Electric Currents - Course Of Physics

Course Of Physics

Chapter I. Introduction And Measurement. Page
Chapter II. Molecular Forces And Motions.
Chapter III. Mechanics or Liquids.
Chapter IV. Mechanics Of Gases.
Chapter V. Force And Motion.
Chapter VI. Work And Energy.
Chapter VII. Heat, Its Production And Transmission.
Chapter VIII. Heat And Work.
Chapter IX. Magnetism.
Chapter X. Static Electricity.
Chapter XI. Electric Currents Produced By VoltaicCells.

Chapter XII. Magnetic Effects Of Electric Currents,And Electrical Measurements.
Chapter XIII. Chemical And Heat Effects Of ElectricCurrents.
Chapter XIV. Induced Currents.
Chapter XV. Sound.
Chapter XVI. Light.
Chapter XVII. Invisible Radiations.
Chapter XVIII. Wireless Telephony And AlternatingCurrents.

237. Sources of Electric Currents.?In studying the production and distribution of static electricity it was seen that if two bodies at different potentials are connected by a copper wire a movement of electricity to the body having the lower potential occurred along the conducting wire. This movement of electricity is called an electric current (Art. 227). A difference of potential is therefore often called an electromotive force (E.M.F.), since it produces the movement of electricity in a conductor. The current between two oppositely charged bodies lasts for so short a time as to be of little or no practical value unless some means are found for continually recharging the bodies. That is, some device must be used to restore the difference in potential as fast as the conducting wire equalizes it. The continual charging of the bodies takes work. In other words, it requires a continual expenditure of some form of energy (which is converted into electrical energy) to produce the electric current. Two forms of energy are commonly used for this purpose.
(A) Chemical energy is employed in voltaic cells for producing electric currents. (B) Mechanical energy is used for the same purpose in the dynamo and similar devices.
238. The voltaic cell is named after Volta, an Italian physicist, who in 1800 invented it. In its simplest form it consists of a strip of copper and a strip of zinc placed in dilute sulphuric acid (one part acid to fifteen or twenty[Pg 268] of water) (Fig. 215). By the use of sensitive apparatus, it can be shown that the copper plate of the voltaic cell has a positive charge and the zinc plate a negative charge. For example, let a flat plate 10 cm. in diameter be placed upon the knob of an electroscope and a similar plate, coated with shellac and provided with an insulating handle, be set upon it to form a condenser. (See Fig. 216.) If now wires from the two plates of a simple voltaic cell be respectively connected to the plates of the condenser, charges from the copper and zinc plates will accumulate upon the two condenser plates. Now remove the wires and lift the upper plate. The "bound" charge upon the lower plate will spread over the leaves and cause them to separate. Upon testing, the charge from the zinc plate will be found to be negative and that from the copper plate, positive. Since a positive charge is found upon the copper plate it is called the positive electrode; the zinc plate is called the negative electrode.

Fig. 215.?Cross-section of a simple voltaic cell.
Fig. 216.?Testing the charges upon the plates of a simple voltaic cell.

239. Test for an Electric Current.?If the copper and zinc plates of a voltaic cell are connected by a wire, a[Pg 269] current of electricity is set up in the conductor. Evidence of the current may be obtained by holding the conducting wire over and parallel to the needle of a magnetoscope. The needle is deflected by the action of the current parallel to it (Fig. 217). This magnetic effect of a current is the means usually employed for the detection and measurement of an electric current. Such a device which detects an electric current by its magnetic effect is called a galvanoscope, in honor of Galvani, who in 1786 was the first to discover how to produce an electric current.

Fig. 217.?The magnetic needle is deflected by the current.

Fig. 218.?Diagram of an electric bell circuit.

