Electric Charge

Discussion

history

Thales of Miletus. Electricity comes from the Greek word for amber — electron (ηλεκτρον). Nothing written by Thales survives today (assuming he "wrote" anything at all).

So many quotes that need to be paraphrased (and verified).

• The most primitive electrical and magnetic phenomena -- the attraction of dry light material such as chaff to rubbed amber, and the attraction of iron to loadstone -- were no doubt observed before recorded history began. However, as far as I can find, these phenomena were not recorded by the Egyptians or any other pre-Greek civilization. The first definite statement is by Thales of Miletus (about 585B.C.) who said loadstone attracts iron because it has a soul. The prevailing view at the time was that movement of any kind indicated life, or a soul, or a god. In fact, it was advanced thinking on Thales' part to think that the loadstone's moving of the iron was caused by itself rather than by the intervention of some god. Actually Miletus was a very multicultural environment -- a flourishing commercial city in Asia Minor (now part of Turkey), trading with Babylon and also Egypt, where Thales travelled. This mixture of cultures meant that there was not a strongly repressive religious orthodoxy, as was often the case in primitive (and not so primitive) societies, so freedom of inquiry was tolerated. Sad to report, this did not lead to a more enlightened political system -- it was a slave based society, with bloody rebellion and repression.
• Pliny also has something to say about amber. Apparently, it was used for the end of the spindle by Syrian women spinners, and they called it the "clutcher". One possibility is that it became electrified as it spun and rubbed against their clothing, so attracting chaff, etc.
• The four types of electrical phenomena known and recognized by the Greeks and Romans were: lightning, St Elmo's fire and other luminous effects, the stunning shock of the torpedo ray -a species of electric fish, and the force of attraction exhibited by rubbed amber-the "amber effect".
• What seems certain is that Thales was revered by contemporaries as the greatest of the wise, and that he introduced the notion of philosophical reasoning which attempted to make nature intelligible by generalizing about the first principles of cosmology. Thales used a method which reasoned from the observations, i.e. phenomena-things which appear, to explain the ultimate principles of nature.
• The testimony of Diogenes Laertius is important because it implies Thales was the first philosopher to attempt a rational interpretation of the behavior of magnets and amber. Thales produced no written documents. What we know is based on second hand knowledge, passed along as a verbal tradition until recorded by Aristotle in his writings.
• In his last dialog, Timaeus, Plato refers to "the wonderful attracting power of amber and the Heraclean stone". This dialog is significant because it represents the earliest preserved complete primary source record of a cosmology in the scientific sense; a theory of the origin of the universe and everything in it, which is derived from first principles and not based on myth. In one stunning paragraph, Plato links lightning, with electricity and magnetism and presents a theory to explain them. In the course of explication of his principle of circular thrust Plato has Timaeus cite examples where the principle applies: "Other examples are any stream of flowing water, the fall of thunderbolts, and the puzzling attraction of amber and loadstone."

The phenomena of charge separation by rubbing is known as triboelectricity. The process of becoming charged by rubbing is known as triboelectrification. The triboelectric series is a list that ranks the tendency of materials to acquire a net positive charge by rubbing. Triboelectricity is a poorly understood phenomena despite being the branch of electricity and magnetism with the longest history.

Benjamin Franklin

• 1752 Experiments with a kite and discovers that lightning is an electrical discharge.
• 1753 Awarded the Copley medal for this discovery, and elected a member of the Royal Society; receives the degree of M.A. from Yale and Harvard.

Ben Franklin: The Plus and Minus. From The First American: The Life and Times of Benjamin Franklin, by H.W. Brands, here is a description of Franklin's ongoing correspondence begun in 1747 with Peter Collinson in London. Collinson was an agent of the Library Society of Philadelphia, and a scholar with scientific interests similar to Franklin's:

"In one of his first letters, Franklin supplied a novel terminology that became standard in analyzing electrical phenomena. Describing a particular apparatus, consisting of bodies labeled A and B, he wrote: 'We say B (and other bodies alike circumscribed) are electrised positively; A negatively. Or rather B is electrised plus and A minus….' At a time when other electricians spoke of two different kinds of electricity — vitreous and resinous — Franklin unified the field by positing a single sort and explaining the opposite properties in terms of a surfeit or a deficit (that is, positive condition or negative) of this single electricity, with uncharged objects being in balance."

Franklin and lightning: Previously, lightning was thought to be the work of supernatural forces. Many European Christians at the time of Franklin believed it to be diabolical in origin, based largely on the interpretation of a single phrase in the Bible.

Ephesians 2: 2 (KJV)
Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience:

Long quote from White.

But the means of baffling the powers of the air which came to be most widely used was ringing of consecrated church bells. This usage had begun in the time of Charlemagne…

Short quote from White.

