Physics Part 40

3. Charging by induction. Explanation.

Exercises

1. What are electric lines of force? Where are they found? What does the arrow mean upon the lines?

2. Name three effects produced by electric fields.

3. Does electrostatic induction occur outside of laboratories? Where?

When?

4. Given a charged rubber rod, how may one charge from it by induction, insulated bra.s.s sh.e.l.ls, giving some a positive and some a negative charge?

5. How may the charges upon the sh.e.l.ls be tested?

6. In charging an electroscope by induction, why must the finger be removed before the gla.s.s rod?

7. Why is it best to have the rubber and gla.s.s rods, used in electrification, warmer than the air of the room in which the experiments are being performed?

8. When a sharp metallic point is held near the k.n.o.b of a charged electroscope the leaves quickly come together. Explain.

9. Might one of the members of your cla.s.s in physics be charged with electricity, if he should stand on a board supported by dry gla.s.s insulators? Explain.

10. If a metal can is charged strongly while standing on an insulator, tests made by means of the proof-plane and electroscope show no charge on the inside. Explain.

(3) ELECTRICAL THEORIES AND DISTRIBUTION OF CHARGES

=222. Franklin's Theory of Electricity.=--We have studied the production of electrification by friction and induction. It will be helpful now to consider some of the theories of electricity. From the ease with which electrification moves, along a conductor, many have imagined that electricity is a fluid. Benjamin Franklin's _One Fluid Theory_ held that a _positive_ charge consisted in an acc.u.mulation or an excess of electricity while a _negative_ charge implies a deficiency or less than the usual amount. This theory led to representing positive electrification by a plus (+) sign and _negative_, by a minus (-) sign.

These signs are in general use to-day. The use and significance of these signs should be clearly fixed in mind.

=223. The Electron Theory.=--Various discoveries and experiments made in recent years indicate, however, that _negative_ electricity consists of little _corpuscles_ or _electrons_ which may pa.s.s readily from one molecule of a conductor to another while their movement through an insulator is much r.e.t.a.r.ded if not entirely prevented. This theory, sometimes called the _Electron Theory_, holds that each atom of a substance has as a nucleus a corpuscle of _positive_ electricity, and surrounding it, minute negative corpuscles or electrons. It is thought that the electrons in the atom are very much smaller than the positive charges and are revolving about the latter with great rapidity.

Ordinarily, the positive and negative charges are equal so that the atom is in a neutral or uncharged condition. By the action of various forces some of the _negative_ corpuscles within a conductor may be moved from molecule to molecule. Thus if a negatively charged rod is brought near a conductor, many electrons stream away to the far end charging it _negatively_, while the nearer end of the conductor is left with fewer electrons than usual along with the fixed positive corpuscles. Hence the near end is positively charged. (See Fig. 198.) On the other hand, if a positive charge is used, it attracts the electrons from the far end, leaving the immovable positive corpuscles there, and that end becomes positively electrified, while the nearer end with its surplus of electrons is, of course, negatively electrified.

The Electron Theory is considered well founded since the electrons have (a) had their _ma.s.s_ determined, (b) their _speed_ measured, (c) their _electric charge_ determined, (d) and their _behavior_ while _pa.s.sing through magnetic_ and _electric fields_ observed. These facts and other experimental evidence have demonstrated the existence of electrons. The positive corpuscle has not been directly observed but is a.s.sumed to exist to account for the effects observed in induction, charging by friction, etc.

=224. Distribution of an Electric Charge upon a Conductor.=--We have applied the electron theory in explaining the phenomenon of electrostatic induction. Let us now use it in studying the distribution of an electric charge upon a conductor. Let a cylindrical metal vessel open at the top and insulated by being placed upon pieces of sealing wax have a charge of negative electricity given it. (See Fig. 201.) On now taking a proof plane and attempting to obtain a charge from the _interior_ of the vessel no result is found, while a charge is readily obtained from the _outside_ of the dish. This result is explained by considering that the electrons are mutually self-repellent and in their attempt to separate as widely as possible pa.s.s to the outer surface of the vessel. This same condition is also true of a dish made of woven wire. If the charged conductor is not spherical in outline, an uneven distribution of the charge is observed. Thus if an _egg-shaped_ conductor is insulated and charged (see Fig. 202), a proof plane touched to the broad end of the body and then to an electroscope causes a certain divergence of the leaves of the latter. If now a charge be taken from the _pointed_ end by the proof plane to the uncharged electroscope, a greater spreading of the leaves than before will be noticed. This indicates that the electricity may be unevenly distributed over the surface of a body. It is found that the _electric density_, as it is called, is greatest where the surface curves most sharply. At a very sharp curve, as at a point, the electric density may be so great that a part of the charge escapes into the air. (See Fig. 203.) For this reason electric conductors on which it is desired to _keep_ an electric charge have round surfaces and all sharp points and corners are avoided. While conductors, such as lightning rods, which are designed to facilitate the escape of electric charges, are provided with a number of sharp points at the end or elsewhere. At such points, air particles are drawn forcibly against the point and after being charged are driven away strongly, creating the so-called _electrical wind_ which carries away the charge at a rapid rate. (See Fig. 203.)

[Ill.u.s.tration: FIG. 201.--No charge is found inside a hollow vessel.]

[Ill.u.s.tration: FIG. 202.--More charge at the pointed end.]

