Unit 1 - History

Greek observations included, as we now classify them, two types of phenomena: electrical and magnetic. Magnetic effects are generally more reliable to demonstrate, and got more systematic attention by early investigators; however, we put further discussion of them in Unit 3.

The electrical phenomena were most easily demonstrated with the material called amber, and our term 'electricity' comes from the Greek word 'elektron' which is translated as 'amber.' In brief, the first observations were that if amber is rubbed, it will (for a time) exert a detectable attractive force on any nearby small object. At some point, someone must have had two such pieces of amber near each other, and learned that in this case they repel each other.

Eventually, it has been learned that an effect is possible if any two dissimilar objects are rubbed; amber is just one of the easiest (though it's not the cheapest good candidate, which is why it is seldom used for demonstrations; some plastics do well). Fabrics also show effects, some more than others; hence, 'static cling,' since it is essentially impossible to keep fabrics in clothing from rubbing.

Everyday effects tend to dissipate, unless rubbing maintains them; and different materials manifest effects in different ways. Eventually it was recognized that the better materials for demonstrating were ones such that the property appeared only where the rubbing occurred; in metals the property spread throughout, which usually diluted the effect too far. This distinction is now expressed in the terms 'insulator' or 'insulating' for the one type and 'conductor' or 'conducting' for the other type which includes all metallic materials. Further, dry air is fairly insulating, but moisture in it allows conduction, so that the 'electricity' can leak in or out at a rate that is affected by the weather.

By the mid-1700s, investigators were getting a fairly detailed picture, including quantitative data. Properties of insulators and of conductors had been used to enable an investigator to work with reproducible amounts of electricity, and with fairly large amounts. A critical step was taken by Ben Franklin, who recognized that all the observations about which types of rubbed objects attracted each other, and which repelled, could be systematized by assuming that there was some single sort of electrical stuff with these properties:
it cannot be created or destroyed, only moved around;
all materials have some of it;
rubbing can transfer this stuff, from 'losers' to 'gainers';
the stuff can flow in response to forces;
materials normally have the 'right' amount, and in that state experience at most what could be called secondary effects;
if two objects each have an excess, or each have a deficiency, they repel, but if one has an excess and the other is deficient, they attract.

Since the effect is, evidently, due to excess or deficiency, and these are naturally associated (especially by businessmen like Franklin was then; his political reputation came later) with positive and negative. So Franklin established our designations of 'positive' and 'negative' for electrical charge and everything associated with them. However, every time he tried to compare something before and after rubbing, to see whether its charge type was excess or deficient - he couldn't measure any change. But using signed numbers would be so much easier, so he made a guess, and made it known (so that other people could use this short way of describing what kind of charge they were working with), and we use his labels today.

We now know that his labels were backward, that what is normally transferred by rubbing is negative, so that excess is negative and deficiency is positive, but no one is going to go back and rewrite all the earlier books to put it right. And we also know that there is actually more than one kind of thing that can be transferred to produce a positive or negative charge effect, but the essence of Franklin's idea remains:
all electrical effects can be expressed in terms of net amounts (positive or negative) and locations of a single quantity called electrical charge;
various amounts of this quantity brought to the same place act on others as a single quantity given by addition of signed numbers;
net electrical charge cannot be created or destroyed, only moved;
like-sign charges repel, opposite-sign charges attract.

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Format for this page adapted with permission from pages
constructed by Dr. Lionel D. Hewett for his course
Modern Physics 1.