CHAPTER XVII.
ELECTROMOTIVE FORCE.
296. Electromotive Force. It has been stated that a galvanic cell has the power to charge one of its plates positively and the other negatively; this power is called electromotive force, and, for short, E. M. F. is written. The E. M. F. of a cell depends upon the kinds of plates used and their condition, the chemicals used in the exciting fluids, etc. The greater the E. M. F. of a cell the greater its power to force the current through wires, etc. The E. M. F. of a cell does not depend upon the size of its plates, as will be seen by later experiments.
297. Unit of E. M. F.; The Volt. A certain amount of E. M. F. has been taken as the standard, and, in honor of Volta, it has been called the volt. The E. M. F. of the two-fluid cell used in Exp. 113 is not far from 1 volt. If a certain cell has the power to keep up twice the difference of potential between its terminals that the Daniell cell has, we say that it has an E. M. F. of about 2 volts.
Voltmeters are instruments to measure E. M. F.
EXPERIMENT 116. To see if the E. M. F. of a cell depends upon the materials used in its construction.
Apparatus. Tumbler two-thirds full of dilute sulphuric acid (258); strips of zinc, Zn (No. 60); copper strips, Cu (No. 67); iron strip, I (No. 76); lead strip, L (No. 77); carbon rod (No. 64); the galvanoscope, G V (No. 58); 2 wires with connectors (§ 226), so that the plates can be changed quickly; the wooden cross-piece, W C P (No. 70).
Arrange as in Fig. 90. The metal strips are all of the same size; they may be held with the hand firmly against W C P, in order to have them the same distance apart in each trial. They should be lowered to the bottom of the tumbler in each case, in order to have them acted upon by the same amount of acid. Place G V properly.
298. Directions. (A) With Zn and Cu connected to G V as shown (Fig. 90), take the reading in degrees, and note in which direction (east or west) the N end of the needle is deflected. Tabulate results, as shown in Fig. 91, filling in each column of your table made out on paper.
(B) In like manner try the following combinations in the order given, in each case connecting the first-mentioned plate with the left-hand binding-post, L, of G V. For (B) use zinc-iron.
(C) Use zinc-lead; (D) iron-copper; (E) iron-lead; (F) lead-copper; (G) copper-carbon.
| PART | PLATES. | LIQUID. | DEFLECTION. | CURRENT IN CELL FROM | |
| (A) | ZINC. | COPPER. | DIL. SULP. ACID. | 65° WEST. | CU TO ZN |
| (B) | |||||
| (C) | |||||
299. Note. Some of the combinations produce but slight currents. In case G V is not delicate enough to show clearly which way the current passes, use the astatic galvanoscope in its place for such combinations.
300. Discussion. Exp. 116 clearly showed that different combinations of metals in the acid have different powers of pushing electricity through the galvanoscope. Although some of the pairs of metals furnished so weak a current that it was necessary to use the astatic galvanoscope to study the current, all produced some current, and from the results can be formed an electromotive series (§ 301). The strength of acid, condition of plates, etc., affect the E. M. F. of a cell.
301. Electromotive Series. All metals are not acted upon to the same degree by dilute acid. From the results of Exp. 116 it is seen, part (B), that iron is electronegative to zinc; that is, the current in the cell flows from zinc to iron. Part (D) showed that iron is electropositive to copper, as the current flowed from iron to copper in the cell. It is possible to arrange the metals in a series, one below the other, in such a way that any one will be electronegative to those above it and electropositive to those below it; that is, the list should have the most electropositive metal at the top, and the one least acted upon by the acid at the bottom. Make such a list from your results. The farther the metals used are apart in the list, the greater will be the E. M. F. of the cell. Good carbon is acted upon the least of all, so zinc and carbon are better than zinc and copper.
EXPERIMENT 117. To see whether the electromotive force of a cell depends upon its size.
Apparatus. Galvanoscope; two glass tumblers; dilute acid; two wooden cross-pieces; two copper and two zinc strips, the same size as those used for Exp. 112. (See § 275). These materials will form two simple cells like Fig. 85. Have about 3 in. of acid in one tumbler, and but 1 in. in the other. The plates of one cell will then be about 2½ in. in acid, and those of the other cell only ½ in. in acid. This gives us the same effect as a large and a small cell.
302. Directions. (A) Join the large and small cells with G V so that their currents will oppose each other. To do this, join the two zinc plates by means of a wire and connectors. With two other wires connect the two copper plates with the galvanoscope binding-posts, and watch for any indication of current. Does one cell oppose the other?
303. Discussion. The E. M. F. of a cell, then, does not depend upon the size of its plates. The small piece of zinc—that is, the one in but little acid—had the same potential as the large piece; they must have had, as they were joined. The large cell will give a stronger current, under certain conditions, than the small one; but this depends upon other things than E. M. F. (See experiments under Current Strength.) A zinc-copper cell, like the one just used (Exp. 117), has the same voltage as one of the same kind would have, even though it were made as large as a barrel.