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Electric Power Distribution
Page 5 of 8 (73 Questions and Answers)

864. Why does copper conduct electric currents better than steel and lead? Why do copper and aluminum seem to conduct about the same? - L
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A metal's conductivity is related to how far an electron can coast through the metal before suffering a collision that reduces its kinetic energy. Since an electron can collide with an impurity in the metal or a region of local disorder, the first task in obtaining a good conductor is to make a pure and uniform metal. Increased temperature also enhances these inelastic collisions, so keeping a metal cool improves its conductivity. Finally, different metals exhibit different couplings between the electrons and the metal ions from which those electrons came. Copper and aluminum have relatively weak electron-ion couplings while steel and lead have stronger couplings. The stronger the coupling, the more likely is a collision between an electron and an ion. Because of their weaker couplings, the electrons in copper and aluminum suffer far fewer collisions per centimeter than the electrons in steel and lead. That's why copper and aluminum are better conductors of electricity than steel and lead. The coupling in copper is only slightly weaker than that in aluminum, so they have similar conductivities. However, aluminum's tendency to form a very hard, insulating oxide coating (aluminum oxide or "alumina" is the mineral sapphire) makes it a bit tricky to use in wiring.

876. If voltage shocks you, why does current kill you?
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Your skin is a very good electric insulator and it prevents any current from passing through your body as long as that current doesn't have much voltage. A higher voltage (the electric equivalent of "pressure") is required to push charge through your skin. But once the charge is inside you body, it moves through you quite easily—your body fluids are essentially salt solutions and are relatively good conductors of electricity.

However, a small current passing through your body won't cause injury. It takes about 0.030 amperes or 30 milliamperes to cause a life-threatening disturbance to your "electric system." The small currents associated with static electricity are not enough to cause trouble, even through they easily pass through your skin. So high voltages are needed to break through your protective barrier—your skin—in order to give you a shock, but large currents are needed to injury you.


877. Is it safe to live near high-power lines?
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It's probably fine. While the high-tension wires do create modest alternating electric and magnetic fields, there is no credible evidence that these fields cause any injury and no one has proposed a convincing mechanism whereby those fields could affect biological tissue.

878. Why is it that the same transformers seem to always be hit by lightning?
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Lightning tends to strike elevated objects that acquire large charges that are opposite to those of the clouds. Since transformers are often elevated and they are connected to wires that allow them to become highly polarized when a charged cloud passes overhead, transformers are good targets for lightning.

879. With reference to power generation and transmission, can you please explain "Volt Amp Reactance" (VAR, kVAR, MVAR). What is meant by "importing/exporting VAR's"? What is meant when a plant is "consuming/producing VAR's"— ID, Northern Territory, Australia
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In most situations of AC electric power generation or AC electric power consumption, the current flowing through the circuit is in phase with (or, more simply, directly proportional to) the voltage across the circuit. But that isn't always the case. In situations involving reactive components (e.g., capacitors and inductors), it's possible for the current and voltage to be out of phase with one another. If the current and voltage are a full 90° out of phase, there is no average power flowing through the circuit. I believe that VAR is a reference to this portion electricity in the circuit—the portion for which the voltage and current are 90° out of phase. While this portion of the electricity doesn't transfer any power, it does place demands on the power transmission system. I think that the distinctions between "importing" and "exporting" and between "consuming" and "producing" are related to the phase ordering of the current and voltage (whether a device is acting as a capacitor or an inductor). In one case, the voltage leads the current by 90° and in the other the current leads the voltage by 90°.

890. I'm doing a science fair project on electricity and I need to know how to make a homemade hot dog cooker. - BE
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Although I have never done it myself, I understand that it is possible to run electric power directly from the power line through a hot dog and to use the resistive heating that occurs as electric current struggles to pass through the hot dog to cook that hot dog. While I can't recommend doing this and caution anyone trying it to be extremely careful with the electricity (i.e. seek adult supervision from someone who is experienced with the safe handling of electricity), I believe that it can be done. My understanding is that you should carefully connect each wire of an electric power cord (unplugged!) to its own nail (choose an uncoated steel nail to avoid toxic materials). You should then insert one nails into each end of the hot dog and place that hot dog on a safe, nonconducting surface where no one and nothing can touch it. Finally, you should plug the electric cord into an electric socket that is properly connected to a working circuit breaker. I would recommend using a socket protected by a ground-fault interrupter (GFI) such as are used in modern bathrooms (the ones with a "test" and "reset" button). (As you can see, I don't want anyone hurt!) I'm not sure how quickly the hot dog will cook, but I'd expect it to be quite fast. Be sure to unplug the cord before getting anywhere near the hot dog.

