Читать книгу The Study of Elementary Electricity and Magnetism by Experiment - Thomas M. St. John - Страница 9

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100. Some Varieties of Electricity. Static electricity does not seem to "flow in currents" as readily as some other varieties; its tendency is to stand still, hence the name, static. The simplest way to produce it is by friction. Thermo electricity is produced by changes in temperature. When certain combinations of metals become hotter or colder, a current is produced. Voltaic or Galvanic electricity is produced by chemical action. Batteries give this variety. Induced electricity is produced by other currents, and by combinations of magnets and moving coils of wire, as in the dynamo. This is, by far, the most important variety of electricity, and the dynamo is the most important producer of it.

Each of the above varieties of electricity will be studied experimentally with simple apparatus.

EXPERIMENTS 51–52. To study electrification by friction.

Apparatus. Ebonite sheet, E S (No. 26); flannel cloth, F C (No. 30). See what is said in preface about static electricity.

101. Directions. (A) Examine E S. Note that its surface is not smooth, like that of ordinary hard-rubber combs. Can you think of any reason for this?

(B) Hold its flat surface near your face, then near the back of your hand. Do you feel anything unusual?

(C) Lay E S upon a flat board, or uncovered wooden table, and slide it about. Can you easily pick it up?

(D) Place E S flat upon the table again; keep it from sliding about with your left hand, and rub it vigorously for a minute with F C. Does E S act exactly as it did before in (B) and (C)?

(E) Repeat the experiment in a dark room.

(F) Thoroughly electrify E S, and see if it will cling to the wall strongly enough to support its own weight.

102. Discussion; Electrified and Neutral Bodies. The ebonite sheet became electrified or charged; and as the electrification was produced by friction, we may say that the action of the ebonite indicated the presence of frictional electricity. No one can tell just why the ebonite acted so queerly, but we can learn a great deal by experimenting. Bodies which are not charged are said to be neutral. The table, chairs, earth, etc., are neutral. We may consider that a neutral body has been discharged.

103. Force; Resistance; Work; Potential Energy; Electrification. It takes force to raise water into a tank placed on the roof. In raising the water, work has to be done, and we have to do the work; but when we once have the water in the tank we have accomplished something. The water has potential energy; that is, on account of its high position, we can make it do some work by simply turning a stop-cock so that the water can run out and turn a water-wheel, for example.

It takes force to vigorously rub a piece of ebonite with a flannel cloth, for resistance has to be overcome; that is, work has to be done. Several things are accomplished by this work; heat is produced, for we can feel that the ebonite gets warm; we can hear sounds and see sparks. The simple muscular exertion on our part has been changed to heat, light, and sound. The most wonderful part of it all, however, is that we have electrified or charged the ebonite. We did the work at first, and now the ebonite has the power to do something, as you will soon see. Electrification is, then, a sort of potential energy.

104. Heat and Electrification. We say that heat passes to or from a body to make it hot or cold. Heat produces the sensation of warmth, but heat isn't warmth. We can force a cold body to become hot; in other words, we can get it into a hot condition in various ways, such as rubbing it, hammering it, or by placing it near or in contact with another hot body.

Electrification is, also, a condition or state into which we can force a body; but electrification isn't electricity. We know whether a body is hot or cold by its effects upon us, upon thermometers, and upon other bodies. We can tell, also, whether a body is electrified or not by the way it acts, and, in certain cases, by the sound, heat, and light which accompany the electrification.

Do not get the idea that an electrified body is covered with a layer of electricity just as a board is covered with a layer of paint.

Fig. 28.

EXPERIMENT 52.

105. Directions. Repeat Exp. 51, but in place of the ebonite, use hot tissue-paper, hot brown paper, hot newspaper, or a hot silk handkerchief. Rub your hand vigorously over them. Do these become charged?

EXPERIMENTS 53–54. To study electrical attractions.

Apparatus. The ebonite sheet, E S (No. 26); flannel cloth, F C (No. 30); small pieces of dry tissue-paper, T P (No. 31); thread (No. 32).

106. Directions. (A) Thoroughly electrify E S as before, then lift and hold it in the air. (Fig. 28.)

(B) See what the paper and thread will do when held loosely near E S.

107. Discussion. Exp. 53 shows that an electrified body attracts neutral ones. This much was known about electricity over 2,000 years ago. They didn't have ebonite then, but some of the educated men of Greece knew that amber would attract light bodies after being rubbed. The Greek word for amber is elektron, and from this has been made the word electricity.

EXPERIMENT 54.

108. Directions. Charge a sheet of hot paper by friction; lift it, by its opposite ends, and lower it over small pieces of tissue-paper placed on the table. What happens to the little pieces?

EXPERIMENT 55. To study mutual attractions.

Apparatus. The support and its attachments (See § 109); support wire, S W (No. 36); silk thread, S T (No. 33), or a rubber band, R B (No. 45); ebonite rod, E R (No. 28); flannel cloth, F C (No. 30); wire swing, W S (No. 37).

Tie one end of S T to W S, Fig. 29; tie the other end of S T to S W; adjust W S by bending, if necessary, so that it will securely hold E R. It will be found convenient to use a rubber band instead of S T; if you do, let W S straddle one end of R B (Fig. 33), and hang the other end of R B upon S W.

