at the same time. With FM radio, the receivers are clever enough to distinguish one radio wave from another, but they can’t determine which broadcast they’re supposed to follow. Instead, they lock onto whichever wave is strongest and will often flip back and forth from one station to the other as their signal strengths fluctuate.

The only way to avoid interference completely is to choose a radio frequency that no one else nearby is using. That way your transmission is certain to be stronger than any other at the same frequency and your receiver will follow only your broadcast. If you have no choice but to share a particular frequency, then you must use some encoding scheme such as digital transmission so that your receiver can tell when it’s receiving a broadcast from your transmitter and not from some other transmitter. Your receiver looks for your personal encoding scheme and won’t respond to that of some other transmitter. However, if that other transmitter is strong enough, it will probably prevent your receiver from detecting your transmission. That trick of overwhelming a receiver with a second transmission is the principle behind jamming of a radio transmission.

What is analog? I hear about digital audio being better than analog, but nobody …

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What is analog? I hear about digital audio being better than analog, but nobody defines what analog is. — DG, Houston, TX

In analog audio, the air pressure fluctuations of sound at the microphone are represented by a continuously variable physical quantity such as an electric current, a voltage, or a magnetization. Thus as the air pressure at a tape recorder’s microphone rises during one moment of a song, an electric current in the recorder will rise and a region of a magnetic tape surface will become particularly strongly magnetized in a particular direction. Overall, each value of air pressure is converted to a particular value of the physical quantity.

The problem with analog recording is that when the sound is recreated, any defect in the physical quantity representing air pressure will lead to an imperfection in the reproduced sound. For example, if the magnetization of the recording tape has changed slightly due to how it was stored, the sound that the tape recorder produces won’t be exactly the same as the sound that the microphone heard. Digital recording avoids this problem by recording the information as bits. The physical quantity such as magnetization is representing bits (which take only two possible values) rather than the air pressure itself (which can take a broad range of values). Minor changes in the physical quantity representing these bits won’t change the bits. Thus imperfections in the recording or playback process won’t affect the sound quality.

I have to do an experiment for school on the electromagnetic properties of iron,…

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I have to do an experiment for school on the electromagnetic properties of iron, steel, and aluminum. The only problem is that I am not too sure what I should be testing. Any ideas? — CP, Nassau, Bahamas

Iron and steel (not stainless) are ferromagnetic metals, meaning that they are intrinsically magnetic. While this magnetism is normally hidden by the formation of millions of tiny, randomly oriented magnetic domains, it becomes apparent when you hold a magnet near the iron or steel: they are attracted! Aluminum has no intrinsic magnetism and is not attracted to a magnet. There are far more non-magnetic metals than magnetic ones. Why don’t you try to see which metals will stick to a magnet. Only the ferromagnetic ones will. Even common stainless steel is non-ferromagnetic.

How much water power do you need to turn on a light bulb? How much wind power do…

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How much water power do you need to turn on a light bulb? How much wind power does it take to turn on a light bulb? Can artificial light make a solar paneled car run? If so, how bright? — BB, Stafford Springs, CT

If you are trying to light a 60 watt bulb, you must deliver 60 watts of electric power to it (unless you are willing to have it glow relatively dimly). So the answers to your questions are 60 watts of waterpower and 60 watts of windpower. But you are probably more interested in how much water or wind is needed to run those power sources. An efficient water generator that produces 60 watts of power lowers about 6 liters (or one and a half gallons) of water about 1 meter (or 3 feet) each second. An efficient wind generator that produces 60 watts of power stops about 1 cubic meter (or 32 cubic feet) of air moving at 36 km/h (or 21 mph) each second. Finally, a solar powered vehicle needs at least several hundred watts of power to operate. Since solar panels are only about 20% energy efficient and artificial light sources are also only about 10 to 50% energy efficient, it would take thousands of watts of artificial lighting to operate a solar powered car. Not very practical.

How does an electronic dimmer work? I know that a regular household dimmer works…

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How does an electronic dimmer work? I know that a regular household dimmer works through resistance coils, but I read that electronic dimmers actually clip the A.C. cycle. Is this why you read the voltage output of an electronic dimmer the voltage remains the same even when it is dimmed down? Why can electronic dimmers dim fluorescents and arc lamps, but resistive dimmers cause those lamps to flicker? — KG, New York, NY

Electronic dimmers do clip the AC cycle. They use transistor-like devices called triacs to switch on the current to a lamp part way into each half-cycle. By shortening the time that power is delivered to the lamp, the dimmer reduces the total energy delivered to the lamp during each half-cycle and the lamp dims. But while a triac turns on easily, the only way to turn it off is to get rid of any voltage drop across it. The dimmer uses the alternating current itself to turn off the triac—the voltage of the power line naturally goes to zero at the end of each half-cycle and the triac turns off. The triac then waits until the dimmer restarts it, sometime into the next half-cycle.

Since the dimmer messes up the waveform of the electric current flowing through the lamp circuit, what you measure with a voltage meter depends on how that meter works. Since many AC voltmeters just measure peak voltage and assume that they are looking at a pure sinusoidal current, they don’t give you an accurate sense for what is really happening to the voltage of this clipped waveform as a function of time. Unless an electronic dimmer is turned way down, the peak voltage it delivers will be close to the normal power line peak, a fact which tricks the voltage meter into reading a high value and which allows a properly designed fluorescent lamp to continue operating normally but at a dimmer level.

How does a light switch work?