891. Why is alternating current better than direct current? — MK, California
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The genius of George Westinghouse and Nikola Tesla in the late 1800's was to realize that producing alternating current made it possible to transfer power easily from one electric circuit to another with the help of an electromagnetic device called a transformer. When an alternating electric current passes through the primary wire coil of a transformer, the changing magnetic and electric fields that this current produces transfer power from that primary current to the current passing through another coil of wire—the secondary coil of the transformer. While no electric charges move between these two wires, electric power does. With the help of a transformer, it's possible for a generating plant to move power from a large current of relatively low energy electric charges—low voltage charges—to a small current of relatively high-energy electric charges—high voltage charges. This small current of high voltage electric charges can move with relatively little power loss through miles and miles of high voltage transmission lines and can go from the generating plant to a distant city without wasting much power. Upon arrival at the city, this current can pass through the primary coil of another transformer and its power can be transferred to a large current of relatively low voltage charges flowing through the secondary coil of that transformer. The latter current can then deliver this electric power to your neighborhood. A transformer can't transfer power between two circuits if those circuits operate with direct current. Edison tried to use direct current in his power delivery systems and fought Westinghouse and Tesla tooth and nail for years. Edison even invented the electric chair to "prove" that alternating current was much more dangerous than direct current. Still, Westinghouse and Tesla won out in the end because they had the better idea.

944. A charge coupled device converts light (photonic energy) into electric energy. What is the underlying mechanism that makes this happen? — PM, Belfast, Ireland
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As in any photoelectric cell, the energy from a single particle of light—a photon—is used to raise the energy of an electron in a diode and to propel that electron from one side of the diode to the other. In this process, the light energy is partly converted to electrostatic potential energy and partly to thermal energy. Since a diode only carries current in one direction, the electron is unable to return to its original side. In a photoelectric cell, the electron flows through a circuit to return to the other side of the diode and provides energy to that circuit. In a charge coupled device, a complicated charge shifting system transfers the electrons to a detector that registers how much light was absorbed.

996. How does electricity get to my home?
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The electricity you receive comes from a distant power plant. A generator in that power plant produces a substantial electric current of medium high voltage electric charge. This current is alternating, meaning that its direction of flow reverses many times a second—120 reversals per second or 60 full cycles of reversal (over and back) in the United States. This alternating electric current flows through the primary coil of wire in a huge transformer at the power plant, where it produces an intense alternating magnetic field. When a magnetic field changes with time, it produces an electric field and, in the transformer, this electric field pushes electric charges around a second coil of wire in the transformer, the secondary coil. The effect of this transformer is to transfer power from the current in the primary coil of the transformer to the current in the secondary coil of the transformer. Thus the generator's electric power moves along to the current passing through the secondary coil of the transformer. However, the secondary coil has far more turns of wire than the primary coil and this gives each charge passing through that coil far more energy than the charges had in the primary coil. Although the current passing through that secondary coil is relatively small, it acquires an enormous voltage by the time it leaves the secondary coil. The transformer has produced this high voltage power needed for efficient power transmission to a distant city.

This high voltage electric current passes through the countryside on high voltage transmission wires. The value of using a small current of high voltage charges is that wires waste power in proportion to the square of the electric current they are carrying. Since the current in the transmission wires is small, they waste relatively little power.

When this current reaches your town, it passes through a second transformer, which transfers its power to yet another electric current. This current is large and, because it passes through a coil that has few turns of wire, it acquires only a medium high voltage when it flows through the secondary coil of the new transformer. Electricity from this second transformer flows toward your neighborhood through medium high voltage wires. Finally, near your home there is a third and final transformer that extracts power from the medium high voltage current and transfers that power to a very large current that acquires a low voltage when it flows through the secondary coil of the final transformer. It is this very large current of low voltage charges that flows through appliances in your home and those of your neighbors. That final transformer is often visible as a large gray drum on a utility pole or a green box in someone's yard.


1014. How do I make a battery that will charge using wind power? — K
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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.


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