ScienceWeek

SCIENCE POLICY: ON THE ELECTRIC POWER GRID

The following points are made by C.W. Gellings and K.E. Yeager (Physics Today 2004 December):

1) Thomas Edison (1847-1931) is best known for his inventions --particularly the incandescent lamp -- but his contributions toward the development of the US electric power grid are often underappreciated. Edison and his team designed the entire electrical system down to the wall outlet and in 1881 established the first power company. Edison's system was in the Wall Street section of New York City. Even today, vestiges of it supply DC power to about 2000 customers.

2) The current US electricity grid remains a mystery to most people. Its ubiquity and high reliability over the past 50 or more years has rendered it nearly invisible, more a backdrop for the workings of modern society than its central nervous system --at least until the lights go out. The blackout of 14 August 2003 brought the operation of the grid momentarily into prominence and raised questions about how it works and why it fails. How could a small local problem bring the lives of 50 million people to a standstill in a matter of minutes?

3) The grid represents the high-voltage transmission system that connects bulk power generation with the medium-voltage distribution systems that supply most consumers. (Some large, typically industrial consumers are connected directly to the grid.) In Canada and the US, the grid includes more than 200 000 miles of high-voltage lines that operate at or above 230 kilovolts and ultimately serve more than 300 million consumers. The US electricity delivery system, which consists of the grid and the downstream distribution system, is a $360 billion asset.

4) Physically, the grid is a network of wires and other devices, collectively called circuits, that weave together electrical loads (clusters of consumer demand) with the sources of electrical power generation. It predominantly comprises three-phase AC circuits, though the US grid includes a number of high-voltage DC lines as well. Three-phase circuits are systems with three equal voltages having a fixed phase difference of 120 . Those voltages result from generators that have three sets of equally spaced magnets.

5) AC circuits predominate in the US transmission system because they are compatible with transformers -- devices that can step up voltage before electricity is transported or step it down before electricity is distributed to consumers. Transmission voltages in the US are typically 115, 138, 230, 345, or 500 kV, although there are a few extra-high voltage lines at 765 kV. The 230-kV system represents the backbone of the US electricity grid.

6) A variety of sources, including dammed water, nuclear reactions, oil, coal, and natural gas, are exploited to generate electrical energy, typically at medium voltages of 13-25 kV. Higher voltages would potentially be more efficient, but to obtain them, one would need to insulate generators differently. That would unreasonably complicate generator construction.

References (abridged):

1. G. Constable, B. Somerville, A Century of Innovation: Twenty Engineering Achievements That Transformed Our Lives, Joseph Henry Press, Washington, DC (2003)

2. Consortium for Electric Infrastructure to Support a Digital Society, Value Assessment, EPRI, Palo Alto, CA (2001)

3. Electric Power Research Institute, Summer 2002 Eastern Interconnection Pre-Season Study and New Tools for Community Activity Room, EPRI, Palo Alto, CA (2002)

4. Electric Power Research Institute, Electricity Technology Roadmap: 2003 Summary and Synthesis, EPRI, Palo Alto, CA (2003)

5. Electric Power Research Institute, Electricity Sector Framework for the Future, vols. 1 and 2, EPRI, Palo Alto, CA (2003)

Physics Today http://www.physicstoday.org

HISTORY OF PHYSICS: ELECTRICITY AND MAGNETISM

The following points are made by Dan Falk (citation below):

1) For two centuries following the publication of Newton's /Principia/, the mechanical world-view held sway. Scientists eagerly applied Newton's laws to problems in astronomy, physics, and engineering with spectacular success. Perhaps the best example is a prediction made by the English astronomer Edmond Halley (1656-1742). Observing a bright comet in 1682, Halley was able to use Newton's laws to work out its orbit, and predicted that it would return 76 years later. And sure enough, the space-faring chunk of rock and ice reappeared in the sky in 1758, right on cue. (Halley died in 1742, but is immortalized in the comet that now bears his name.)

