Category: electric power distribution

Is it true that Tesla invented a way to send electrical power without the use of power lines? If so, how? – BS

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.

How far can electricity be transferred over wires from a power station before the loss factor is too great? — JD, New York NY

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.

Do sparks generated by Tesla coils shock humans? If not, why not? – AW

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.

How does the power/frequency of the earth’s magnetic field compare to the magnetic fields of electrical appliances? — MC, Independence, KA

Although I haven’t been able to find detailed lists of the magnetic fields near common appliances (such lists do exist), those fields are unlikely to be stronger than the earth’s own magnetic field. That’s because the magnetic fields in most appliances are created by electric currents and you must be quite near a relatively large current before the magnetic field of that current exceeds 0.5 gauss, the strength of the earth’s magnetic field. But while an appliance’s magnetic field is likely to be no greater than that of the earth, the appliance’s magnetic field does change with time. It reverses each time that the alternating current from the power line reverses. In the United States, that’s 120 reversals per second (60 full cycles of reversal, over and back, each second).

What are watts and amps? – NS

The watt is the standard unit of power—that is, it’s the way in which we measure how much energy is being transferred to or from sometime each second. 1 watt is equivalent to 1 joule of energy per second. A 100 watt light bulb consumes 100 joules of electric energy each second. Anytime energy moves from one place to another, you can determine how much power is flowing. For example, the food energy in a jelly donut is about 1 million joules, so if you eat 1 jelly donut in 100 seconds, you receive 10,000 watts of power. Since your body only consumes about 100 watts of power while you are resting, it will take you 10,000 seconds to use up all that food energy.

The amp (or ampere) is the standard unit of electric current—that is, its the way in which we measure how many electric charges flow past a certain point each second. 1 amp is equivalent to 1 coulomb of electric charge per second. Since 1 coulomb of electric charge is the charge on 6,240,000,000,000,000,000 protons, even a current of only 1 amp means that a great many electric charges are passing each second. The current passing through a 100-watt light bulb is roughly 1 amp on average, while the current used in starting a car is about 100 amps.

How do power lines work and what is the purpose of all the electrical things you see behind the fences with signs saying “Warning: High Voltage”?

Electric power is distributed over long distance using high voltages and relatively low currents. Since the amount of power that flows through a wire is equal to the product of its voltage (the amount of energy carried by each unit of electric charge) and its current (the number of units of electric charge that flow through the wire each second), the electric company can distribute its power either as a large current at low voltages or a small current at high voltages. But it turns out that the amount of power that’s wasted by electricity as it flows through a wire is proportional to the square of the current in that wire. Thus the more current that flows through a wire, the more power that wire turns into thermal energy (or heat). To minimize this energy loss, the power company uses transformers to convert the electricity to small currents at very high voltages for transmission cross country. Near each community, there is then a power substation at which this very high voltage power is converted to lower voltage forms. Even in neighborhoods, they use medium currents at moderately high voltages to avoid power wastage. Only in the vicinity of your home is the electricity finally converted by transformers to a large current at low voltage for safe delivery to your appliances. You’ve probably seen those final transformers as the gray oil-drum sized units on utility poles or the green boxes on front lawns. But despite all this effort to minimize power loss, something like 6% of the electric power generated in this country is lost in the delivery process.

How does power get from the plant to my house? Where do the voltages go up and down?

The voltage is stepped up at the power plant so that a small current of very high voltage charges (high energy per charge) can carry enormous power across the countryside. When this current arrives at your city, its voltage is stepped down so that a medium current of medium high voltage charges can carry that same enormous power through your city. Finally, near your house, its voltage is again stepped down so that a large current of low voltage charges can carry this power into your house. Naturally, you do not use all of the power from the power plant yourself, so it is distributed among all of the buildings in the city.

When going from 12 volts to 240 volts, is the point that with higher voltage the power transfer proceeds with fewer particles?

Yes. If you use higher voltages, you can transfer the same amount of power with a small current of charged particles. The energy lost in the transmission through wires increases as the square of the amount of current through those wires so reducing that current is very important.

How is AC converted in certain items to DC?

These devices use diodes, which are one-way devices for current. They only allow the current to flow a certain direction and block its flow the other way. With the help of some charge storage devices called capacitors, these diodes can stop the reversals of AC and turn it into DC. Those little black battery eliminators that you use for household electronic devices contain a transformer, a few diodes and a capacitor or two.

A step-up transformer has a secondary coil with many, many turns. As the current in the primary circuit flows back and forth, it creates a reversing electric field around the iron core of the transformer. This electric field pushes charges through the secondary coil so that it travels around and around the core. Each charge goes around many times, picking up more energy with each passage. By the time the charge leaves the transformer, it has lots of energy so its voltage is very high.

When you say that a transformer can change a small current with a high voltage into a large current with a low voltage, where do those extra charges come from?

A transformer involves two completely separate circuits: a primary circuit and a secondary circuit. Charges circulate within each circuit, but do not move from one circuit to the other. If the primary circuit of a transformer has a small current flowing through it and that current experiences a large voltage drop as it flows through the transformer’s primary coil, then the primary circuit current is transferring power to the transformer and that power is equal to the product of the primary circuit current times the voltage drop. The transformer transfers this power to the current flowing in the secondary circuit, which is an entirely separate current. That current may be quite large, in which case each charge only receives a modest amount of energy as it passes through the secondary coil. As a result, the voltage rise across the secondary coil is relatively small. The power the transformer is transferring to the secondary circuit current is equal to the product of the secondary circuit current times the voltage rise.