=225. Lightning and Electricity.=--The fact that lightning is an electrical discharge was first shown in 1752 by Benjamin Franklin, who drew electric charges from a cloud by flying a kite in a thunderstorm.

With the electricity which pa.s.sed down the kite string he performed a number of electrical experiments. This discovery made Franklin famous among scientific men everywhere. Franklin then suggested the use of lightning rods to protect buildings from lightning. These rods act as conductors for the electric discharge and thus prevent it from pa.s.sing through the building, with the risk of overheating some part and setting the latter on fire. The points provided at the top of lightning rods are believed to aid in preventing strokes of lightning by the _silent discharge_ of the so-called electric wind which tends to quietly unite the charges in the clouds and on the earth beneath.

[Ill.u.s.tration: FIG. 203.--Electrical wind produced by a pointed conductor.]

[Ill.u.s.tration: FIG. 204.--Electrical whirl. The reaction from the electrical wind causes it to revolve.]

[Ill.u.s.tration: FIG 205. The wire screen protects the electroscope.]

The charge in an electrified cloud acts inductively upon the earth beneath, attracting an opposite charge to the objects below. The discharge from the cloud often pa.s.ses to the objects beneath, such as trees or buildings. _Thunder_ is believed to be due to the sudden expansion of the air when intensely heated by the electric discharge and its sudden contraction, like a _slap_, as the track instantly cools.

Thunder at a distance is usually followed by rumblings due to changes in the intensity of the sound mainly due to reflections of sound waves from clouds and other reflecting surfaces.

=226. An electric screen= is a device for cutting off the influence of an electric charge. Faraday found that if a sensitive electroscope is surrounded by a wire mesh screen (see Fig. 205), no evidence of electrification could be found inside. In other words, a network of conductors on a building makes the best protection against lightning, provided it is connected to the earth by good conductors at several places.

Important Topics

1. Electrical theories. Evidences for electron theory.

2. How is the theory used in explaining induction?

3. Charges, and distribution on conductors (effect of shape).

4. Lightning: cause, effects, lightning rods.

Exercises

1. In what respects is Franklin's one-fluid theory like the electron theory? In what respects different?

2. Consider two sh.e.l.ls charged by induction from an electrified rubber rod, one positively and one negatively. Explain the process, using the ideas of the electron theory.

3. Should the metal top of an electroscope have sharp corners? Explain.

4. Would a tall steel tower have the same need of a lightning rod as a brick chimney of the same height? Explain.

5. Will a solid sphere hold a greater charge of electricity than a hollow one of the same diameter? Explain.

6. If a positively charged cloud floats over a tree which is a good conductor of electricity will the tree be charged? Show diagram.

Explain.

(4) POTENTIAL, CAPACITY AND THE ELECTRIC CONDENSER

=227. Conditions Causing a Movement of Electricity.=--In the study of conductors and insulators it was observed that an electric charge moved along the conducting rod to the electroscope. This _movement_ of _electricity_ along a conductor is a result of great practical importance. We will now consider the conditions that produce the "flow"

or "current" of electricity. Let two electroscopes stand near each other. Charge one, _C'_ (Fig. 206), strongly and charge the other slightly. If now a light stiff wire attached to a stick of sealing wax be placed so as to connect the tops of the electroscopes, the leaves of _C_ will partly close while those of _D_ will open slightly, thus indicating a movement of electricity from _C_ to _D_ along the wire. The movement was from a place of greater degree of electrification to one of less.

[Ill.u.s.tration: FIG. 206.--Electricity flows from high to low potential.]

=228. Potential.=--The _potential_ of an electrified body is its _degree_ of _electrification_. Therefore, it is said that electroscope _C_ mentioned above has a greater potential than electroscope _D_. The movement of electricity is from a place of greater or _high_ potential to one of lesser or _low_ potential. If two bodies are at the _same_ potential there will be found no movement of electricity between them. A _difference_ of _potential_ between two points connected by a conductor is therefore the _necessary condition_ for an electric current. Just as heat is transmitted along a conductor from a place of high to one of lower temperature, so electricity is transmitted along a conductor from a place of high to one of low potential. Thus potential in electricity corresponds to temperature in heat. One is the "degree of electrification," the other, "the degree of hotness."

[Ill.u.s.tration: FIG. 207.--Air pressure apparatus to ill.u.s.trate electrical pressure.]

=229. Electrical pressure= is a term sometimes used for difference of potential. To better understand electrical pressure consider three round tanks (Fig. 207) containing air. _A_ is a tank holding air at 10 lbs.

pressure per square inch, above atmospheric pressure, _B_ is open to the air and hence is at atmospheric pressure while _C_ has a partial vacuum, with 10 lbs. less pressure than that of the atmosphere. If the valve at _D_ or _E_ is opened a flow of air sets up until the pressures are equalized. While if the pump at _P_ is working a difference in pressure is easily maintained. Tank _A_ corresponds to an insulated body charged to a high _positive_ potential; tank _B_, open to the air, a body connected to the earth; while tank _C_ represents a body having a _negative_ potential. The earth is said to have _zero potential_.

Now just as compressed air will be pushed into the atmosphere (as from _A_ to _B_) while air at atmospheric pressure will if possible be forced itself into a partial vacuum (as from _B_ to _C_), so electricity at a positive potential will tend to move to a place at zero potential, while that at zero potential tends to move to a place of negative potential.