240. The Electric Circuit.?The entire conducting path along which a current of electricity flows is called an electric circuit. In the case of a voltaic cell, the circuit includes not only the wires connecting the plates but also the plates themselves and the liquid between them. When some device or apparatus is to receive current from the cell, it is attached to the plates and wires so that the device is a part of the electric circuit. Separating the circuit at any point is called breaking or opening the circuit, while connecting the ends of an open circuit is called making or closing the circuit. A device for opening and closing a circuit is called a key or switch. The electric circuit used in ringing a door bell is familiar to most boys and girls. This circuit is open most of the time. It is closed by pressing the push-button at the door, and the flow of current through the electric bell causes the latter to ring. Such a circuit is represented in Fig. 218. Here C is the[Pg 270] voltaic cell, the two lines representing the plates of the cell. A cross-section view of the push-button (P), shows how the circuit is closed, (B) is the bell. Wherever current electricity is used the device in which it is employed forms a part of an electric circuit extending back to some electric generator. This generator must be able to continually produce an E.M.F., or a difference of potential between its terminals, in order that the movement of electricity may be continuous.

Important Topics

(a) Electric generators: (1) voltaic cell uses chemical energy; (2) dynamo uses mechanical energy.
(b) Electric circuits: (1) open, (2) closed, (3) key and switch.
(c) Voltaic and galvanic electricity (names).
(d) Galvanoscope, uses.

Exercises

1. In what two ways are static and current electricity alike? In what two different?
2. Draw a diagram of an electric bell circuit at your home. Give the location of the electric bell, the electric generator and the push-button. Show the connecting wires, and explain briefly how the circuit is operated.
3. Represent some other electric circuit, naming the generator and other devices in the circuit.
4. Look up the work of Volta and Galvani and write a statement of the electrical discoveries and inventions made by them.

(2) The Voltaic Cell and its Action

241. The simple voltaic cell consists of a strip of copper and a strip of zinc placed in dilute sulphuric acid. (See Fig. 219.) A short time after placing the plates in the acid, bubbles of a gas (hydrogen) appear on the surface of the zinc. These bubbles increase in size and some rise to the surface of the liquid. Nothing appears upon the[Pg 271] copper plate. If the tops of the plates are connected by a wire, an electric current is set up through the wire and the cell, and bubbles of gas also appear upon the copper as well as on the zinc. In a short time the surface of the copper becomes coated with bubbles and the current becomes much weaker. If the plates are left in the acid for some time the zinc is found to be eaten away, having been dissolved in the acid through chemical action. The copper, however, remains practically unaffected.

Fig. 219.?A simple voltaic cell.

242. How the Current is Produced.?To maintain the electric current a continual supply of energy is required. This is furnished by the chemical action of the acid upon the zinc. The chemical action is in several respects like combustion or burning, by means of which chemical energy is transformed into heat energy. In the voltaic cell the chemical action of the acid upon the zinc transforms chemical energy into electrical energy. The E.M.F. or difference of potential may be considered as originating at the surface of the zinc where the chemical action takes place. At this point the zinc has the lower and the liquid in contact with it the higher potential. The molecules of the acid are believed to be separated or broken up into two parts called ions; one ion, the SO₄ or sulphion, combines with the zinc forming zinc sulphate, the other, or hydrogen (H) ion, passes over to the copper plate, and accumulates on the surface of this plate giving it a positive charge. It is therefore called the positive ion.[Pg 272] The sulphion ion, or SO₄ ion, carries a negative charge to the zinc. It is therefore called the negative ion.
243. The Direction of the Current.[K]?Beginning at the surface of the zinc the direction of the movement of positive electricity may be traced through the liquid to the copper plate, to the wire, to the zinc plate, to the starting point, thus completing the electric circuit. When the circuit is closed it is found that the movement of electricity starts in all parts of the circuit at practically the same instant.

Fig. 220.?A comparison of a voltaic cell and circuit to a water pump and connecting pipes.

