Electricity

Since 1879, when Thomas Alva Edison (1847–1931) flipped the first switch to light Menlo Park, New Jersey, the use of electrical power has become nearly universal in the United States.

What is Electricity?

Electricity is a form of energy resulting from the movement of charged particles, such as electrons (negatively charged subatomic particles) and protons (positively charged subatomic particles). For example, static electricity is caused by friction: when one material rubs against another, it transfers charged particles. The zap you might feel and the spark you might see when you drag your feet along the carpet and then touch a metal doorknob demonstrate static electricity—electrons being transferred between you and the doorknob.

Electric current is the flow of electric charge; it is measured in amperes (amps). Electrical power is the rate at which energy is transferred by electric current. A watt is the standard measure of electrical power, named after the Scottish engineer James Watt (1736–1819). The term wattage refers to the amount of electrical power required to operate a particular appliance or device. A kilowatt (kW) is a unit of electrical power equal to 1,000 watts, and a kilowatt-hour (kWh) is a unit of electrical work equal to that done by 1 kW acting for one hour.

Electrical Capacity

The generating capacity of an electrical power plant, which is measured in watts, indicates its ability to produce electrical power. A 1,000-kW generator running at full capacity for one hour supplies 1,000 kWh of power. That generator operating continuously for an entire year will produce nearly 8.8 million kWh of electricity (1,000 kW C2 24 hours per day C2 365 days per year). However, no generator can operate at 100% capacity during an entire year because of legal restrictions and downtime for routine maintenance and outages. On average, about one-fourth of the generating capacity of an electrical plant is not available at any given time.

Electricity demands vary daily and seasonally, so the continuous operation of electrical generators is usually not necessary. Utilities depend on steam, nuclear energy, and large hydroelectric plants to meet routine demand. Auxiliary gas, turbine, internal combustion, and smaller hydroelectric plants are used during short periods of high demand.

An Electric Power System

An electric power system has several components. Figure 8.1 illustrates a simple electric system. Generating units (power plants) produce electricity, transmission lines carry electricity over long distances, and distribution lines deliver the electricity to customers. Substations connect the pieces of the system together, and energy control centers coordinate the operation of all the components.

U.S. Electricity Usage

In 2007 net generation of electricity totaled nearly 4.2 trillion kWh. Table 8.1 shows that electricity use in the United States—measured by the retail sales of utility companies—has increased nearly every year since 1949. In the thirty years between 1977 and 2007, retail sales of electricity nearly doubled, from 1.9 trillion kWh to 3.7 trillion kWh.

According to the Energy Information Administration (EIA), in Annual Energy Review 2007 (June 2008, http://www.eia.doe.gov/aer/pdf/aer.pdf), coal has been and continues to be the most-used raw source for electricity production in the United States. It accounted for approximately 2 trillion kWh of electricity in 2007, supplying 49% of the net amount of 4.2 trillion kWh of electricity generated. (See Figure 8.2.) Natural gas was the second-largest source of net generation of electricity (893 billion kWh, or 21%), followed by nuclear power (807 billion kWh, or 19% of the total) and hydroelectric power, a renewable energy source (248 billion kWh, or 6%). Very little electricity (103 billion kWh, 2.5%) was generated by all other renewable sources combined, such as geothermal, solar, and wind power.

From 1949 to the early 1990s the industrial sector was the largest consumer of electricity in the United States. (See Table 8.2.) Since then sales to the residential sector have been higher. Beginning in 1998 sales in the commercial sector became higher than sales in the industrial sector. In 2007 about 1.4 trillion kWh went to residential users, 1.3 trillion kWh to commercial customers, and 1 trillion kWh to industrial users.

The consumption of electricity in general is growing because electricity is being used increasingly to perform tasks that were once done with coal, natural gas, or human muscle: manufacturing steel, assembling cars, and milking cows. Electricity is used extensively in technology fields, such as the computer industry, and residential and commercial customers need electricity to run appliances and machinery, such as air conditioners.

The Electric Bill

The cost of electricity is affected by the amount of energy used to create the electricity and move it to the consumer. In 2007, for example, about 42.1 quadrillion British thermal units (Btu) of energy were consumed by U.S. utilities to generate 14.9 quadrillion Btu of electricity. (See Figure 8.3.) After accounting for energy used by the power plants themselves, only 14.2 quadrillion Btu were net generation—the amount available for transmission to customers. Almost 27.2 quadrillion Btu were lost when fuel was converted, and about 1.3 quadrillion Btu were lost during transmission and distribution (labeled “T & D losses” in Figure 8.3). In the end, for every three units of energy that were converted to create electricity in 2007, slightly less than one unit actually reached the end user.

Between 1960 and 1970 the price of electricity declined, but it began to increase during the 1970s because of an oil embargo by the Organization of the Petroleum Exporting Countries. (See Figure 8.4.) From the mid-1980s to 2002 the price of electricity dropped because prices of energy resources declined. After that time, electricity prices began to climb, and prices often varied by location. As Figure 8.5 shows, in 2006 electricity was the most expensive in Hawaii, Massachusetts, New York, Connecticut, Rhode Island, New Hampshire, Alaska, and California, respectively. According to the EIA, in Electric Power Annual 2006 (November 2007, http://www.eia.doe.gov/cneaf/electricity/epa/epa.pdf), the average price of electricity sold to the residential sector was 10.4 cents per kWh in 2006, whereas the commercial sector paid 9.4 cents per kWh. Industrial users paid only 6.1 cents per kWh because the huge amounts of electricity they use allowed them to receive volume discounts. The average price for all sectors across the United States in 2006 was 8.9 cents per kWh.

Deregulation of Electric Utilities

Regulated by the government for decades, electric utilities in some states have passed through a controversial shift toward unregulated markets and increased competition. In 1978 Congress passed the Public Utilities Regulatory Policies Act, which required utilities to buy electricity from private companies when that would be cheaper than building their own power plants. The Energy Policy Act of 1992 gave other electricity generators greater access to the market, resulting in widespread debates about regulatory, economic, energy, and environmental policies. State public utility commissions conducted proceedings and crafted rules related to competition.

California was a leader in deregulation. In the summer of 2000, however, the state experienced rolling electrical blackouts, and electricity bills doubled for many customers. Fearful of similar blackouts and price spikes, most other states had slowed or stopped their efforts to deregulate their electricity markets by the spring of 2001. At that time, twenty-four states and the District of Columbia had begun deregulation. Then, during an investigation of Enron Corporation, documents were found that showed how Enron’s electricity traders had boosted profits with strategies that added to electricity costs and congestion on transmission lines. As a result, public confidence in power companies, in general, and deregulation,

in particular, eroded. As of February 2003, only seventeen states plus the District of Columbia were actively engaged in restructuring their utilities. (See Figure 8.6.) In addition, five states had delayed deregulation, and California had suspended its restructuring activities. Restructuring was not active in twenty-seven states.

