Tag: electric power distribution

I’m doing a science fair project on electricity and I need to know how to make a homemade hot dog cooker. – BE

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.

Why is alternating current better than direct current? — MK, California

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.

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

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°.

If voltage shocks you, why does current kill you?

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.

Is it safe to live near high-power lines?

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.

Why is it that the same transformers seem to always be hit by lightning?

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.

Why does copper conduct electric currents better than steel and lead? Why do copper and aluminum seem to conduct about the same? – L

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.

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

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.

What is a kVA? Can you convert watts to kVA? – M

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.

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.