Physics Part 48

=281. Electrolysis.=--A solution from which a deposit is made by an electric current is called an _electrolyte_. The plates or other objects by which the current enters or leaves the electrolyte are called the _electrodes_. The electrode by which the current enters is called the _anode_ (_an_ = in) while the electrode by which it leaves is the cathode (_cath_ = away). The process by which an electric current decomposes a solution and deposits a substance upon an electrode is called _electrolysis_. The _current_ always flows within the cell from _anode to the cathode_. (See Fig. 262.) The metal goes with the current and is found deposited upon the cathode.

=282. Theory of Electrolysis.=--The action going on in an _electrolytic_ cell has been carefully studied. The _theory of electrolysis_, which is supported by much experimental evidence, supposes that many of the molecules in a _dilute_ solution of a substance "split up" into two parts called "ions," one ion having a positive, the other a negative charge. In a dilute solution of sulphuric acid, the _positive_ ion is of hydrogen, while the _negative_ ion is the (SO_{4}) or sulphion. These ions bearing electric charges are believed to be the _carriers of the electric current_ through the electrolyte.

The positive ions move with the current from the anode to the cathode, while the negative ions apparently are repelled by the cathode and appear upon the anode. Evidence of the acc.u.mulation of the two kinds of ions at the two electrodes is furnished by the _electrolysis_ of water, described below.

=283. Electrolysis of Water.=--Two gla.s.s tubes (Fig. 263), _H_ and _O_, are attached at the bottom to a horizontal gla.s.s tube. To the latter is also connected an upright tube _T_. At the lower ends of _H_ and _O_ are inserted, fused in the gla.s.s, platinum wires, _A_ and _C_. The tubes are filled with a weak solution of sulphuric acid. The tops of _H_ and _O_ are closed with stopc.o.c.ks, _T_ being open; a current of electricity is sent in at _A_ and out at _C_. A movement of the ions at once begins, the positive hydrogen ions appearing at _C_. These acc.u.mulate as bubbles of hydrogen which rise to the top of _H_ and displace the liquid. At the same time bubbles of oxygen appear at _A_. These rise in _O_ and also displace the liquid which rises in _T_. After the action has continued some time it may be noticed that the volume of hydrogen is just twice that of the oxygen. This was to have been expected since the formula for water is H_{2}O. The nature of the gas in _H_ or _O_ may be tested by opening the stopc.o.c.k and allowing the gas to escape slowly. The hydrogen gas can be lighted by a flame while the oxygen gas will cause a spark upon a piece of wood to glow brightly, but does not burn itself.

[Ill.u.s.tration: FIG. 263.-Electrolysis of water; oxygen collects in _O_, hydrogen in _H_.]

=284. Evidence that ions are necessary to conduct a current in a liquid= is furnished by the following experiment. A quart jar is carefully cleaned, and half filled with distilled water. Two pieces of zinc 5 cm.

square are soldered to pieces of rubber-insulated No. 14 copper wire.

The zincs are placed in the distilled water (Fig. 264) and the wires are connected to a 110 volt circuit with a 16 candle-power incandescent lamp in _series_ with the cell, as in the figure. If the zincs have been carefully cleansed and the water is pure, no current flows as is shown by the lamp remaining dark. If a minute quant.i.ty of sulphuric acid or of common salt is placed in the water the lamp at once begins to glow. Ions are now present in the liquid and conduct the current. That some substances in solution do not form ions may be shown by adding to another jar of pure water some glycerine and some cane sugar, substances resembling the acid and salt in external appearance but which do not _ionize_ when dissolved as is shown by the lamp remaining dark after adding the glycerine and sugar. The acid and salt are of _mineral_ origin while the glycerine and sugar are _vegetable_ products. This experiment ill.u.s.trates the principle that the water will conduct only when it contains ions.

[Ill.u.s.tration: FIG. 264.--The current pa.s.ses only when ions are present in the liquid.]