By the last half of the first decade of the 2000s it was becoming widely known that the deregulation of electric utilities, which had occurred in some states as indicated in Figure 8.6, had done little to bring electric prices down. A comparison of Figure 8.5 and Figure 8.6 shows that many of the states in which restructuring and deregulation activity was active in 2003 were states in which electricity prices were the highest in 2006. John Funk reports in “Electric Deregulation: A Legacy of Problems; Consumers Never Got Expected Benefits” (Plain Dealer [Cleveland, Ohio], December 8, 2007) that by late 2007 only eleven states were still deregulated.

International Electricity Usage

World Production

The EIA notes in Annual Energy Review 2007 that in 2005 approximately 17.3 trillion kWh of electricity were generated around the world: 11.5 trillion kWh from fossil fuels, 2.9 trillion kWh from hydroelectric power, 2.6 trillion kWh from nuclear power sources, and 0.4 trillion kWh from wood, waste, wind, and other sources. (See Figure 8.7.) The United States accounted for nearly 4.1 trillion kWh (23%); China, 2.4 trillion kWh (14%); Japan, 1 trillion kWh (6%); and Russia, 904 billion kWh (5%). Figure 8.8 shows net generation of electricity by the type of fuel used and by regions of the world.

World Consumption

In International Energy Annual 2005 (October 2007, http://www.eia.doe.gov/iea/elec.html), the EIA explains that total world electricity consumption increased from 7.3 trillion kWh in 1980 to 15.7 trillion kWh in 2005. Asia and Oceania used 5.1 trillion kWh (32%) in 2005; North America, 4.5 trillion kWh (29%); Europe, 3.2 trillion (21%); and Eurasia, 1.1 trillion (7%). Central and South America used about 4% of the world’s electricity, and the Middle East and Africa each consumed about 3%.

Trends in the U.S. Electrical Power Industry

In Annual Energy Outlook 2008 (June 2008, http://www.eia.doe.gov/oiaf/aeo/pdf/0383(2008).pdf), the EIA predicts that from 2006 to 2030 total electricity consumption will grow at a rate of 1.1% annually. Even though demand will be high for products that use electricity, the efficiency of those products will be high, thereby tempering the demand. Population shifts to warmer climates are expected to increase the use of electricity for cooling.

The demand for electricity in the United States has always been related to economic growth. However, electricity use is expected to grow more slowly than the gross domestic product (a measure of economic growth). Figure 8.9 shows how electricity sales are related more to economic growth than to population growth. Note that the phrase “five-year moving average” means that each point on the graph is an average for that year’s data plus the previous four years’ data. This type of averaging is used to determine long-term averages without the weight of cyclical influences such as the weather.

The rate of growth of consumption carries financial risks for electric companies. If the industry underestimates future needs for electricity, consumers may experience power shortages or losses. By contrast, excessive projections of the nation’s needs may mean billions of dollars spent on unneeded equipment.

The EIA estimates that the United States will need 263 gigawatts (GW) of new generating capacity from 2007 to 2030 to meet growing demand for electricity and to replace aging power plants, most of it after 2015. From 2007 to 2030, 45 GW of capacity are expected to be taken out of production, mainly old fossil-fired plants that are not competitive with newer types of fossil-fired plants.

According to the EIA, the high cost of natural gas, petroleum, and coal as well as increasing prices for new electricity generating capacity led to a jump in electricity prices from 2000 through 2006. Furthermore, electricity prices are forecast to continue increasing through 2009. In 2009 electricity prices are expected to be at an annual average of 9.3 cents per kWh (in 2006 dollars). By 2015, however, as new sources of natural gas and coal are brought on line, electricity prices should fall to 8.5 cents per kWh. After 2015 natural gas and petroleum prices will rise, but electricity producers will rely more on coal and renewables for power generation. The result will be a slow rise in electricity prices to 8.8 cents per kWh in 2030.

Continued concerns about pollution and global warming could result in tightened environmental emission standards, which could, in turn, affect electrical utility expansion, supply, and prices. Advances in solar and wind turbine technology could make renewable sources of electrical power more economical. Some energy experts and environmentalists claim that increased efficiency and conservation efforts are the most sensible alternatives to new construction or to the burning of more fossil fuels in existing plants. Using this idea to its fullest, the former vice president and Nobel Peace Prize winner Al Gore Jr. (1948–) challenges the nation in “A Generational Challenge to Repower America” (July 17, 2008, http://blog.algore.com/2008/07/) to generate all electricity in the United States using only non carbon based renewable resources such as wind, solar, and geothermal power by 2018.

chapter 8
ELECTRICITY

Since 1879, when Thomas Edison flipped the first switch to light Menlo Park, New Jersey, the use of electrical power has become nearly universal in the United States.

ELECTRICITY DEFINED

Electricity is a form of energy resulting from the movement of charged particles, such as electrons (negatively charged subatomic particles) and protons (positively charged subatomic particles). Static electricity is caused by friction, when one material rubs against another and transfers charged particles. The zap you might feel and the spark you might see when you drag your feet along the carpet and then touch a metal doorknob is static electricity—electrons being transferred between you and the doorknob.

Electric current is the flow of electric charge; it is measured in amperes (amps). Electrical power is the rate at which energy is transferred by electric current. A watt is the standard measure of electrical power, named after the Scottish engineer James Watt. The term "wattage" refers to the amount of electrical power required to operate a particular appliance or device. A kilowatt is a unit of electrical power equal to one thousand watts, while a kilowatt-hour is a unit of electrical work equal to that done by one kilowatt acting for one hour.

Electrical Capacity

The generating capacity of an electrical plant, measured in watts, indicates its ability to produce electrical power. A one-thousand-kilowatt generator running at full capacity for one hour supplies one thousand kilowatthours of power. That generator operating continuously for an entire year could produce 8.76 million kilowatt-hours of electricity (1,000 kilowatts × 24 hours/day × 365 days a year). However, no generator can operate at 100% capacity during an entire year because of downtime for routine maintenance, outages, and legal restrictions. On average, about one-fourth of the potential generating capacity of an electrical plant is not available at any given time.

