ie-Physics
Experiment IV-6
Electric Charge
Amber, the hardened sap from various trees such as pine, was known to the ancient Greeks. They viewed amber to have an unique virtue: when rubbed by cloth it can attract nearby objects, an effect for which they coined the term electrik. Magnetic rocks seem to have a similar attraction for iron. This was noted by Homer in his Odyssey. (The Sailors had to find a way past a magnetic mountain where nails pulled from boats caused sinking.) Magnetic lodestones hung from threads were used by travelers and later by sailers to provide orientation when there was no visual landmarks. But there was little new interest in these phenomena until William Gilbert of Colchester (1544-1603, at right from title page of de Magnete), physician to Queen Elizabeth I of England, had the time to review the phenomena, make a careful study, and publish in 1600 AD de Magnete. Gilbert noted
- that amber is not unique, but rather a large class of substances showed the same effects when induced by rubbing.
- that lodestones require no stimulus of friction such as was needed for amber, glass and sulphur.
- that lodestone attracts only magnetizable substances whereas electrified bodies attract everything.
- that magnetic attraction between two bodies is not affected by placing between the bodies a sheet of paper or cloth, or immersing the bodies in water, whereas the electric attractions is readily destroyed by such screens.
- that the magnetic forces tends to align bodies in definite orientations, whereas electric force merely tends to heap them together in shapeless clusters.
- Gilbert conclude that in contrast to magnetic bodies, the electric phenomena is due to something of a material nature which the friction liberates from the body. For all these reasons, electric and magnetic effects must be different phenomena and therefore should be studied independently.
Electric sparks and their effects became the rage at royal parties in Europe. In 1734 Charles-Francois de C. Du Fay (1667-1736 or 1698-1739 sources differ) distinguished two different types of electrical effects. Du Fay named the electric charge created by rubbing pitchy substances such as amber and rubber with fur such as wool resinous and that created by rubbing of glass with silk vitreous.
Benjamin Franklin (1706-1790 shown at left) in Philadelphia in the American colonies, far from the established civilization of Europe, proposed that only one kind of electricity was required. He believed the two effects could be explained as an excess of electric fluid or a deficiency. Franklin observed that sparks, although very fast, seemed to jump from glass rubbed with silk to sealing wax rubbed with wool. So Franklin called glass (previously called vitreous) which seemed to have the excess charge positive and wax (previously called resinous) negative. (The spark actually moves in the opposite direction. Franklin's accidental reversal of names will probably never be corrected.)
Experiment
Needed supplies: Scotch™ Magic Mending tape (not cellophane tape; other tape may work but has not been tried)
- Stick a hand long length of transparent tape to a table top. Except for an end used for a handle, press the tape down firmly to increase adhesion. Using the handle, pull the tape free.
- Test if an electric change has been produced by seeing if the non-sticky side held near a very small piece of paper will attract the paper. (Try other kinds of tape if this tape produces no attraction.)
- Test to determine if a piece of paper will attract another piece of paper. Is the paper charged?
- Suspend the charged tape (perhaps by its sticky handle from a hanging cupboard) then hold a second piece of tape similarly charged nearby. Is there any attraction or repulsion? Does it matter if non-sticky sides are near, if one is reversed, or if both sticky sides are near?
- Stick a new piece of tape to the table top and a second piece of tape on top of the first. Pull the first free so the two pieces of tape remain together. Check against the hanging tape to see if the pair is charged. Run the nonsticky side of the pair several times back and forth over a metal water faucet (or your lips). Metal objects (and the salt solution in your body) are conductors of electric charge. Test against the hanging tape to see if the pair is still charged or whether the charge ran off to ground via the plumbing. If there is no effect, its charge is said to be neutral.
- Carefully pull the two paired tapes apart. Test each by bring it in turn near the hanging tape. If you find one has a different kind of charge, also hang it as a testing tool.
- If you have other non-metallic objects (such as plastic eating utensils, combs, hard rubber objects, or glass items) try rubbing them with various kinds of cloth, fur, or hair and checking them against the hanging tape.
How many kinds of electric charge must exist?
Finally, record your procedures, measurements, and findings in your journal. If you need course credit, use your observations recorded in your journal to construct a formal report
References
- Edmund Whittaker, A History of the Theories of Æther & Electricity, Harper, 1910.