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How does a light switch work? — AB, Tulsa, OK

A light switch controls the flow of electricity through a circuit—a complete, unbroken loop through which electric charges can move. When the light switch is on, these electric charges can move in an endless loop. This loop starts with a trip to the power company—actually to the power transformer near your home—where the charges pick up electric energy. They then flow through wires to the light switch, then to the light bulb where they deliver their electric energy, and finally back to the power company to obtain more energy. The same charges complete this loop over and over again. The loop is called a circuit.

But when you turn off the light switch, you open or break the circuit. One of the wires connecting the power company to the light bulb suddenly has a gap in it and the current of electric charges can no longer flow. The switch itself actually contains two separated wires and a mechanical device that connects them only when the switch is in its on position. The precise structure of the mechanical switching device differs from switch to switch, but the behavior is always the same: the switch disconnects the two wires—and thus breaks the circuit—whenever you turn the switch off.

How does a magnetic train work? How can I make an experiment with it for a schoo…

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How does a magnetic train work? How can I make an experiment with it for a school project? — AASE, Quito, Ecuador

There are many techniques for supporting a train on magnetic forces, but the simplest and most promising involves electrodynamic levitation. In this technique, the train has a strong magnet under it and it rides on an aluminum track. The train leaves the station on rubber wheels and then begins to fly on a cushion of magnetic forces when its speed is high enough. Its moving magnet induces electric currents in the aluminum track and these currents are themselves magnetic. The train and track repel one another so strongly with magnetic forces that the train hovers tens of centimeters above the track.

To demonstration this effect, you can lower a very strong magnet above a rapidly spinning aluminum disk. In my class, I spin a sturdy aluminum disk with a motor and lower a 5 cm diameter disk magnet onto its surface. I hold the magnet firmly with a strap made of duct tape, so that the magnet won’t fly across the room or flip over as it descends. Instead of touching the spinning disk, the magnet floats about 2 cm above it. If you try this experiment, don’t spin the aluminum disk too fast or it will tear itself apart. It should spin about as fast as an electric fan on high speed. Also, be careful with the magnet, because it will experience magnetic drag forces as well as the magnetic lift force. If you don’t hold tight, it will be yanked out of your hand.

For a simpler experiment that anyone can do, float an aluminum pie plate in a basin of water and circle one pole of a strong magnet just above its surface. The pie plate will begin to spin with the magnet. You are again inducing currents in the aluminum, making it magnetic. While the forces here are too weak to lift the magnet in your hand, they are enough to cause the pie plate to begin spinning, even though you never actually touch it. This technique is used in many electric motors. That’s physics for you—the same principles just keep showing up in seemingly different machines.

In steam generation, wouldn’t it be more economical to heat a small boiler and f…

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In steam generation, wouldn’t it be more economical to heat a small boiler and feed it just enough water for it to maintain its optimal steam generating temperature than to heat a huge boiler as is normally done? — MF, Gillette, WY

Not really. Once you have heated the water to its steam generating temperature, all of the heat you add goes into converting water into steam. The presence of more or less water just doesn’t make any difference. The extra water requires no extra heat while the boiler is making steam. And having that extra water does act as a buffer in case you add too much or too little heat for a short while. That’s probably why most boilers have a bit more water than they need over any short period of time. Furthermore, it’s not always easy to add water to a boiler when the boiler’s pressure is very high.

How does reverse osmosis work? – MC

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How does reverse osmosis work? – MC

Normal osmosis in water is a process in which pure water flows through a semi-permeable membrane to dilute a concentrated solution on the other side. It is driven by statistics—it’s much more likely for a water molecule on the fresh water side to pass through the membrane than it is for a water molecule on the concentrated solution side to pass through the membrane. There are simply more water molecules trying to cross the membrane from the fresh water side! In fact, water molecules will continue to flow from the fresh water side to the concentrated solution side until the solution has been highly diluted or an accumulation of pressure on the solution side slows the passage of water and brings it to a halt.

Reverse osmosis occurs when the pressure on the solution side is raised so high that the movement of water reverses directions. If you squeeze the concentrated solution hard enough, you can drive additional water molecules from that solution through the semi-permeable membrane and into the fresh water on the other side. The raised pressure on the solution changes the statistics, making it more likely for water molecules to go from the solution side to the fresh water side. This technique is used to purify water in homes and to desalinate water in desert countries.

How do stalactites and stalagmites form in caves?

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How do stalactites and stalagmites form in caves? — GS, Conroe, TX

They form when various minerals come out of solution in water and crystallize on the surfaces of a cave. To understand how this process occurs, we must look at the interface between the water and the cave surface. Whenever water is in contact with a mineral surface, there is a chance that an atom of the surface will suddenly leave the surface and dissolve in the water. If there are atoms already dissolved in the water, there is also a chance that one of them will suddenly come out of solution in the water and attach to the surface. Atoms leave and return to cave surfaces all the time as water drips from the ceiling of a cave to its floor.

What is important for the growth of stalactites and stalagmites is that more atoms stick to the cave surfaces than leave those surfaces. That is exactly what happens and it does so because the water has already picked up more than enough dissolved atoms before it reaches the stalactite. Either because of temperature changes or because of evaporation, the water that runs across the cave roof and down the sides of a stalactite deposits more atoms on the stalactite’s surface than it removes. The same goes for the stalagmite after the water drips down to the cave floor. As the atoms build up on the cave surfaces, the stalactites grow down and the stalagmites grow up.