Physics Part 39

(1) ELECTRIFICATION AND ELECTRICAL CHARGES

=213. Electrical Charges.=--The ideas gained in the study of magnetism are of a.s.sistance in the study of electricity in giving some fundamental ideas and principles that will often be referred to as a basis for comparing the actions of magnetized and electrified bodies. The process of electrifying a body is very different from that of magnetizing it.

Thus if a rubber comb or rod be rubbed with a woolen cloth the object rubbed is able to attract to itself light bits of paper, thread, etc.

This peculiar attraction was noticed and recorded by the ancient Greeks, 600 B.C., when it was found that amber when rubbed would attract light objects to itself. For a long time it was supposed that amber was the only substance showing this property. Dr. William Gilbert, however, discovered that the electrified condition could be produced by rubbing a great variety of substances. He named the _result_ produced, _electrification_, after the Greek name for amber (_elektron_). A body like hard rubber or amber which will attract light objects when rubbed is said to be _electrified_, or to have been given a _charge_ of electricity.

=214. Law of Electric Action.=--Let a vulcanite rod be electrified by rubbing with a woolen cloth until it will attract light objects; then place it in a wire stirrup suspended by a silk thread. If a second vulcanite rod is similarly electrified and brought near the first, the two will be found to repel. (See Fig. 186.) If now a gla.s.s rod be rubbed with silk and brought near the suspended rod, the two will _attract_. This difference in behavior indicates a difference in the electrification or charge upon the rods. The two charged vulcanite rods repelling and the charged gla.s.s and vulcanite attracting indicate _the law of electric action_. _Like charges repel each other and unlike charges attract each other._ Extensive experiments with all kinds of substances indicate that there are but two kinds of electrical charges.

The electrical charge upon gla.s.s when rubbed with silk or wool is called _positive_, and that upon hard rubber or vulcanite when rubbed with wool is called _negative_.

[Ill.u.s.tration: FIG. 186.--Repulsion of like charges.]

[Ill.u.s.tration: FIG. 187.--An aluminum foil electroscope.]

[Ill.u.s.tration: FIG. 188.--A proof plane.]

=215. The Electroscope and its Uses.=--An electroscope is a device employed to test the presence of an electrical charge. The _aluminum foil electroscope_ consists of a flask closed by a rubber stopper through which pa.s.ses a rod which ends at the top in a ball or plate and below is attached two narrow leaves of thin aluminum-foil. Ordinarily the two leaves hang close together and parallel but if a charged body is brought near the electroscope the leaves spread apart at the bottom.

(See Fig. 187.) The _kind of charge_ upon a body may be determined with an electroscope as follows: Make a _proof-plane_ by sealing a small metal disc on the end of a hard rubber rod. (See Fig. 188.) Touch the disc of the proof-plane first to a charged rubber rod and then to the top of the electroscope. The leaves of the latter will separate showing that the electroscope is charged. This charge remains after the proof-plane is removed. If the charged vulcanite rod is brought near the electroscope, the leaves separate further That is, a charge _like_ that on the electroscope makes the leaves separate further. But if an _unlike_ charge, as that on a positively charged gla.s.s rod, is cautiously brought near, the leaves will be seen to move together.

[Ill.u.s.tration: FIG. 189.--Rod with woolen cap.]

=216. Two Charges are Produced at the Same Time.=--A closely fitting woolen cover or cap some 3 in. long is made for the end of a vulcanite rod. A silk thread attached to the cap enables one to hold the latter while the rod is turned within it. (See Fig. 189.) If the rod bearing the cap is held near a charged electroscope little or no effect is noticed. If now the cap is removed by the silk thread and held near the electroscope, it will be found to be positively charged while the rod is negatively charged. The fact that no result is seen when the cap and rod are together, indicates that one charge neutralizes the other. In other words, _the charges_ must _be equal_. This ill.u.s.trates the truth that _when electrification is produced by friction, the two objects rubbed together acquire equal and opposite charges_.

=217. Charging by Contact and Conduction.=--If a small pith ball is suspended by a silk thread, a charged rod brought near is at first attracted, but after contact is repelled (see Fig. 190) showing that the ball has become charged with the same kind of electrification that is upon the rod. That is, a charge given to an object by _contact_ with a charged body is of the _same kind_ as that upon the charged one. The proof-plane in Art. 215 carries the same kind of charge that is upon the rod it is charged from. Some substances have the ability to transfer charges of electrification. These are called _conductors_, those that do not conduct electrification are _insulators_. The conducting power of a body is readily tested by placing one end of a rod of the material upon the top of an electroscope and the other end upon an insulated support, as in Fig. 191. If now a charge be put in contact with the body of _a_, the electroscope will show by its leaves whether the rod tested conducts or not. The leaves separate instantly when conducting substances are tested, while no action results with insulators. In testing some materials for conductivity the leaves are found to diverge gradually.

Such bodies are said to be _poor_ conductors. All degrees of conductivity are found. The metals are the best conductors. The best insulators are rubber, mica, sh.e.l.lac, gla.s.s, silk, porcelain, paraffin, and oils.

[Ill.u.s.tration: FIG. 190.--The pith ball charged by contact is repelled.]

[Ill.u.s.tration: FIG. 191.--Testing for conductivity.]

Important Topics

1. Positive and negative changes. Law of electric action.

2. Electroscope and its uses.

3. Conductors and insulators.

Exercises

1. Is air a conductor? Give reasons for your answer.

2. Mention two points of likeness and two points of difference between magnetism and electrification.

3. If you were testing the electrification of a body with a charged pith ball suspended by a silk thread, would attraction or repulsion be the better test? Give reasons.