Electricity demands vary daily and seasonally, so the continuous operation of electrical generators is not necessary. Utilities depend on steam, nuclear, and large hydroelectric plants to meet routine demand. Auxiliary gas, turbine, internal combustion, and smaller hydroelectric plants are normally used during short periods of high demand.

An Electric Power System

An electric power system has several components. Figure 8.1 illustrates a simple electric system. Generating units (power plants) produce electricity, transmission lines carry electricity over long distances, and distribution lines deliver the electricity to customers. Substations connect the pieces of the system together, while energy control centers coordinate the operation of all the components.

U.S. ELECTRICITY USAGE

In 2003, net generation of electricity totaled 3.8 trillion kilowatt-hours. Table 8.1 shows that electric utility retail sales (electricity use) in the United States has increased almost every year since 1949.

In the United States coal has been and continues to be the largest raw source for electricity production, accounting for slightly more than half of the electricity (about 52%) generated in 2003. (See Figure 8.2 and Figure 8.3.) Nuclear power was the second largest source of electricity (approximately 20%), followed by natural gas (14%), hydroelectric power (a renewable energy source, 7%), and petroleum (3%). Very little electricity (2%) was generated in the United States by all other renewable sources combined, such as geothermal, solar, and wind power.

The Energy Information Administration (EIA) of the U.S. Department of Energy (DOE), in its Annual Energy Review 2003 (2004), noted that coal accounted for the

FIGURE 8.1

generation of approximately 2 trillion kilowatt-hours of electricity in 2003, while natural gas contributed 629 billion kilowatt-hours and petroleum only 118 billion kilowatt-hours in all sectors. Nuclear power accounted for 764 billion kilowatt-hours, and hydroelectric generation totaled 275 billion kilowatt-hours. All other renewable energy sources, including geothermal, wood, municipal waste, wind, and solar energy, produced only 84 billion kilowatt-hours in 2003.

From 1949 through the early 1990s the industrial sector was the largest consumer of electricity in the United States, but since then sales to the residential sector have generally been higher because of changing economic factors. In 2001 sales in the commercial sector became higher than in the industrial sector as well. (See Table 8.2.) In 2003 about 1.3 trillion kilowatt-hours went to residential use, 1.0 trillion kilowatt-hours to industrial customers, and 1.1 trillion kilowatt-hours to commercial users.

Consumption of electricity in general is growing because electricity has increasingly taken over the tasks formerly done with coal, natural gas, or human muscle: manufacturing steel, assembling cars, and milking cows. Electricity is being used extensively in technology fields, such as the computer industry. Residential and commercial use of electricity is increasing with new appliances, air conditioners, computers, and many other developing applications.

THE ELECTRIC BILL

The price paid by a consumer for electricity includes the cost of converting energy into electricity from its original form, such as coal, as well as the cost of delivering it. In 2000, according to the EIA's Annual Energy Review 2003 (2004), consumers paid an average of $20.04 per million Btu for the electric power delivered to their residences, compared to only $5.68 per million Btu for natural gas and $12.01 per million Btu for motor gasoline.

TABLE 8.1

FIGURE 8.2

The unit cost of electricity is high because of the amount of energy expended in creating the electricity and moving it to the point of use. In 2003, for example, about 39.6 quadrillion Btu of energy were consumed by electric utilities to generate electricity in the United States, but 13.1 quadrillion Btu was the net generation, after accounting for energy used by the power plants themselves. (See Figure 8.2.) Most of the remaining 25.8 quadrillion Btu was lost during the energy conversion process. Additionally, about 1.2 quadrillion Btu is lost during the transmission and distribution process (T & D losses). In the end, for every three units of energy that are converted to create electricity, slightly less than one unit actually reaches the end user.

Between 1960 and 1970, the price of electricity declined, but it began to increase during the 1970s because of the OPEC (Organization of the Petroleum Exporting Countries) oil embargo. (See Figure 8.4.) From the mid-1980s to 2003 the price of electricity, in general, dropped because of the decline in energy resource prices. Prices varied depending upon the location. As Figure 8.5 shows, in 2002 electricity was most expensive in the New England states, New York, New Jersey, Pennsylvania, California, Nevada, Alaska, and Hawaii. According to the EIA's Electric Power Annual 2002 (December 2003), the average price of electricity sold to the residential sector was 8.5 cents per kilowatt-hour in 2002, while the commercial sector paid 7.9 cents per kilowatt-hour. Industrial users paid less per kilowatt-hour, 4.9 cents in 2002, because the huge amounts of electricity they use allow them to receive volume discounts. The average price for all sectors across the United States in 2002 was 7.2 cents.

DEREGULATION OF ELECTRIC UTILITIES

Regulated for decades as "natural monopolies," much like the railroad and telecommunications industries, electric

FIGURE 8.3

utilities are in the midst of a radical, highly controversial shift toward unregulated markets and increased competition. In 1978 Congress passed the Public Utilities Regulatory Policy Act (PURPA; PL 95-617), which required that utilities buy electricity from private companies when that would be a lower-cost alternative to building their own power plants. The Energy Policy Act of 1992 (PL 102-486) gave other generators greater access to the market, resulting in a flurry of activity in state and federal legislatures as a host of interest groups debated regulatory, economic, energy, and environmental policies. State public utility commissions conducted proceedings and designed rules related to competition in the electric utility industry.

California was a leader in deregulation activities. In the summer of 2000, however, the state experienced rolling blackouts and electricity bills doubled for many. Fearful of the blackouts and price spikes that afflicted California, by the spring of 2001 most other states had slowed or stopped their efforts to deregulate their electricity markets. At that time, twenty-four states and the District of Columbia had begun deregulation. Then, during an investigation of Enron Corporation, documents were found that showed that Enron electricity traders used strategies that added to electricity costs and congestion on transmission lines for their own profit. These revelations lowered public confidence in power companies in general. As a result of these events and an eroding confidence in deregulation, only seventeen states plus the District of Columbia were actively engaged in restructuring activities as of February 2003, as shown in Figure 8.6 from the

FIGURE 8.4

Energy Information Administration (EIA). In addition, five states had delayed the restructuring process, and one state (California) suspended its restructuring activities. Restructuring was not active in twenty-seven states.

TABLE 8.2

INTERNATIONAL ELECTRICITY USAGE

World Production

In 2002 approximately 15.3 trillion kilowatt-hours of electricity were generated around the world. (See Figure 8.7.) As reported in the EIA's Annual Energy Review 2003 (2004), the United States accounted for 25% of this production; China, 10.4%; Japan, 6.5%; and Russia, 5.8%. Figure 8.8 shows net generation of electricity by type and by region of the world.