244. The production of the current may be illustrated by describing a device for producing a continuous circulation of water. Thus let Cu and Zn represent two pipes connected by two horizontal tubes, one at V provided with a valve and one at P with a rotary Pump. (See Fig. 220.) Suppose the pipes filled to the level of V and the pump started. The pump will force water from Zn to Cu, through P, the level falling in Zn and rising in Cu. If the valve V is open the water will flow back through V as long as the pump is working. If V is closed, the level in Cu will rise as high as the driving force of the pump can send it. If now V is opened, the pump will maintain the water in circulation from Cu to Zn through V. In the illustration, the tubes Cu and Zn correspond to the conducting plates of copper and zinc of a voltaic cell. The pump P represents the chemical action which produces[Pg 273] the electrical pressure. The upper pipe represents the part of the circuit outside of the cell, the valve V corresponds to an electric key or switch which is used to open and close the electric circuit.
245. Polarization.?In the simple voltaic cell, after the circuit is closed, bubbles of hydrogen collect upon the copper plate. This accumulation of hydrogen gas is called polarization. It acts as a non-conducting layer upon the surface of the plate and seriously interferes with the movement of electricity from the liquid to the copper plate not only in the simple voltaic cell but in many others as well. Some voltaic cells are made entirely free from this defect, either (a) by the removal of the hydrogen as fast as it is formed, or (b) by the use of such chemicals that no hydrogen is produced.
246. Local Action.?It is noticed that when a strip of zinc is placed in dilute acid that bubbles appear upon the surface of the zinc. The appearance of these bubbles indicates that some of the hydrogen ions carrying positive electricity have moved to the zinc plate. Careful examination of the plate after it has been in acid shows numerous black spots upon it. These are bits of carbon. They are always found in ordinary zinc. Small electric currents are set up which run from molecules of pure zinc into the liquid and back to the carbon particles, thus forming small closed circuits. (See Fig. 221.) The formation of these circuits from and to the zinc is called local action. This action is a defect in voltaic cells since a part of the current is thus kept from passing through the main outside circuit, and the zinc may be consumed even when no outside current is flowing.

Fig. 221.?Local action.

247. Amalgamation.?Local action is prevented by coating the zinc with mercury. This process is called amalgamation. The mercury covers the entire surface of the[Pg 274] plate in the acid. Its action is to dissolve pure zinc and bring it to the outer surface where it is acted upon by the acid. The carbon particles are kept covered so that no local currents can be formed as long as the bits of carbon are below the surface. Amalgamation therefore prevents local action.

Important Topics

The Simple Voltaic Cell
1. Two plates: zinc, copper; electrolyte, dilute sulphuric acid.
2. Ions: hydrogen, positive: sulphion, negative.
3. Current, where and how produced, direction, illustration.
4. Polarization: cure, local action, cure.

Exercises

1. Write in your own words an account of the production of an electric current by the simple voltaic cell. Use sketches.
2. Which plate has the higher potential? How is it produced?
3. Would you expect to get an E.M.F. by forming a cell of two copper plates? Why?

(3) Practical Voltaic Cells

248. Advantages of Voltaic Cells.?Many forms of voltaic cells have been devised. Several of the more common of these will be described and their electro-chemical action explained.
At the present time voltaic cells are employed only where small currents are needed, such as for electric bells and induction coils. Where more than a small amount of current is required, the dynamo and the storage battery have generally taken their place as sources of electric current.
The advantages of voltaic cells as electric generators are: (a) they are inexpensive, (b) they are easily taken from place to place, (c) they may be ready for instant use.
The most desirable voltaic cell would be one having the[Pg 275] following qualities: (a) High electromotive force, (b) no polarization or local action, (c) very low internal resistance, (d) small expense, both as to first cost and upkeep.

Fig. 222.?The Leclanch? cell, "wet" type.

249. The Leclanch? cell is the one commonly used for ringing door bells. It has two plates: one of zinc and the other of carbon. These are placed in a solution of sal ammoniac (Fig. 222). Take up the desirable qualities mentioned at the end of the preceding paragraph. (a) It may be shown that this cell has a good E.M.F. about 1.5 volts. (b) It polarizes easily yet it recovers well when left upon open circuit. Usually a substance called manganese dioxide is mixed with the carbon. This acts as a depolarizer, that is, it combines with the hydrogen to form water. (c) Its resistance varies and is often considerable. (d) The expense for upkeep is small, since a 5-cent rod of zinc, and a 5-cent charge of sal ammoniac will keep the cell in action on a bell circuit from six months to a year or more. It is well suited for use on open circuits that is, where the circuit is open the greater part of the time and is closed only occasionally; as in ringing door bells, operating telephones, and other devices whose circuits are usually open.
250. The Dry Cell.?Many forms of Leclanch? cells are made. One of these is called the dry cell (See Fig. 223.) In this cell the zinc plate is made into a jar or can and contains the other materials. At the center of the cell is a rod of carbon and manganese dioxide. The space between the carbon and zinc is filled with a porous material such as sawdust or plaster of Paris. A strong solution of sal ammoniac fills the porous material. The[Pg 276] top of the cell is sealed with pitch or wax to prevent evaporation. The great advantage of this cell is that it may be used or carried in any position without danger of spilling its contents. Dry cells are often used to operate the spark coils of gas and gasoline engines. The Leclanch? cell described in Art. 249 is commonly known as the "wet cell."