2) Yet the mechanical picture, described by Newton's law of gravity and his laws of motion, did not seem to cover everything. In particular, the nature of electrical and magnetic forces was still a mystery. What little was known, in fact, had been known to the ancient Greeks two millennia earlier. For example, Thales of Miletus (c.625-547 BC) had noted that certain black rocks had the power to attract metals such as iron; the Greeks called these stones magnets, after the region of Magnesia, in Asia Minor, where they were commonly found. They also knew that rubbing certain materials, like amber, caused them to attract lightweight objects like cork, paper, or bits of hay.

3) In the Middle Ages, someone discovered that if a lightweight magnet were suspended so that it was free to rotate, it aligned itself in a north-south direction -- and the mariner's compass was born. (The discovery dates from around the 11th century, and may have originated in China.) And yet, in terms of the underlying theory, progress was painstakingly slow. In 1600, the English scientist William Gilbert (1544-1603), physician to Queen Elizabeth, summed up what was then known about electricity and magnetism in a grand treatise, /de Magnete/. Gilbert claimed --correctly -- that the Earth itself was a natural magnet; he also coined the word electricity from "elektron", the Greek word for amber.

4) However, Gilbert's attempts to link electricity and magnetism to other forces of nature failed. Like Johannes Kepler (1571-1630), Gilbert sought a link between magnetism and the motion of the planets around the Sun. It was, on the surface, plausible enough; he had already shown that the Earth had a magnetic field, and we know today that the Sun and many of the other planets do as well. With the work of Newton, however, it became clear that gravity, not magnetism, governed planetary motion.

5) After Gilbert, another two centuries slipped by with little progress in the realm of electricity and magnetism. The slow pace of discovery is perhaps not surprising. First, in the wake of Newton's great discoveries, both electricity and magnetism were seen as intriguing but not particularly important phenomena; secondly, until the dawn of the 19th century, the tools needed to study them in detail simply did not exist.

6) Some kind of link between electricity and magnetism had long been suspected, but the evidence was mostly anecdotal. In 1731, for example, lightning struck the kitchen of an English tradesman; when the dust had settled, he found that some of the knives and spoons had the power to pick up nails and other small bits of iron: they had become magnetized. In 1752, the American inventor and statesman Benjamin Franklin (1706-1790) flew a kite during a lightning storm -- and demonstrated the link between lightning and electricity.

7) But electricity is a slippery subject: you can't study what you can't store, and electric charges have a way of dissipating before they can be measured and analyzed. For many years, the only way to store electric charge was with a "Leyden jar" -- a sealed glass container lined with metal. The first scientists to examine charges quantitatively began the study of electrostatics -- the forces between stationary charges. They quickly realized there were two kinds of charge; we call them simply "positive" and "negative." Opposite charges were seen to attract one another, while similar charges repelled. Before long, an inverse-square law (analogous to that of gravity) was found to govern the strength of the force between charges. The rule is usually called Coulomb's Law after the French scientist Charles-Augustin de Coulomb (1736-1806); however, Joseph Priestley (1733-1804), a British clergyman and chemist known primarily for his work with gases, independently discovered the inverse-square law at about the same time. Another Englishman, Henry Cavendish (1731-1810), also made important contributions to electrostatics, though he's better known for isolating the element hydrogen and measuring the strength of gravity with great precision.

8) The next breakthrough came, as sometimes happens in science, by sheer accident. In 1786, the Italian physiologist Luigi Galvani (1737-98) touched the leg of a dissected frog with an electrical charge and observed a violent contraction. He thought the effect originated in the animal's organic tissue, but it was actually the salt within the tissue, in concert with Galvani's metal electrodes, that was responsible. His discovery led to the invention of the electrochemical battery. Another Italian, the physicist Alessandro Volta (1745-1827), took the next step. In 1800, he produced the "voltaic pile" -- a stack of alternating layers of silver, zinc, and cardboard which, when placed in an electrical circuit, produced a continuous stream of electricity. The quantitative study of electric current had begun.

9) The Emperor Napoleon, sensing the promise in this string of discoveries, called for more research, suggesting that prizes be awarded for advances in understanding electricity. "Galvanism, in my opinion, will lead to great discoveries," he declared. And indeed it did.

Adapted from: Dan Falk: Universe on a T-Shirt: The Quest for the Theory of Everything. Arcade Publishing 2004, p.72.

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