. What causes different types of lightning? — CR, Tokyo, Japan
Cloud to ground lightning is caused by difference in electric charges between the cloud and the ground. Cloud to cloud lightning is caused by difference in electric charges among the clouds themselves. While I'm not at all expert on the subject, I would guess that the various different types of lightning discharges are caused by differences in the distances between charged objects, by variations in the local electric conductivity of the air and clouds during a discharge, and by the sizes and shapes of the clouds and ground.
. What is a superconducting magnet? — JS, Montreal, Quebec
Electric currents are magnetic. That's the basis for electromagnets—if you run an electric current around a coil of wire, that coil of wire will develop a north magnetic pole at one end and a south magnetic pole at the other end. But an electromagnet made with normal copper wires consumes electric power all the time. The current passing through those wires wastes energy because of friction-like effects in the copper and the wires become hot. The electromagnet also needs a power source to keep its current flowing.
However, a superconducting electromagnet is one in which the wires are superconducting—the current passing through them doesn't waste any power. Once a current has been started in a coil of superconducting wire, it flows forever. Since it doesn't waste any power, that current needs no source of power and produces no thermal energy. In fact, you can buy superconducting magnets with the current already started at the factory. As long as the wires are kept cold (as they must be to remain superconducting), the current will continue to flow and the coil will remain magnetic forever.
. What would be a legitimate form of propulsion for magnetic trains? — DS, Kenton, OH and MB, Willows, CA
The most sensible propulsion system for a magnetically levitated train would be a linear electric motor. This motor would consist of electromagnets on the train and electromagnets on the track. By turning these electromagnets on and off at carefully chosen moments, they can be used to pull or push the train forward for propulsion or backward for breaking. The timing is important because, for propulsion, the magnet on the train must always be attracted toward the track magnet in front of it and repelled by the track magnet behind it. For breaking, this relationship must be reversed.
. How do I make a battery that will charge using wind power? — K
Any rechargeable battery will do for this job, although I'd recommend using a lead-acid battery. To charge it, you need a wind-powered DC generator. You can make such a generator by attaching a DC motor to the blades of a fan and providing some weather-vane mechanism to ensure that the fan always points into the wind. The wind will then cause the fan to spin, and with it the motor. Wind energy will become mechanical energy and that will in turn become electric energy. The DC motor will act as a generator and will produce electric power.
To make this generator recharge the battery, you first need to ensure that the motor can generate a voltage that's at least 20% higher than the voltage of the battery while the wind is blowing at its usual rate. If it can't, you need a higher voltage motor or a lower voltage battery. Now you should connect the negative output wire of the generator to the negative terminal of the battery and use a power rectifier (a power diode) to connect the positive output wire of the generator to the positive terminal of the battery. You need this diode to prevent the battery from sending its power into the motor and making the fan turn when the wind isn't blowing hard. If the fan starts turning when you've inserted the diode, you have it installed backward. When correctly inserted, the diode will prevent the battery from operating the fan so that the fan can only charge the battery. When the wind starts blowing and the fan starts turning, it will charge the battery.
. How does a transformer lessen voltage? — C
When you send an alternating current through the primary coil of wire in a transformer, that current produces a magnetic field in the transformer. Because the current in the primary coil is changing with time—it's an alternating current—this magnetic field is changing and changing magnetic fields are accompanied by electric fields. In the transformer, this electric field pushes electric charges around the secondary coil of wire in the transformer. Since these electric charges are pushed in the direction they are traveling, work is being done on them and their energies are increasing. However, in the transformer you mention, the secondary coil of wire has fewer turns in it that the primary coil of wire. As a result, the charges don't receive as much energy per charge (as much voltage) as the charges in the primary coil are giving up. This type of transformer, in which the secondary coil has fewer turns of wire than the primary coil, is called a step-down transformer and reduces the voltage of an alternating current.
. How did wire recorders work? — MW, San Diego, CA
The original recording scheme invented by Poulson used a wire as the recording medium, rather than a tape. It recorded audio information as the magnetization of a steel wire in much the same way that a modern tape recorder records audio information as the magnetization of iron particles on the surface of a plastic tape. Both devices record the air pressure changes associated with sound as magnetization changes in a magnetizable surface—the higher the air pressure, the deeper the magnetization in a particular direction; the lower the air pressure, the deeper the magnetization in the opposite direction.
. I'm a poor student and can't afford the deposit for a telephone line. Is there any kind of telephone or radio that I can use to communicate with other people? — AG, Tulsa, OK
Yes, you can use a radio to communicate with your friends, but they will also have to have radios. Amateur radio has been popular almost since the invention of radio and the most accessible version of this hobby, citizen band or CB radio, was extremely popular in the 60's and 70's. You can still buy CB radios and communicate with friends directly through the air, but the general interest in CB radio has waned in recent years. Unfortunately, you can't make your friend's radio ring to alert them to begin listening. You'll have to anticipate your "call." Also, there is no privacy on conventional radio—any nearby person with a similar radio can listen in.
. What does the inside of a radio look like and what is the difference between AM and FM?
These days, radios just look like electronic circuit boards inside. You'd have some trouble telling the difference between a radio and a computer. AM and FM are both techniques whereby the radio station tells your radio which way to move the diaphragm of its speaker and by how much, in order to make sound. In the AM or Amplitude Modulation technique, the station raises or lowers the power of its radio wave to tell your radio to move its speaker diaphragm toward you or away from you, respectively. The higher the power of the radio wave, the more your radio pushes its diaphragm toward you. In the FM or Frequency Modulation technique, the station raises or lowers the frequency of its radio wave slightly to tell your radio to move its speaker diaphragm toward you or away from you, respectively. The more it raises the frequency of its radio wave, the more your radio pushes its diaphragm toward you.
. Have you made RF leakage measurements on a sample of microwave ovens? I understand that the FDA requires that if measured 5 cm away from any of the oven's surfaces, the RF leakage must be less than 1 mW/cm2
for new ovens and less than 5 mW/cm2
over the oven's life time. I'm just curious what actual measurements reveal about a "typically used" oven. — S
I've measured several ovens and have only found one that leaks a measurable amount of microwave power. That leaker is an oven that I've used in countless demonstrations and have taken apart several times (it appears on page 514 of my book). Considering the abuse that poor oven has had, it's doing pretty well. At a talk I gave yesterday, I couldn't get it to leak more than about 1 mW/cm2 even though I was measuring microwave power directly on the edge of the oven door—the most vulnerable point in the oven. Given that this oven's door sags several millimeters as the result of its rough treatment, that's not bad. In short, I doubt that there are many leaky microwave ovens around that haven't been dropped, crushed in shipping, or otherwise suffered serious mechanical injury.
. Is it possible isolate a room or part of it totally from microwaves? — DMJ
Because conducting surfaces reflect electromagnetic waves, you can shield a room from electromagnetic waves by enclosing it in conducting surfaces. For example, a room surrounded by metal mirrors will be completely black inside because light won't be able to enter it. Furthermore, if the electromagnetic waves that you're trying to exclude have reasonably long wavelengths, you can put holes in the conducting surfaces because electromagnetic waves can't pass through holes in a conducting surface if those holes are substantially smaller than their wavelengths. So, to shield your room from microwaves, I'd suggest enclosing it in copper screening with holes that are no more than a few millimeters in diameter. Many scientific experiments are performed in such screen rooms, which are generally called Faraday cages.