How Everything Works
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Electric Power Distribution
Page 4 of 8 (73 Questions and Answers)

713. Do sparks generated by Tesla coils shock humans? If not, why not? - AW
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A Tesla coil is radio-frequency transformer that produces small currents of very high-energy electric charges. A radio frequency alternating current passes through the primary coil of this transformer and it induces a current in the secondary coil of the transformer. The frequency of the alternating current must be extremely high because there is no iron in the core of the transformer to store energy during a cycle, so that each cycle must be very brief. Because the alternating current flowing out of the secondary coil of the transformer has a very high frequency, it travels over the surface of a conductor, rather than through its center. Thus when you allow that current to pass through you, it goes along your skin and not through your body. As a result, you barely feel its passage except perhaps as surface heating (however, it can cause what is called an "RF burn" in some cases.) Also, the current from a typical Tesla coil is very small so it would barely be noticeable even if it went through your body.

719. How far can electricity be transferred over wires from a power station before the loss factor is too great? — JD, New York NY
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That depends on the electricity's voltage. The transmission lines carrying the electricity are important parts of the overall electric circuit. They waste electric power as they carry current and the amount of power they waste is proportional to the square of the current they carry. The purpose of high voltage transmission lines is to send as small a current as possible across the countryside so that the wires waste as little power as possible. This reduction in current is possible if each electric charge moving in that current carries a large amount of energy—the current must be one that consists of high voltage charges. In short, higher voltage transmission lines employ smaller currents and waste less power than lower voltage transmission lines.

When Thomas Edison set out to electrify New York City, he used direct current of the highest practical household voltage. Nonetheless, his relatively low voltage power transmission lines wasted so much power that he had to scatter generating plants throughout the city so that no home was far from a power plant. But when George Westinghouse and Nicola Tesla realized that using alternating current and transformers to temporarily convert the household power to high voltages and small currents, they were able to send power long distances without wasting electricity. That realization eventually destroyed Edison's direct current electric system and gave us the modern alternating current system. It's now common to send electric power several hundred miles through high voltage transmission lines. At those distances, perhaps half the power is lost en route. I doubt that transmission of power more than 1,000 miles is practical.


740. How dangerous is the radiation from high voltage power lines? - K
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Probably not very dangerous. The radiation itself is so weak that it can't cause significant heating in your body (as the microwaves used in diathermy treatment do) and so low frequency that it can't do chemical damage (as the X-rays from a CT scan do). The only possible source of trouble is the small electric and magnetic fields from the power lines and there is still no credible evidence that these affect biological tissue. Moreover, there are sound physical arguments why those fields should not be able to affect biological tissue. Only in rare cases of an organ that is devoted to sensing magnetic fields (e.g., in migratory birds) is there any reasonable interaction between tissue and small magnetic fields.

741. Is it true that Tesla invented a way to send electrical power without the use of power lines? If so, how? - BS
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Yes. Tesla found that the alternating electromagnetic fields around a large high frequency transformer could propel currents through wires or lamps that were located at a moderate distance from the transformer. But this technique of using the alternating fields near a transformer to provide power aren't very practical—there is too much power wasted through radiation or in heating things that aren't meant to be heated.

772. Does an electric blanket produce enough EMF to affect the body and possible increase the risk of cancer? - FL
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The electromagnetic fields (EMF) produced by the currents in an electric blanket are very weak and it takes a pretty sensitive electronic device to detect them. You body is not nearly so sensitive and I still haven't seen any credible explanation for how these fields could cause any injury to biological tissue. I strongly suspect that all the concern about EMF is just hysteria brought about by a few epidemiological flukes or mistakes.

785. How does electricity get from the generating station to the outlet in my living room? — JJ, Arlington, MA
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The generating station uses a large generator to transfer energy from a giant turbine to an electric current flowing through a coil of wire. Current from this generating coil then flows through the primary coil of a huge transformer, where it transfers its energy to the magnetic core of the transformer. The current then returns to the generator to obtain more energy.

The magnetic core of the transformer transfers its energy to a second current—one that is passing through the secondary coil of the transformer. Because this current consists of far fewer electric charges per second, each charge receives a very large amount of energy. This large energy per charge gives the current a high voltage and it flows very easily through a high voltage transmission line. Because the amount of power that a wire loses is proportional to the square of the current passing through it, this high-voltage, low-current electricity wastes very little power in the transmission line on its way across country to your city. When the current reaches your city, it passes through another transformer and its energy is transferred to a third current. The cross country current then returns through the transmission line to the original power station to obtain more energy from the first transformer.

This third current involves more charges per second, so each charge carries less energy and the voltage is lower. This medium voltage electricity travels to your neighborhood before passing through a final transformer. This final transformer is probably either a gray metal can on a utility pole or a green box on a nearby lawn. In passing through the final transformer, the current transfers its energy to a current which then enters your home. This last current delivers energy to your appliances and lights and then returns to the final transformer to obtain more energy.


786. How much does it cost to run a regular 60 to 100 watt light bulb per minute or per hour? — JM, Smithfield, ME
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Electricity typically costs about 7 cents per kilowatt-hour. Over the course of an hour, a 100-watt light bulb will use 100 watt-hours or 0.1 kilowatt-hours, at a cost of about 0.7 cents. That's about 0.012 cents per minute.

809. 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
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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.


837. How does 240-volt electricity work in house wiring? If each "hot" wire in a circuit from the central wiring panel is at 120 volts with respect to neutral/ground, how are devices that use 240 volts wired? — GK, Ottawa, Ontario
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Most homes receive power through three wires: two power wires and one neutral wire. Each power wire is at 120 volts AC with respect to the neutral wire, meaning that its electric potential fluctuates up and down with respect to the neutral wire and behaves as though, on average, it were 120 volts away from the potential of the neutral wire. But the fluctuations of the two power wires are opposite one another—when one power wire is at a positive voltage relative to the neutral wire, the other power wire is at a negative voltage relative to the neutral wire. If you compare the two power wires to one another, you'll find that they behave as though, on average, they are 240 volts away from one another. Thus home appliances that need 240 volts are powered by the two power wires, rather than one power wire and one neutral wire.

838. What is a kVA? Can you convert watts to kVA? - M
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kVA is the product of kilovolts (kV) times amperes (A) and is a measure of power. In fact, if you multiply the voltage in volts delivered to an electric heater by the current in amperes sent through that heater, you will obtain the electric power in watts consumed by the heater. Thus the heater's power consumption in watts is the same as the product of its voltage times its current, or its kVA. However, there are many devices that don't behave like an electric heater. The heater is purely resistive, while many other devices such as motors are both resistive and reactive. Reactive devices don't obey Ohm's law and may not draw their peak currents at times of peak voltage. Therefore, the power in watts consumed by a reactive device isn't the same as the product of its current times its voltage, or its kVA.

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