. Why does regular water freeze faster than salt water? — CD, Crown Point, IN
When salt dissolves in water, its individual sodium positive ions and chlorine negative ions are carried about by the water molecules. Each of these ions is wrapped in a solvation shell of water molecules. These solvation shells and the salt ions themselves interfere with the water's ability to crystallize into ice. The ice crystals that form when salt water freezes rarely include the salt ions so the water molecules must abandon the salt ions in order to crystallize. Because of the attraction between the salt ions and the water molecules, and because of the loss of randomness that comes with forming pure ice crystals in the midst of salty water, you must lower the temperature of salt water below the freezing temperature of pure water before that salt water will begin to freeze into ice. When ice does begin to form, it will be relatively pure water crystals and the remaining water will become increasingly saltier. If you're ever lost in the winter without a supply of fresh water, look for sea ice—even though it forms from salt water, it contains very little salt.
. What is an analog clock? How do you attach it to batteries? — HB
An "analog" clock is a clock that has an hour hand and a minute hand. Twenty years ago, virtually all clocks were analog clocks but nowadays electronics has made it easier to display time with digits ("digital" clocks) than with hands ("analog" clocks). However, there are some clocks and wristwatches that still use moving mechanical hands to display the time. Most of these devices use quartz crystal oscillators to control electronic pulsing devices that drive electric motors that advance the hands. In such clocks, the batteries power the oscillators and the motors. You connect them as you would any electronic device: you form a string of batteries with the correct voltage, attach the negative lead from the clock to the negative terminal of the battery string, and attach the positive lead from the clock to the positive terminal of the battery string.
There are also some analog clocks in which the hands are just lines on a computer display, an arrangement that strikes me as silly. Finally, long ago there were two interesting types of analog electric clocks: the electric clocks that used the AC power line to run synchronous electric motors to advance their hands and the electric clocks that were used in automobiles. The automobile clocks were actually mechanical clocks, with mainsprings and everything, but they were wound by electromagnetic devices. Every minute or two, this device would give the spring a small wind and you would hear a click.
. How can you run a clock off of a potato?
The classic technique is to insert two dissimilar metal strips into the potato in order to build a simple battery. You can then run an electronic clock with the power provided by that battery. But the energy in that battery is coming from chemical reactions of the metals and not really from the potato. If you really want to use a potato as the power source for a clock, you should dry the potato out and burn it. You can use the heat of the fire to run a steam engine or to generate electricity.
. How does a car horn work? — CP
While some modern car horns are actually specialized computer audio systems, the old-fashioned electromagnetic car horns are still common. An electromagnetic horn uses an electromagnet to attract a steel diaphragm and turns that electromagnet on and off rhythmically so that the diaphragm vibrates. In fact, it uses the diaphragm's position to control the power to the electromagnet. Whenever the diaphragm is in its resting position or even farther from the electromagnet, a switch closes to deliver electric current to the electromagnet. The electromagnet then attracts the diaphragm's center. But when the diaphragm moves closer to the electromagnet, as the result of this attraction, the switch opens and current stops flowing to the electromagnet. Because of this arrangement, the diaphragm moves in and out and turns the electromagnet off and on as it does. The diaphragm's tone is determined by the natural resonances of its surface.
. What is a Zobel network in an audio amplifier and how does it work? Is it an effective device or not? — CV, Cape Town, South Africa
My understanding is that a Zobel network consists of a resistor in series with a capacitor and that the capacitor is normally connected to ground. When you attach the free end of this network to a wire carrying an audio signal, the network acts like a frequency-dependent load. At very low frequencies, the capacitor has plenty of time to charge through the resistor and the network has little effect on the audio signal—it acts as though it weren't there. At very high frequencies, the capacitor has no time to charge through the resistor and behaves like a wire. As a result, the network acts as though it were just the resistor connecting the audio signal wire to ground. So the impedance of the Zobel network varies from infinite at low frequencies to become equal to the resistance of the resistor at high frequencies. The crossover between these two behaviors is related to the RC time constant. I think that Zobel networks are used in audio amplifiers to dampen out high frequency oscillations that might occur in the absence of loads at high frequencies.
. I heard recently of someone with a pacemaker who went near a microwave oven and his pacemaker faulted, with him needing urgent medical attention. How did this happen? I also know of someone currently undergoing chemotherapy, who was told by his doctor not to eat food from a microwave oven. Why?