World Consumption

According to the EIA in Annual Energy Review 2003 (2004), world total electricity consumption continued to increase, rising from 353.3 quadrillion Btu in 1993 to 411.6 quadrillion Btu in 2002. North America, Central America, and South America accounted for 34% of the world's total consumption in 2002; Asia and Oceania, 28%; Western Europe, 18%; and Eastern Europe and the former USSR, 13%. The Middle East and Africa used 5% and 3%, respectively. (See Figure 8.9.)

FIGURE 8.5

FUTURE TRENDS IN THE U.S. ELECTRIC INDUSTRY

In Annual Energy Outlook 2004 the EIA predicted that from 2002 to 2025, residential U.S. electricity consumption will grow at a rate of 1.4% annually. This compares with 7% growth per year during the 1960s. Several factors led to this decreased growth of electricity consumption, including increased market saturation of electric appliances and improvements in efficiency. Commercial demand is expected to grow by 2.2% per year because of growth in commercial floor space, while industrial demand will likely increase by 1.6% per year as industrial output rises.

Historically, the demand for electricity in the United States has been related to economic growth. This relationship will continue, but electricity use is expected to grow more slowly than the gross domestic product (GDP), a measure of economic growth. Figure 8.10 shows how electricity sales are related more to economic growth (the GDP) than to population growth. Also, the phrase "five-year moving average" means that each point on the graph is an average of that year and the previous four years'data. This type of averaging is often done to get a better idea of what the real long-term averages would be, without heavy influences on data from cyclical influences, like business or weather cycles.

The issue of electric growth is important and carries financial risks for electric companies. If the industry underestimates future needs for electricity, it could mean power shortages or losses. However, excessive projections of the nation's needs could mean billions of dollars spent on unneeded equipment.

The EIA estimated that the United States will need 356 gigawatts of new generating capacity from 2002 to

FIGURE 8.6

2025 to meet growing demand for electricity and to replace retiring units, most of it after 2010. From 2002 to 2025, sixty-two gigawatts of capacity will most likely be retired, accounting for nearly all old fossil-fired plants that are not competitive with newer types of fossil-fired plants.

The EIA's Annual Energy Outlook 2004 projected that electricity prices will decrease by 8% from 2002 to 2008 and will then remain somewhat stable until 2011. From 2011, however, electricity prices are expected to increase gradually by 0.3% per year to 2025.

Continued concerns about acid rain and global warming could result in tightened environmental emission standards, which may have an impact on electrical utility expansion decisions, prices, and supply. Continued advances in solar and wind turbine technology could make renewable sources of electrical power more economical in the future. Some energy experts and environmentalists claim that increased efficiency and conservation efforts are the most sensible alternatives to new construction or to the burning of more fossil fuels in existing plants.

FIGURE 8.7

FIGURE 8.8

FIGURE 8.9

FIGURE 8.10

Electricity

Electricity is a form of energy caused by the presence of electrical charges in matter. All matter consists of atoms, and atoms themselves contain charged particles. Each proton in an atomic nucleus carries one unit of positive electric charge, and each electron circling a nucleus carries one unit of negative electric charge. Electrical phenomena occur when electrons escape from atoms. The loss of one or more electrons (negative charges) from an atom leaves behind a positively charged fragment known as a positive ion. An electric current is produced when a mass of electrons released from atoms begins to flow.

Static and current electricity

Electrical phenomena can be classified in one of two general categories: static electricity or current electricity. The term static electricity refers to the behavior of electrical charges at rest. Suppose you hang two ping-pong balls from silk threads so that they are about 2 inches (5 centimeters) apart. Then imagine that each ball is rubbed with a piece of wool. The two balls become electrically charged with the same sign. Because like charges repel each other, the two balls will swing away from each other because of the static charge on each one.

Current electricity refers to the behavior of electrical charges in motion. In order for charged particles to flow, some pathway must be provided

for them. That pathway is called an electric circuit. An electric circuit typically consists of a source of electricity, such as a battery; an appliance that operates on electric energy, such as a toaster; a meter that measures the flow of electrons, such as a galvanometer; and metal wires connecting those parts of the circuit.

Electric charge

The two kinds of electric charges—positive and negative—have the same magnitude (size, force, or intensity) but opposite effects. The magnitude of an electric charge has been measured very accurately and been found to be 1.602189 × 10⁻¹⁹ coulomb. The unit used in measuring electric charge (coulomb; C) was named after French physicist Charles Augustin de Coulomb (1736–1806), an early authority on electrical theory. The coulomb is a fundamental property of matter, like the mass of an electron, the gravitational constant, and the speed of light.

Since a single positive charge and a single negative charge have the same magnitude, their combination produces a net charge of zero. That is, +1.602189 × 10⁻¹⁹ C + −1.602189 × 10⁻¹⁹ C = 0. All atoms normally have equal numbers of protons and electrons and are, therefore, electrically neutral. This fact explains the absence of electrical phenomena in everyday life. A person walking across ordinary grass normally does not get a shock because grass, dirt, and air are all made of electrically neutral atoms.

Only when electrons or protons begin to accumulate do electrical events occur. One such effect can be observed when a person shuffles across a carpet. Friction transfers charges between shoe soles and carpet, resulting in the familiar electrical shock when the excess charge sparks to a nearby person. Lightning is another phenomenon caused by the accumulation of electric charges. At some point, those charges become so large that they jump from one cloud to another cloud—or between ground and cloud—producing a lightning bolt.

Electric fields

Any charged particle alters the space around it. For comparison, think of any object in space, such as a planet. The region around that object (in this case, the space around the planet) is affected by the object's (the planet's) presence. We call that effect gravity. A second object placed in the gravitational field of the first object is attracted to the first object. A space probe sent in the direction of another planet, for example, is pulled toward that planet's surface by gravitational attraction.

Electric charges have similar effects. Imagine a ping-pong ball carrying a negative charge is suspended in the air by means of a silk thread. Then, a second ping-pong ball is placed in the vicinity of (or near to) the first ball. The second ping-pong ball will be attracted to or repelled by the first ping-pong ball. The second ball experiences a force of attraction or repulsion caused by the nature of the electric charge on the first ball. The region in space over which that force exists is called an electric field.