=285. Laws of Electrolysis.=--These were discovered by Faraday in 1833, and may be stated as follows: _I. The ma.s.s of a substance deposited by an electric current from an electrolyte is proportional to the intensity of current which pa.s.ses through it._

_II. The ma.s.s of any substance deposited by a current of uniform intensity is directly proportional to the time the current flows._

These laws have been used as a basis for defining and measuring the unit of current flow, the _ampere_. (See Art. 264.)

=286. Instances of Electrolysis.=--(a) Medicines, especially those containing a mineral substance, are sometimes introduced into the human body by electrolysis. (b) Water and gas pipes are sometimes much weakened by the effects of electric currents in the earth, especially return currents from street railways. Such currents use the metal pipes as a conductor. At the place where the current leaves the metal and enters the ground, it removes metallic ions from the pipe. This process continuing, the pipe becomes weakened and at length breaks. (c) _Copper_ is purified by the use of electric currents that remove the copper from ore or from other metals and deposit it upon electrodes. _Electrolytic_ copper is the purest known. (d) _Aluminum_ is obtained by the use of large currents of electricity that first heat the material used until it melts and then deposit the metal from the fluid material by electrolysis. These results are called chemical effects of the current since by the use of electric currents substances are changed chemically, that is, they are separated into different chemical substances.

Important Topics

1. Electrolysis, electroplating, anode, cathode, ion.

2. Theory of electrolysis--evidence: (a) electrolysis of water; (b) conductivity of acid and water.

3. Laws of electrolysis.

4. Practical use of electrolysis.

Exercises

1. A dynamo has an E.M.F. of 10 volts. What is the resistance in the circuit when 20 amperes are flowing?

2. How much silver will be deposited in an hour by this current?

3. Name five objects outside of the laboratory that have been acted upon by electrolysis. How in each case?

4. Why is table ware silver plated? Why are many iron objects nickel plated?

5. How is the electrolysis of water pipes prevented?

6. Two grams of silver are to be deposited on a spoon by a current of 1 ampere. Find the time required.

7. How long will it take to deposit 20 g. of silver in an electroplating bath if a current of 20 amperes is used?

8. If 1000 g. of silver are deposited on the cathode of an electrolytic reduction plant in 10 minutes, what is the current intensity employed?

(2) THE STORAGE BATTERY AND ELECTRIC POWER

=287. Differences Between Voltaic and Storage Cells.= Voltaic cells in which electric currents are produced by the chemical action between metal plates and an electrolyte are often called _primary batteries_. In voltaic cells one or both plates and the electrolyte are used up or lose their chemical energy in producing the current and after a time need to be replaced by new material, the _chemical energy_ of the electrolyte and of one of the plates having been _transformed_ into electrical energy.

A different proceeding obtains with another type of cell. This is called a _storage battery_, or an acc.u.mulator. In these cells, the same _plates_ and electrolyte are _used_ without change _for extended periods_, sometimes for a number of years. For this reason storage batteries have displaced many other types of cells, and they are now used (a) to operate many telephone, telegraph, and fire-alarm circuits, (b) to work the spark coils of gas and gasoline engines, (c) to help carry the "peak" load upon lighting and power circuits and (d) to furnish power for electric automobiles. Since a storage battery can deliver an electric current only after an electric current from an outside source has first been sent through it, they are often called _secondary batteries_.

=288. Construction and Action of a Storage Cell.=--The common type of storage cells consists of a _number of perforated_ plates made of an alloy of lead and a little antimony. (See Figs. 265, 266, 267.) Into the perforations is pressed a paste of red lead and litharge mixed with sulphuric acid. The plates are placed in a strong solution (20 to 25 per cent.) of sulphuric acid. The plates are now ready to be charged. This is accomplished by sending a direct current from an electric generator through the cell. The hydrogen ions are moved by the current to one set of plates and change the paste to _spongy_ metallic lead. The sulphions move to the other set of plates and change the paste to lead oxide. This electrolytic action causes the two plates to become quite different chemically so that when the cell is fully charged it is like a voltaic cell, in having plates that are different chemically. It has, when fully charged, an E.M.F. of about 2.2 volts. The several plates of a cell being in parallel and close together, the cell has but small internal resistance. Consequently a large current is available.