A pacemaker contains electronic circuits and wires that can act as antennas for microwaves. If a pacemaker is exposed to sufficiently intense microwaves, currents will begin to flow in those wires and circuits, and these currents may cause computational errors to occur or they may cause the circuitry to overheat. But while a pacemaker is far more sensitive to microwave radiation than say your hand is, I'm still surprised that enough microwave radiation leaked out of the oven to cause trouble. I'd suspect a real problem with that oven.
As for the chemotherapy question, I can't think of any reason why the doctor would suggest avoiding cooking food in a microwave oven. Unless I hear otherwise, I would suspect ignorance on the part of the doctor. The doctor may not understand the difference between "microwave radiation" and "gamma radiation".
. Why does microwave radiation affect plant seeds differently? If you microwave sunflower seeds 30 seconds, they germinate faster than if you did not microwave them at all, and yet if you microwave them for 60 seconds, the seeds do not germinate at all. If you do this same experiment with carrot seeds, the non-radiated seeds, the 30 second and 60 second seeds all germinate within 14 days. Why? Is it because the sunflower seeds are larger and absorb more radiation than the smaller carrot seeds? — ST, Mobile, AL
When you expose the seeds to microwave radiation, you are selectively heating portions of the insides of the seeds. Fats and oils don't absorb microwaves well but water does, so the parts of the seeds that become hottest are those that contain the most water molecules. Evidently, heating the water-containing portion of a sunflower seeds slightly cause that seed to germinate faster, but heating that same portion too much sterilizes the seed. That observation indicates that a moderate temperature rise causes the chemical reactions of germination to occur more rapidly while a more severe temperature rise denatures some of the critical biological molecules and kills the seed. The absence of any effect in carrot seeds may indicate that they don't have enough water in them to absorb the microwaves. It may also indicate that they can tolerate higher temperatures without undergoing the chemical reactions of germination and without experiencing damage to their critical molecules.
. When you are looking at something and there is an object partially blocking your view (e.g., a fence or a railing), why with one eye closed does the barrier block your vision but with both eyes open you seem to look through the barrier? — DS
Your brain merges the images it obtains from your two eyes so that you "see" a composite image that is essentially a sum of what both eyes see. When you close one eye so that only the other eye is providing an image to your brain, any object that blocks your view chops a piece out of the distant scene. No light from that portion of the scene reaches your open eye, so you can't see that portion of the scene. But when you have both eyes open, the image observed by one eye can compensate for any missing pieces in the image observed by the other eye. Since the barrier you are looking through chops out a different piece of the distant scene for each of your two eyes, the composite image that your brain assembles from these two individual images will include the whole scene.
. What happens when a speaker blows?
A speaker produces sound by using magnetic forces to push or pull a thin surface—the speaker cone—toward or away from the listener. As the cone moves forward, it compresses the air in front of it and as the cone moves backward, it rarefies the air in front of it. These compressions and rarefactions are what produce sound. But if you try to drive the cone into motions that are too extreme by turning up the volume of an amplifier too high, the cone will reach the limits of its motion. At that point, the cone may tear away from the electromagnetic coil that pushes it back and forth or it may tear away from the supports at its outer edge. The electromagnetic coil may also burn up because of overheating. All of these failures are lumped together as "blowing a speaker."
. I have experimented with passing high voltage arcs through ionic compounds and have observed different colors when I do. An arc through salt (sodium chloride) produces a brilliant yellow light. How does this work? — JB, Lantana, FL
When electric current passes through air as an arc, the air becomes hot enough to vaporize the compounds you expose to it. As a result, there are individual sodium and chlorine atoms moving about in the arc itself. Like all atoms, a sodium atom resembles a tiny planetary system. It has 11 negatively charged electrons orbiting a massive, positively charged nucleus. But unlike our experience with the solar system, the electrons in a sodium atom can only travel in certain allowed orbits or "orbitals." These electrons are normally found in the orbitals with the lowest possible energy. But when charged particles in the arc collide with sodium atoms, they often shift electrons in those atoms to orbitals with more energy. The electrons quickly return to their original orbits and emit their excess energies as light during their returns. In the case of sodium, the final step of the most common return path results in the emission of yellow light with a wavelength of about 590 nanometers. This yellow light is the same one you see in the sodium vapor lamps that are used to light highways and parking lots.
While sodium tends to emit yellow light, other atoms have different orbital structures and emit their own characteristic colors. Copper and barium atoms emit blue/green light while strontium atoms emit red light. These colored lights are the same ones that you see in fireworks.