Coulomb's law

The law describing the force between charged particles was discovered by Coulomb in 1777. Electrical force, Coulomb found, depends on two factors: the electric charge on any two objects and the distance between them. That force can be expressed as an inverse square law. That is, the force between two charged particles decreases as the distance between them increases. When the distance is doubled (increased by 2), the force is reduced by one-fourth (½²). When the distance is tripled, the force is reduced by one-ninth (⅓³). And when the distance is made ten times as great, the forced is reduced by .

Electrical properties

Potential difference. Any collection of electric charges (such as a large mass of electrons) has certain characteristic properties, including potential difference and current flow. Potential difference, also called voltage, is the amount of electric energy stored in a mass of electric charges compared to the energy stored in some other mass of charges.

Imagine a small box into which electrons can be pumped. Pushing the first few electrons into the box is not difficult. But the more electrons in the box, the more difficult it is to add additional electrons. Electrons are all negatively charged, so they repel each other. Adding electron number 1,001 to the box, for example, is difficult because it must overcome the repulsion of 1,000 electrons already in the box. Adding electron number 10,001 is even more difficult.

The more electrons that have been accumulated, therefore, the greater their pressure to escape. The giant spark machines that are sometimes displayed at science fairs illustrate this point. Electrons are added to one of two large metal balls that make up the machine. Normally the air between the two balls is a nonconductor of electricity: it does not permit the flow of electrons from one ball to the other. At some point, however, the number of electrons on the first ball becomes too large to maintain this nonconductive state—the potential difference between it and the second ball is just too great. Many electrons jump all at once from the first ball to the second ball, producing a giant electric spark.

Potential difference is responsible for the operation of all electric appliances. Electric power companies build power plants where huge amounts of electric charge are accumulated; in other words, these plants are capable of providing high voltage electric currents. When a consumer turns on a switch, a pathway for that current is provided. Electric charges

rush out of the power plant, through transmission wires, and into the consumer's home. There they flow through and turn on a microwave, a CD player, a television set, a VCR, or some other electric device.

Electric current. The rate at which electric charges flow through a circuit is called current. The formal definition of current (designated by the symbol i) is the number of electric charges (C) that pass a given point in a circuit (a path of current that includes a power source) per second (t). Mathematically, i = C/t.

The unit of current flow is the ampere (amp, or A), named for French physicist André Marie Ampère (1775–1836). One ampere is defined as the flow of one coulomb (a measurement of electrical charge) of electrons per second.

Electrical resistance. The flow of electrons in a circuit depends on two factors. One factor is the potential difference or voltage in the circuit. The other factor is resistance, a force similar to mechanical friction that reduces the flow of electrons through a material. Nearly all materials have at least some resistance to the flow of electric current. Those with a smaller resistance are said to be conductors of electricity. Those with a greater resistance are called nonconductors, or insulators. The unit of electrical resistance is the ohm (Ω), named for German physicist Georg Simon Ohm (1789–1854).

The amount of current that flows through an electric circuit can be expressed mathematically by a law discovered by Ohm in 1827. Ohm's law says that the amount of current in a circuit is equal to the potential difference in the circuit divided by the electrical resistance, or i = V/r.

Electric power

The most useful way of expressing the amount of work available from an electric current is electric power. Electric power is defined as the product of the voltage and current in a circuit, or: P = V · i. Thus, a circuit with a high potential difference (voltage) and a large current is a source of a large electric power.

Most people are familiar with the unit for electric power, the watt (W). The watt was named for English inventor James Watt (1736–1819). One watt is defined as the product of one volt times one ampere, or 1 W = 1 V × 1 A.

Most electric appliances are rated according to the electric power needed to operate them. Ordinary lightbulbs, for example, are likely to be 25 W, 60 W, or 100 W bulbs. At the end of each month, local electric companies send consumers a bill for the amount of electric power used during that time. The bill is based on the number of kilowatts (thousands of watts) and the price per kilowatt in the consumers' area.

ELECTRICITY

Electrical and electronic devices and machines have become an integral part of contemporary life, ranging from household appliances and computers to huge industrial machines. When home and business owners pay the monthly bill from the electrical power company for the use of all of these items, they are paying for energy very conveniently delivered over electrical wires from the power company.

Although the delivery of electricity to homes and businesses has been possible only during the past century, static electricity was observed by the ancient Greeks over 2,000 years ago. They also noticed natural magnets, called lodestones, found near the town of Magnesia. These were important discoveries because scientists now know that electricity and magnetism are intimately related. Magnetism is used by power companies to produce the electricity used every day.

ELECTRIC CHARGES AND ELECTRIC FORCE

If the humidity is low, it is common to experience a shock when touching a metal doorknob after walking across a carpeted floor. This is static electricity, and it can be studied by rubbing a hard rubber rod on some fur, and then touching the rod to a small metal ball that is suspended on the end of a silk thread. The ball quickly bounces away from the rod and is repelled by it, as shown in Figure 1a. The rod and the ball are electrified or charged with the same type of (net) charge. Further, when a glass rod is rubbed with silk and touched to a second similar ball, the second ball is repelled by the glass rod, shown in Figure 1b. But when the two balls are brought near each other (without touching), they are clearly attracted to each other as shown in Figure 1c.

In the mid-1700s Benjamin Franklin proposed that there are only two kinds of electrical charge, which he called plus and minus. He defined the net charge on the rubber rod to be negative, and the charge on the glass rod to be positive. Further, charges of the same kind are repelled from each other, while opposite charges are attracted to each other. The amount of electrical charge (often represented by the letter q or Q) is measured in coulombs (abbreviated as C) in the Standard International system of units, called S.I. units.

CHARGES IN THE STRUCTURE OF AN ATOM

Electrons particles in matter with a negative charge were discovered around 1900. All matter is made of tiny atoms packed closely together. The structure of the atom was proposed to be like a tiny solar system, with negative electrons revolving in orbits around a very tiny positive nucleus (see Figure 2). The charge ("e") of the electron is a certain fixed amount: only a millionth of a trillionth of 0.16 coulombs (or 0.16 × 10 ^-18 C). The total negative charge of all of the electrons in an atom is exactly the same (but of opposite sign) as the charge of the positive nucleus; thus an atom taken as a whole is normally uncharged or "neutral."

Since a piece of matter (e.g., a piece of rubber or copper) is made up of a great many neutral atoms, the piece is itself normally neutral or uncharged. When a rubber rod is rubbed with fur, some electrons are pulled from the fur onto the rod, giving it extra negative charge (the fur is then deficient of electrons so it is positively charged).