109. The support consists of a support base (S B, Fig. 56), a support rod (S R, Fig. 56), and a support wire (S W, Fig. 29). There is a small hole in one end of S R to receive the wire, S W, and a large hole in the other end to take the short ebonite which holds the insulating table (Fig. 32). A little paper should be wound around the lower end of S R, so that it will stand solidly in the spool which forms a part of the base.

110. Directions. (A) Electrify E R with F C, and place E R in the swing, W S (Fig. 29).

Fig. 29.

(B) Bring your finger near one side of the rubbed end of E R, then near the unrubbed end, and compare the results.

111. Mutual Attractions. A neutral body, like the hand, for example, attracts electrified ones. From Exp. 53, 54, 55, it is seen that the attraction between a neutral and an electrified body is mutual; each attracts the other.

EXPERIMENT 56. To study electrical repulsions.

Apparatus. Same as for Exp. 55; ebonite sheet, E S (No. 26).

112. Directions. (A) Charge E R, and place it in W S, Fig. 29.

(B) Charge E S, and bring it slowly near one side of the charged end of E R.

EXPERIMENT 57. To study electrical repulsions.

Apparatus. A sheet of tissue-paper, T P (No. 31); shears or a knife. Cut T P, as in Fig. 30. Each leg should be about ¼ in. wide and 3 or 4 in. long.

113. Directions. (A) Heat the paper, then place it flat upon the table and electrify it by rubbing it with your hand. You must rub away from the uncut part, or you will break the legs.

(B) Raise T P, holding it by the uncut part. Note the action of legs, and make a sketch of them.

Fig. 30.

Fig. 31.

EXPERIMENT 58. To study electrical repulsions.

Apparatus. Ebonite rod, E R (No. 28); a carbon electroscope, C E, Fig. 31 (see § 114); the support complete (see § 109); small piece of damp tissue-paper.

114. The Carbon Electroscope. Light an ordinary match, and let it burn until it is charred through and through. The black substance remaining is carbon. This is very light; it has, also, another important property which you will soon understand. Tie a small piece of the carbon to one end of a dry silk thread, and fasten the other end of the thread to the support wire, S W, which is fastened to the support (Fig. 31). We shall call this piece of apparatus the carbon e-lec-tro-scope. (See Electroscopes, Chapter XVIII., Apparatus Book.)

115. Directions. (A) Electrify E R, then hold it near the carbon of the electroscope.

(B) Bring the charged rod near little pieces of damp tissue-paper.

116. Discussion of Experiments 56, 57, 58. In 56 the two pieces of ebonite were made of the same material, and both were rubbed with flannel. They must have been similarly electrified. In 57, different parts of the same piece of paper were similarly electrified. In 58, the little piece of carbon took some of the electrification from the charged rod, just as it would take molasses from your finger should your sticky finger touch it. The electrification on the carbon must have been of the same kind as that on the rod. The carbon was charged by contact. We learn, then, that two bodies repel each other when they have the same kind of electrification. Do two charged bodies always repel each other? Is it possible that there are different kinds of electrifications?

EXPERIMENT 59. To study the electrification of glass.

Apparatus. The sheet of glass, G (No. 38), heated (a hot bottle or lamp chimney will do); a piece of silk large enough to rub G. (A silk handkerchief is just the thing, but in case you have no silk, use the flannel cloth, F C, No. 30.)

117. Directions. (A) Vigorously rub the hot glass with the silk (or flannel), also heated.

(B) Test G for electrification by means of little pieces of tissue-paper and the carbon electroscope, Exp. 58.

118. Questions. Will two pieces of electrified glass repel each other? Arrange an experiment to show whether you are right or not. Is the charge on the glass exactly like that on the ebonite? Do you know how to find out?

EXPERIMENT 60. To compare the electrification produced by ebonite and flannel with that produced by glass and silk.

Apparatus. The support (see § 109); wire swing, W S (No. 37); ebonite rod, etc., of Exp. 55 (Fig. 29); the glass, G, and silk of Exp. 59.

119. Directions. (A) Electrify E R, and place it in W S, Fig. 29.

(B) Bring the uncharged glass near E R, noting the action of E R.

(C) Heat and electrify G; bring it near E R, and carefully note whether the attraction between them is stronger or weaker than before, or whether they repel each other.

120. Discussion. We know that the glass was electrified, because it lifted tissue-paper; hence, its charge was not of the same kind as that on the ebonite. Had the electrifications been exactly alike, we should have had a repulsion (Exps. 56, 57, 58).

The exact difference between these two kinds of electrifications is not known. It has been agreed, for convenience, to call that produced by glass and silk a positive electrification. With ebonite and flannel a negative electrification is produced. The sign + is generally written for the word positive, and - for negative. These signs indicate kind, and not more or less, as in arithmetic.

121. Laws. We have learned from the experiments these facts, which are called laws:

(1) Charges of the same kind repel each other; (2) charges of unlike kinds attract each other; (3) either kind of a charge attracts, and is attracted by a neutral body.