In America the earthquake of 1755 was widely ascribed, especially in Massachusetts, to Franklin's rod. The Rev. Thomas Prince, pastor of the Old South Church, published a sermon on the subject, and in the appendix expressed the opinion that the frequency of earthquakes may be due to the erection of "iron points invented by the sagacious Mr. Franklin." He goes on to argue that in Boston are more erected than anywhere else in New England, and Boston seems to be more dreadfully shaken. Oh! there is no getting out of the mighty hand of God."

Three years later, John Adams, speaking of a conversation with Arbuthnot, a Boston physician, says: "He began to prate upon the presumption of philosophy in erecting iron rods to draw the lightning from the clouds. He railed and foamed against the points and the presumption that erected them. He talked of presuming upon God, as Peter attempted to walk upon the water, and of attempting to control the artillery of heaven."

As late as 1770 religious scruples regarding lightning-rods were still felt, the theory being that, as thunder and lightning were tokens of the Divine displeasure, it was impiety to prevent their doing their full work. Fortunately, Prof. John Winthrop, of Harvard, showed himself wise in this, as in so many other things: in a lecture on earthquakes he opposed the dominant theology; and as to arguments against Franklin's rods, he declared, "It is as much our duty to secure ourselves against the effects of lightning as against those of rain, snow, and wind by the means God has put into our hands."

Still, for some years theological sentiment had to be regarded carefully. In Philadelphia, a popular lecturer on science for some time after Franklin's discovery thought it best in advertising his lectures to explain that "the erection of lightning-rods is not chargeable with presumption nor inconsistent with any of the principles either of natural or revealed religion."

In England, the first lightning conductor upon a church was not put up until 1762, ten years after Franklin's discovery. The spire of St. Bride's Church in London was greatly injured by lightning in 1750, and in 1764 a storm so wrecked its masonry that it had to be mainly rebuilt; yet for years after this the authorities refused to attach a lightning-rod. The Protestant Cathedral of St. Paul's, in London, was not protected until sixteen years after Franklin's discovery, and the tower of the great Protestant church at Hamburg not until a year later still. As late as 1783 it was declared in Germany, on excellent authority, that within a space of thirty-three years nearly four hundred towers had been damaged and one hundred and twenty bell-ringers killed.

In Roman Catholic countries a similar prejudice was shown, and its cost at times was heavy. In Austria, the church of Rosenberg, in the mountains of Carinthia, was struck so frequently and with such loss of life that the peasants feared at last to attend service. Three times was the spire rebuilt, and it was not until 1778 - twenty-six years after Franklin's discovery - that the authorities permitted a rod to be attached. Then all trouble ceased.

A typical case in Italy was that of the tower of St. Mark's, at Venice. In spite of the angel at its summit and the bells consecrated to ward off the powers of the air, and the relics in the cathedral hard by, and the processions in the adjacent square, the tower was frequently injured and even ruined by lightning. In 1388 it was badly shattered; in 1417, and again in 1489, the wooden spire surmounting it was utterly consumed; it was again greatly injured in 1548, 1565, 1653, and in 1745 was struck so powerfully that the whole tower, which had been rebuilt of stone and brick, was shattered in thirty-seven places. Although the invention of Franklin had been introduced into Italy by the physicist Beccaria, the tower of St. Mark's still went unprotected, and was again badly struck in 1761 and 1762; and not until 1766 - fourteen years after Franklin's discovery - was a lightning-rod placed upon it; and it has never been struck since.

So, too, though the beautiful tower of the Cathedral of Siena, protected by all possible theological means, had been struck again and again, much opposition was shown to placing upon it what was generally known as "the heretical rod" "but the tower was at last protected by Franklin's invention, and in 1777, though a very heavy bolt passed down the rod, the church received not the slightest injury. This served to reconcile theology and science, so far as that city was concerned; but the case which did most to convert the Italian theologians to the scientific view was that of the church of San Nazaro, at Brescia. The Republic of Venice had stored in the vaults of this church over two hundred thousand pounds of powder. In 1767, seventeen years after Franklin's discovery, no rod having been placed upon it, it was struck by lightning, the powder in the vaults was exploded, one sixth of the entire city destroyed, and over three thousand lives were lost.

units

The unit of charge is the coulomb [C], which is the amount of charge transferred by one ampère of current in one second [As]. It is an unusually large unit for most day-to-day applications. Fractions of a coulomb, like microcoulombs [µC] and nanocoulombs [nC], are more common.

charge density

Sometimes it is more useful to describe the distribution of charge rather than the amount of charge. This is known as the charge density. Similar to mass density, which is usually just called density, it comes in three types depending on the way the charge is spread out (over a volume, an area, or a line) and two versions depending on whether one prefers algebra (average and uniform densities) or calculus (density functions). Charge density uses the same Greek letter symbols that mass density does. The way to tell the difference is through context.