ELECTRIC CIRCUITS AND CURRENT FLOW

There were few applications of electricity (or of magnetism) before the invention of the battery by Alessandro Volta in 1800. A battery can cause charges to move for long periods of time. The movement or "flow" of electrical charge is called a current. A basic battery cell consists of two different metals (called electrodes) immersed in an acid or salt solution (called an electrolyte). Through chemical interactions, one electrode develops extra electrons and becomes negatively charged, while the other develops a deficiency of electrons and becomes positively charged; these are respectively labeled the negative (-) and positive (+) battery cell terminals.

Work (W) is done by a battery whenever it pushes a positive charge (+q) away from the (+) terminal (through space outside the battery) to the (-) terminal. The "potential" or "emf" or "voltage" (V) of a battery is defined as the work done in this process divided by the charge: V= W/q. In S.I. units the voltage V is in volts V to honor Volta. A battery cell often used in flashlights is the "carbon-zinc dry cell." If unused and fresh, the carbon-zinc cell has a voltage of about 1.5 V. Many flashlights use two carbon-zinc cells placed end-to-end or in "series." Strictly speaking, the two cells together constitute a "battery," and the voltage of this battery is 3.0 V. The common 12 V "lead-acid" battery used in automobiles has six cells connected in series with it; each lead-acid cell has a voltage of 2.0 V.

Suppose a long thin metal wire is connected by a pair of thick wires between the terminals of a battery. This is a basic "electric circuit" as shown in Figure 3a. In all metals, each atom permits roughly one of the outer electrons to move quite freely in the material; these are called the "free electrons." In contrast, all electrons of the atoms of good electrical "insulators," such as glass, rubber, and air, are tightly bound to the atoms and are not free to move through the body of the material. The free electrons are repelled from the negative battery terminal and attracted toward the positive terminal, so that a continuous movement of charge (an electrical current) results around this complete path or "circuit." But there are frictional effects (electrons bumping into atoms as they move along) that resist the movement or flow of electrons through the wires, especially in the thin wire. The frictional effects result in the electrical "resistance" (R) of the wire, and cause the wires to heat up, especially the long thin wire.

An important property of this or any electrical circuit is the rate that charge moves past a place in the circuit (e.g., out from or into a battery terminal). The electrical current (I) is defined to be the charge (Q) that flows, divided by the time (t) required for the flow: I = Q/t. In S.I. units the current (I) is in amperes (A).

In the early 1800s, Georg Ohm studied the effect on current of changing the battery voltage as well as the length, cross-sectional area, and material of the wire in an electrical circuit like Figure 3a. He found that the current flow (I) was proportional to the voltage (V) and inversely proportional to the resistance (R) of the wire to this flow. The resistance (R) is quite constant for a given piece of metal wire; Ohm's law states that we may write that I=V/R. The S.I. units of resistance are ohms (Ω). The resistance to current flow of a wire is larger if the wire is longer, but smaller if the wire is thicker (larger cross-sectional area).

The smallest diameter copper wire generally permitted within the walls for U.S. household wiring is called no. 14 wiring; such wire has a diameter of about 1/16 inch (1.63 mm). A 10 foot length (about 3 meters) of no. 14 copper wire has a resistance of about 0.025 ohms; note that a 100 foot length (about 30 meters) has a resistance of 0.25 ohms.

Figure 3b shows the basic electrical circuit of Figure 3a as it is drawn using conventional electrical symbols. Note the symbol for a battery. The connecting wires with negligible resistance are drawn as lines, while an element with significant resistance is drawn as a zigzag line. An electrical switch is a length of metal that can be moved so that a space of air appears in the circuit; the air space has a large resistance (perhaps billions of ohms) so that virtually no current flows through it when it is "open" (as represented by the dashed line in Figure 3b. When the switch is closed the circuit path is complete and current flows around the "closed circuit." It is conventional to draw the current flow direction as from the (+) battery terminal to the (-) terminal. This is the direction that (+) charges would move if they were moving within the wire, and this was assumed to be the case from about 1800 to 1900. It is now known electrons move in the opposite direction, but the old conventional direction is almost universally used. In any case, the current flows in only one direction in this circuit; this situation is called "direct current" or "d.c."

ELECTRICAL POWER AND ELECTRICAL ENERGY

As electrical current flows through a wire of significant resistance, the wire is heated and the wire gives off heat energy. The wire in an incandescent light bulb is heated so much that it glows and gives off light energy as well as heat energy. This energy is supplied by a battery or other source of emf. Further, "power" is the rate that energy is being supplied, or that work is being done: power = energy/time, or power = (work done)/time. In S.I. units power is measured in watts. Using the definition of voltage and of current one can show that when a current flows from a battery (or any source of emf), the battery is delivering power at the rate of the voltage times the current: P = VI. For example, suppose a 115 V battery is supplying 0.87 amperes of current to a light bulb connected to the battery; then the power being used by the light bulb is equal to (115V) × (0.87A) = 100 watts. This particular bulb is a "100 watt light bulb." Note that because 1,000 watts equals one kilowatt, this bulb is using a tenth of a kilowatt or 0.10 kilowatts (0.10 kW) of power. ^F0^Since power = energy/time, it follows that energy = power × time; that is, energy supplied is equal to the power multiplied by the time-interval. The unit of electrical energy unit used by the electrical utility company is (kilowatts) × (hours), or kWh. Suppose a 100 W bulb is turned on for one day or 24 hours. The energy used by the bulb is (0.10 kW) × (24 h) = 2.4 kWh. A typical electrical energy cost in the United States might be 8 cents per kWh, so the cost for using this bulb for an entire 24 hour day would be (2.4 kWh) × (8 cents/kWh) = 19.2 cents.

Note that electrical energy used (and paid for) depends not only on the power consumption by a device or appliance, but also the length of time that it is used in a month. Therefore people are sometimes surprised to discover that there is a modest electric bill even after they have been gone from home on vacation and "everything" had been turned off. However, a number of devices around the home were probably operating, such as electric clocks, the electric motor on the furnace, the refrigerator, and the freezer. There are also many applicances operating on standby mode, such as televisions, answering machines, and cordless appliance. These are sometimes called "phantom power" devices.

MAGNETS, MAGNETIC FIELD, AND ELECTROMAGNETS

Magnets also repel or attract each other. A magnet always has both a north pole (N) and a south pole (S). Again, "opposites attract and likes repel" so that two magnets with north poles close to each other are repelled from each other, even without touching. The influence in the region near a magnet is often pictured as a "magnetic field"; invisible "magnetic field lines" are imagined, and often drawn in a diagram of a magnet. Figure 4a illustrates a steel bar "permanent magnet" with its magnetic field lines.

