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.

What are the relationships between Joules, Coulombs, Amperes, Volts, and Watts?

A Joule is a unit of energy; the capacity to do work. A Coulomb is a quantity of electric charge; equal to about 6,250,000,000,000,000,000 elementary charges. An Ampere is a measure of current; equal to the passage of 1 Coulomb of charge each second. A Volt is a measure of the energy carried by each charge; equal to 1 Joule of energy per Coulomb of charge. A Watt is a measure of power; equal to 1 Joule per second. A current of 1 Ampere at a voltage of 1 Volt carries a power of 1 Watt. That is because each Coulomb of charge carries 1 Joule of energy (1 Volt) and there is 1 Coulomb of charge moving by each second (1 Ampere). That makes for 1 Joule of energy flowing each second (1 Watt).

Why does less current flow through a longer wire?

Wires obey Ohm’s law: the current flowing through them is proportional to the voltage drop across them. But the precise relationship depends on the wire’s length. A short wire will carry a large current even when the voltage drop across it is small because that wire has a small electrical resistance; it does not impede the flow of electricity very much. But a long wire has a large electrical resistance and will only carry a large current if the voltage drop across it is large. If you do not change the source of electrical power (e.g. a battery) and replace short wires with long wires, those wires will not be able to carry as much current.

What causes large electric resistances?

Thin wires or wires made of poor conductors. Some metals are simply better at carrying current without wasting energy than other metals. It has to do with how often a charge bounces off of a metal atom and loses energy. Copper, Silver, and Aluminum are good conductors while stainless steel and lead are pour conductors. Metals tend to become better conductors as you cool them and worse as you heat them. Semiconductors such as carbon (graphite) are poor conductors but have the reverse temperature effect. At low temperature they are poor conductors but become good conductors at high temperature.

Why is direct current so much better than alternating current?

It depends on the situation. You cannot use a transformer with direct current, so in that sense, alternating current is better. But many electronic devices need direct current because they require a steady flow of charges that always head in the same direction. So there are times when you need DC and times when you need AC.