[Ill.u.s.tration: FIG. 265.--The positive plate of a storage cell.]

[Ill.u.s.tration: FIG. 266.--The negative plate of a storage cell.]

[Ill.u.s.tration: FIG. 267.--A complete storage cell.]

[Ill.u.s.tration: FIG. 268.]

About 75 per cent. of the energy put into the storage cell in charging can be obtained upon _discharging_. Therefore the _efficiency_ of a good storage cell is about 75 per cent. Fig. 268 represents a storage battery connected to charging and discharging circuits. The lower is the charging circuit. It contains a dynamo and a resistance (neither of which are shown in the figure) to control the current sent into the cell. The charging current enters the positive pole and leaves by the negative pole. The current produced by the cell, however, flows in the _opposite_ direction through it, that is, out from the positive and in at the negative pole. This current may be controlled by a suitable resistance and measured by an ammeter. Storage cells have several advantages: (a) They can be charged and discharged a great many times before the material placed in the perforations in the plates falls out.

(b) The electrical energy used in charging the plates _costs less_ than the plates and electrolyte of voltaic cells. (c) Charging storage cells takes much _less labor_ than replacing the electrolyte and plates of voltaic cells. (d) Storage cells produce _larger currents_ than voltaic cells. The two princ.i.p.al _disadvantages_ of storage cells are that (a) they are _very heavy_, and (b) their initial _cost_ is _considerable_.

[Ill.u.s.tration: FIG. 269.--The Edison storage cell.]

[Ill.u.s.tration: FIG. 270.--The plates of the Edison storage cell.]

=289. The Edison storage cell= (Figs. 269 and 270) has plates of iron and nickel oxide. The electrolyte is a strong solution of pota.s.sium hydroxide. These cells are lighter than lead cells of the same capacity and they are claimed to have a longer life.

=290. Energy and Power of a Storage Cell.=--In a storage cell, the electrical energy of the charging current is transformed into _chemical_ energy by the action of electrolysis. It is this chemical energy that is transformed into the energy of the electric current when the cell is discharged. The _capacity_ of storage cells is rated in "ampere hours,"

a 40 ampere hour cell being capable of producing a current of 1 ampere for 40 hours, or 5 amperes for 8 hours, etc. The production and extensive use of electric currents have made necessary accurate methods for measuring the _energy_ and _power_ of these currents. To ill.u.s.trate how this is accomplished, let us imagine an electric circuit as represented in Fig. 268. Here four storage cells in series have an E.M.F. of 8 volts and in accordance with Ohm's law produce a current of 2 amperes through a resistance in the circuit of 4 ohms. Now the work done or energy expended by the current in pa.s.sing through the resistance between the points _M_ and _N depends_ upon three factors (1) the E.M.F.

or _potential difference_; (2) the _current intensity_ and (3) the _time_. The energy is measured by their product. That is, _electrical energy_ = _potential difference_ _current intensity_ _time_. This represents the electrical energy in _joules_, or

Joules = volts amperes seconds, or _j_ = _E_ _I_ _t_.

In the circuit represented in Fig. 268 the energy expended between the points _M_ and _N_ in 1 minute (60 seconds) is 8 2 60 = 960 joules.

=291. Electric Power.=--Since power refers to the _time rate_ at which work is done or energy expended, it may be computed by dividing the electrical energy by the time, or the _electrical power_ = _volts_ _amperes_. The power of 1 joule per second is called a _watt_.

Therefore,

Watts = volts amperes, or Watts = _E_ _I_.

Other units of power are the _kilowatt_ = 1000 watts and the _horse-power_ = 746 watts. In the example given in Art. 290 the power of the current is 8 2 = 16 watts, or if the energy of the current expended between the joints _M_ and _N_ were converted into mechanical horse-power it would equal 16/746 of a horse-power. Electrical energy is usually sold by the _kilowatt-hour_, or the amount of electrical energy that would exert a power of 1000 watts for one hour, or of 100 watts for 10 hours, or of 50 watts for 20 hours, etc.

Important Topics

1. The storage battery, its construction, electrolyte, action, uses, advantages, disadvantages.