In the early 1800s H. C. Oersted discovered that an electric current flowing through a coil of wire produces a magnetic field, as shown in Figure 4b. A long coil of wire, often wound on an iron core to enhance the magnetic field, is called a "solenoid" or "electromagnet" and attracts iron or steel objects, just like a permanent magnet.

ELECTRIC GENERATOR

Shortly after Oersted's discovery, Michael Faraday found that changing the number of magnetic field lines within a coil of wire induces an emf or voltage in the coil so that a current flows if there is a closed circuit. Thus not only can a current produce a magnetic field, but a changing magnetic field can generate a current. Figure 5a shows the basic idea of an electric generator. A coil consisting of a single turn of wire is pictured; this is twirled (rotated) in front of a bar magnet so that the magnetic field lines pass through the coil, first from one side and then the other side. The changing magnetic field within the coil results in an alternating emf or voltage that is measured between the terminals A and B, and a current that reverses directions (i.e., an alternating current or a.c.) flows through the attached "load" resistance.

The alternating voltage V _AB measured between terminals A and B is graphed in Figure 5b. If the coil is turned at 60 cycles per second, then the frequency of alternation is 60 cycles per second (called 60 Hz in S.I. units). This is the standard frequency for commercial a.c. power in the United States (in Europe the standard is 50 Hz). By increasing the number of turns (or loops) in the coil, or by increasing the magnetic field strength, it is possible to increase the amount of emf or voltage generated (e.g., the peak voltage V _peak illustrated in Figure 5b). A typical power company generator might generate 10,000 V _rms. This is much too large to be safe for household use, so the generated voltage must be "transformed" down to about 115 V _rms that is present at a regular household outlet. (But first it is actually transformed up to much higher voltage to reduce energy lost in transporting the electrical energy.)

Note that a.c. voltages (also a.c. currents) are usually measured and quoted as "r.m.s." The r.m.s. value is 70.7 percent of the peak value. This is done so that the formula to calculate electrical power in the a.c. case is the same as for the d.c. case stated earlier: power is the voltage times the current (provided both are rms values).

It is important to realize that it requires effort to turn the coil of an electrical generator if current is being supplied by the generator; that is, work must be done to turn the generator coil. Conservation of energy requires that the energy used to turned the coil (i.e., mechanical energy input to the generator) is at least as much as the electrical energy produced by the generator in a given period of time. Electrical generators have relatively high efficiency, so that the electrical power output is perhaps 90 percent of the mechanical energy input, with the remaining 10 percent lost in various heating effects in the generator. Most commercial (utility company) electrical generator coils in the United States are turned by steam engines that burn coal to obtain the energy to generate the electricity.

TRANSFORMERS AND DELIVERY OF ELECTRIC POWER

Alternating voltage (called a.c. voltage) can be quite readily changed to a different value through the use of an electrical transformer. Note that an electrical transformer will not transform a d.c. voltage to another d.c. voltage value; this is a principal reason why commercial electricity in the United States (and most of the world) is almost always a.c. rather than d.c. A transformer consists basically of two coils of wire, with one coil often wound on top of the other. Electrical voltage is supplied to the coil called the "primary" coil, while voltage is output from the "secondary" coil. Then the basic transformer relationship is that the ratio of the secondary voltage to primary voltage is the same as the ratio of secondary-coil-turns to primary-coil-turns.

Suppose 10,000 V _rms (e.g., from an electrical power plant generator) is input to a transformer where the number of secondary turns is 75 times more than the number of primary turns. Then the voltage from the secondary will be 75 × 10,000 V _rms = 750,000 V _rms. This very high voltage level is actually produced by the secondary of the transformers at a modern commercial electrical power plant; the 750,000 V _rms is connected to the high voltage transmission lines (thick wire cables) that are used to transport the electrical power over long distances (perhaps hundreds of miles). The high voltage is used to reduce the energy lost to heating of the transmission wire resistance by the electrical current. The power lost to heating of the transmission wire is proportional to the square of the current flowing, so that the losses mount rapidly as the current flowing increases. But the power transmitted is equal to the transmission line voltage times the current flowing. For a given power, the current flowing will be much less (and energy lost in the transmission process is much less) if the transmission line voltage is very large. At the output end of the transmission line, a transformer is used again to step the voltage down to a level appropriate for local distribution (e.g., 2,400 V _rms), and a neighborhood transformer is finally used to step the voltage down to 220V _rms and 110 V _rms for use in homes and businesses.

Dennis Barnaal

See also: Electric Power, Generation of; Electric Power Transmission and Distribution Systems; Electricity, History of; Transformers.

BIBLIOGRAPHY

Aubrecht, G. (1995). Energy, 2nd ed. Englewood Cliffs, NJ: Prentice-Hall.

Faughn, J.; Chang, R.; and Turk, J. (1995). Physical Science, 2nd ed. Fort Worth, TX: Harcourt College Publishers.

Giancoli, D. (1998). Physics, 5th ed. Upper Saddle River, NJ: Prentice-Hall.

Priest, J. (1991). Energy: Principles, Problems, Alternatives, 4th ed. Reading, MA: Addison-Wesley Publishing Co.

Electricity

Electrical charge

Electric fields

Coulomb’s law and the forces between electrical charges

Current

Voltage

Resistance

Ohm’s law

Electrical power

Alternating current and direct current

Resources

Electricity consists of all phenomena resulting from electrical charges at rest and in motion. Our present-day understanding of electrical principles has developed from a long history of experimentation. Electrical technology, essential to modern society for energy transmission and information processing, is the result of our understanding of electricity.

Electrical charge

Electrical charge is a property possessed by a few of the fundamental particles that make up matter. Electrical charge is either positive or negative. Charges with the same sign repel while unlike charges attract. The unit of electrical charge is the coulomb, named for Charles Coulomb, an early authority on electrical theory.

The most obvious sources of electric charge are the negatively-charged electrons from the outer parts of atoms and the positively-charged protons found in atomic nuclei. Electrical neutrality is the most probable condition of matter because most objects contain nearly equal numbers of electrons and protons. Physical activities that upset this balance will leave

an object with a net electrical charge, often with important consequences.

Excess static electric charges can accumulate as a result of mechanical friction, as when someone walks across a carpet. Friction transfers charge between shoe soles and carpet, resulting in the familiar electrical shock when the excess charge sparks to a nearby person.

Many semiconductor devices used in electronics are so sensitive to static electricity that they can be destroyed if touched by a technician carrying a small excess of electric charge. Computer technicians often wear a grounded wrist strap to drain away an electrical charge that might otherwise destroy sensitive circuits they touch.