4. Have you ever produced electrification by friction outside of a laboratory? Explain.

5. Are the rods upon which we produce electrification by friction, conductors or insulators? How do you explain this?

6. Are conductors or insulators of the greater importance in practical electricity? Explain.

(2) ELECTRIC FIELDS AND ELECTROSTATIC INDUCTION

[Ill.u.s.tration: FIG. 192.--An electric field about a positively charged sh.e.l.l.]

[Ill.u.s.tration: FIG. 193.--A "detector."]

=218. Electrical Fields.=--In our study of magnetism we learned that a magnet affects objects about it by its magnetic lines of force. In a similar way it is a.s.sumed that a charged body produces electrical effects upon its surroundings by _electric lines of force_. For example, the attraction that a charged body exerts upon light objects through short distances or the influence of a charge upon an electroscope several feet away, is said to be due to the _electric field_ about the charged body. (See Fig. 192.) The presence of the electric lines of force may be shown by placing a perforated, slender, diamond-shaped piece of tissue paper upon a light gla.s.s pointer (Fig. 193). When placed in an electric field the tissue paper "detector" places itself parallel to the lines of force. Electric lines of force are said to extend from a positive to a negative charge. (See Fig. 194.) The direction shown by the arrow upon the lines is that along which a small positive charge tends to move. Electric lines of force unlike those from magnets are _not_ continuous. They extend from a positive charge to a negative charge. Therefore each positive charge is connected by lines of force to a negative charge somewhere. These ideas of electric fields are of much a.s.sistance in explaining many electrical effects. Electrical fields between _oppositely_ charged sh.e.l.ls will be found similar to Fig.

194, while between sh.e.l.ls with like charges, fields are found as in Fig.

195.

[Ill.u.s.tration: FIG. 194.--Electric field between unlike charges.]

[Ill.u.s.tration: FIG. 195.--Electric field between like charges.]

=219. Electrostatic Induction.=--If a charged body is brought near an aluminum-foil electroscope, the leaves separate. (See Fig. 198.) The nearer the charge is brought the wider the leaves spread, but when the charge is removed, the leaves collapse showing that nothing was given to the electroscope. It was simply affected by the charge in its vicinity.

_This production of an electrified condition in a body by the influence of a charge near it is called electrostatic induction._ Placing insulators, such as a sheet of gla.s.s, between the charge and the electroscope does not affect the result, which is apparently brought about by the action of the electric lines of force. These lines of force extend without difficulty _through uncharged insulators_ and terminate often at the surface of a conductor, where their influence causes a charge to acc.u.mulate. _Charged insulators_, however, do affect inductive action. This may be noticed by using a sensitive electroscope.

[Ill.u.s.tration: FIG. 196.--Production of two charges by the influence of a third charge.]

[Ill.u.s.tration: FIG. 197.--The two charges separated.]

=220. Electrical Separation by Induction.=--The action just described may be ill.u.s.trated further by taking two insulated, uncharged bra.s.s sh.e.l.ls, _A_ and _B_. (See Fig. 196.) Bring a charged vulcanite rod near sh.e.l.l "_A_" while the sh.e.l.ls are touching each other. Then remove sh.e.l.l _B_ (Fig. 197) while the rod remains near _A_. On testing the sh.e.l.ls for electrification, _A_ is found to possess a positive charge. This action is in some respects similar to magnetic induction, for if one places a north-seeking pole near a piece of iron, the iron develops by induction a south-seeking pole at the end nearest the magnet and a north-seeking at the other end. There is, however, one striking difference. If the magnetized iron be separated into two parts, each part is a complete magnet possessing two unlike poles; while if the object affected by electrostatic induction is separated into two parts _one part_ has a _positive_ charge and the other a _negative_ charge.

[Ill.u.s.tration: FIG. 198.--Effect of a charged rod near an electroscope.]

[Ill.u.s.tration: FIG. 199.--When a finger is touched to the top of the electroscope, the repelled negative charge escapes.]

[Ill.u.s.tration: FIG. 200.--The electroscope is now positively charged.]

=221. Charging a body by induction= is easily accomplished. To charge an aluminum-foil electroscope by induction bring _near_ (say 10 cm.) from the top of the electroscope a charged rubber rod. (See Fig. 198.) The separated leaves show the presence of the repelled or _negative_ charge, the _positive_ charge being on the disc at the top. If while the charged rod is held near, the metal top of the electroscope is touched by the finger the leaves at once fall together showing that the repelled negative charge has escaped from the electroscope (Fig. 199). On removing _first_ the _finger_ and next the charged rod, the positive charge spreads over the metal parts of the electroscope, as is shown by the separation of the leaves (Fig. 200). The electroscope is now _charged positively_ by induction. If the charged rubber rod is brought to about 30 cm. from the electroscope, its leaves tend to move together. If a body charged similarly to the electroscope or _positively_, is moved toward the electroscope the leaves separate further. This behavior of the electroscope enables one to determine the _kind_ of charge upon a body.

Two principles of _electrostatic_ induction may now be stated: (1) Two _equal_, _unlike_ charges are always produced by _electrostatic_ induction.

(2) If the body affected by induction is connected to the earth by a conductor, the repelled or "_free_" charge is conducted away from the body while the "_bound_" charge is held by the inducing charge.

These principles apply in every case of induction.

Important Topics

1. Electric lines of force. Characteristics (3).

2. Electrostatic induction. Principles (2).