Electric fields

Charged particles alter their surrounding space to produce an effect called an electric field. An electric field is the concept we use to describe how one electric charge exerts force on another distant electric charge. Whether electric charges are at rest or moving, they are acted upon by a force whenever they are within an electric field. The ratio of this force to the amount of charge is the measure of the field’s strength.

An electric field has vector properties in that it has both a unique magnitude and direction at every point in space. An electric field is the collection of all these values. When neighboring electrical charges push or pull each other, each interacts with the electric field produced by the other charge.

Electric fields are imagined as lines of force that begin on positive charges and end on negative charges. Unlike magnetic field lines, which form continuous loops, electric field lines have a beginning and an ending. This makes it possible to block the effects of an electric field. An electrically conducting surface surrounding a volume will stop an external electric field. Passengers in an automobile may be protected from lightning strikes because of this shielding effect. An electric shield enclosure is called a Faraday cage, named for Michael Faraday.

Coulomb’s law and the forces between electrical charges

Force, quantity of charge, and distance of separation are related by a rule called Coulomb’s law. This law states that the force between electrical charges is proportional to the product of their charges and inversely proportional to the square of their separation.

The Coulomb force binds atoms together to form chemical compounds. It is this same force that accelerates electrons in a TV picture tube, giving energy to the beam of electrons that creates the television picture. It is the electric force that causes charge to flow through wires.

The electric force also binds electrons to the nuclei of atoms. In some kinds of materials, electrons stick tightly to their respective atoms. These materials are electrical insulators that cannot carry a significant current unless acted upon by an extremely strong electrical field. Insulators are almost always nonmetals. Metals are relatively good conductors of electricity because their outermost electrons are easily removed by an electric field. Some metals are better conductors than others, silver being the best.

Current

The basic unit of electric current is the ampere, named for the French physicist Andre Marie Ampere. One ampere equals 1 coulomb of charge drifting past a reference point each second.

Voltage

Voltage is the ratio of energy stored by a given a quantity of charge. Work must be performed to crowd same-polarity electric charges against their mutual repulsion. This work is stored as electrical potential energy, proportional to voltage. Voltage may also be thought of as electrical pressure.

The unit of voltage is the volt, named for Alessandro Volta. One volt equals one joule for every coulomb of electrical charge accumulated.

Resistance

Ordinary conductors oppose the flow of charge with an effect that resembles friction. This dissipative action is called resistance. Just as mechanical friction wastes energy as heat, current through resistance dissipates energy as heat. The unit of resistance is the ohm, named for Georg Simon Ohm. If 1 volt causes a current of 1 ampere, the circuit has 1 unit of resistance. It is useful to know that resistance is the ratio of voltage to current.

Mechanical friction can be desirable, as in automobile brakes, or undesirable, when friction creates unwanted energy loss. Resistance is always a factor in current electricity unless the circuit action involves an extraordinary low-temperature phenomenon called superconductivity. While superconducting materials exhibit absolutely no resistance, these effects are confined to temperatures so cold that it is not yet practical to use superconductivity in other than exotic applications.

Ohm’s law

Ohm’s law defines the relationship between the three variables affecting simple circuit action. According to Ohm’s law, current is directly proportional to the net voltage in a circuit and current is inversely proportional to resistance.

Electrical power

The product of voltage and current equals electrical power. The unit of electrical power is the watt, named for James Watt. One watt of electrical power equals 1 joule per second. If 1 volt forces a 1-ampere current through a 1-ohm resistance, 1 joule per second will be wasted as heat. That is, 1 watt of power will be dissipated. A 100-watt incandescent lamp requires 100 joules for each second it operates.

Electricity provides a convenient way to connect cities with distant electrical generating stations. Electricity is not the primary source of energy, rather it serves

KEY TERMS

Ampere— A standard unit for measuring electric current.

Conductors— Materials that permit electrons to move freely.

Coulomb force— Another name for the electric force.

Electric field— The concept used to describe how one electric charge exerts force on another, distant electric charge.

Generator— A device for converting kinetic energy (the energy of movement) into electrical energy.

Insulator— An object or material that does not conduct heat or electricity well.

Joule— The unit of energy in the mks system of measurements.

Ohm— The unit of electrical resistance.

Semiconductor devices— Electronic devices made from a material that is neither a good conductor or a good insulator.

Volt— A standard unit of electric potential and electromotive force

Watt— The basic unit of electrical power equal to 1 joule per second.

as the means to transport energy from the source to a load. Electrical energy usually begins as mechanical energy before its conversion to electrical energy. At the load end of the distribution system the electrical energy is changed to another form of energy, as needed.

Commercial electrical power is transported great distances through wires, which always have significant resistance. Some of the transported energy is unavoidably wasted as heat. These losses are minimized by using very high voltage at a lower current, with the product of voltage and current still equal to the power required. Since the energy loss increases as the square of the current, a reduction of current by a factor of 1/100 reduces the power loss by a factor of 1/10, 000. Voltage as high as 1, 000, 000 volts is used to reduce losses. Higher voltage demands bigger insulators and taller transmission towers, but the added expense pays off in greatly-reduced energy loss.

Alternating current and direct current

Direct current, or DC, results from an electric charge that moves in only one direction. A car’s battery, for example, provides a direct current when it forces electrical charge through the starter motor or through the car’s headlights. The direction of this current does not change.

Current that changes direction periodically is called alternating current, or AC. Our homes are supplied with alternating current rather than direct current because the use of AC makes it possible to step voltage up or down, using an electromagnetic device called a transformer. Without transformers to change voltage as needed, it would be necessary to distribute electrical power at a safer low voltage but at a much higher current. The higher current would increase the transmission loss in the power lines. Without the ability to use high voltages, it would be necessary to locate generators near locations where electric power is needed.

Southern California receives much of its electrical power from hydroelectric generators in the state of Washington by a connection through an unusually long DC transmission line that operates at approximately one million volts. Electrical power is first generated as alternating current, transformed to a high voltage, then converted to direct current for the long journey south. The direct-current power is changed back into AC for final distribution at a lower voltage. In certain applications, such as this one, the use of direct current more than compensates for the added complexity of the AC to DC and DC to AC conversions.

Resources

BOOKS

Gibilisco, Stan. Electricity Demystified. New York: McGraw-Hill Professional, 2005.

Keljik, Jeff. Electricity 3: Power Generation and Delivery. 8th ed. Clifton Park, NY: Thomson Delmar Learning, 2005.

Donald Beaty