Fig. 223.?The Leclanch? cell, "dry" type.

Fig. 224.?The Daniell cell.

251. The Daniell Cell.?This cell is often used in laboratories, and on closed circuits such as those connected with fire and burglar alarms and telegraph lines. It has two plates of zinc and copper placed in two different liquids which are kept separated by a porous clay cup (Fig. 224). The zinc rod is kept in a solution of zinc sulphate contained in the porous cup. The copper plate is in a solution of copper sulphate filling the rest of the glass jar. Unlike the Leclanch? cell, this one must be kept upon a closed circuit to do its best work, as the two liquids mix when the circuit is open. Taking its qualities in order, (a) its E.M.F. is about one volt, (b) it has no polarization since copper instead of hydrogen is deposited upon the copper plate. Therefore a uniform E.M.F. may be obtained from it, making it especially useful in laboratory experiments and tests. (c) Its resistance is considerable and (d) it is more expensive to operate than the Leclanch?. It is sometimes used upon closed circuits outside of laboratories[Pg 277] as in burglar and fire alarms, although in recent years, the storage battery is taking its place for these purposes.
252. The Gravity Cell.?Fig. 225 is like the Daniell cell in most respects, except that in this cell, the zinc plate is held at the top of the jar in a solution of zinc sulphate while the copper plate is at the bottom, surrounded by a solution of copper sulphate. The solutions mix but slowly as the copper sulphate solution is denser and remains at the bottom. This cell like the Daniell must also be kept upon closed circuit. On account of its simplicity and economy it is often used to operate telegraph instruments. Its qualities are similar to those of the Daniell cell.

Fig. 225.?The gravity cell.

253. Symbol for Voltaic Cells.?In electrical diagrams, the symbol employed to represent a voltaic cell is a short thick line near to and parallel to a longer thin one. As in Fig. 226. If several cells are to be represented the conventional symbol of the combination is represented as in Fig. 227. A single cell and a group of cells are each frequently called a battery.

Fig. 226.?Diagram of a single cell.
Fig. 227.?Diagram of a group of cells.

254. Effects of Electric Currents.?Having studied some of the devices for producing an electric current, let[Pg 278] us now consider some of the effects caused by it. These effects will be studied under three heads: (a) Magnetic, (b) Chemical, and (c) Heat effects. Devices or articles showing these effects known to most high school students are respectively: (a) the electromagnet (b) electro-plated silver ware and (c) electric heaters, such as electric flat irons, electric toasters, etc. The magnetic effect of an electric current was first detected by Oersted at the University of Copenhagen in 1819. It may be observed by holding a wire carrying a current from a voltaic cell above and parallel to the needle of a magnetoscope. The needle is at once deflected (Fig. 228). If the current is reversed in direction the magnetoscope needle is deflected in the reverse direction. This simple device is the most common means for detecting an electric current. It therefore constitutes a galvanoscope. (See Art. 239.)

Fig. 227.?Diagram of a group of cells.Fig. 228.?A galvanoscope.

Important Topics

1. Leclanch? cells, (a), wet, (b), dry, construction, advantages, uses.
2. Daniell and gravity cells, construction, advantages, uses.
3. Three effects of electric currents, illustrations.
4. The galvanoscope, uses.

Exercises

1. Explain how the direction of current in a wire can be determined by a compass.
2. Would you expect to obtain a current from a zinc and copper cell containing a solution of common salt? Perform the experiment.
3. What conditions in a voltaic cell will give a steady electromotive force.
4. What conditions in a voltaic cell will give a strong electromotive force.
5. Name three different electric circuits that you know exist. Which are open and which are closed circuits?
6. Are voltaic cells used in your home? If so, for what purpose are they used? On open or closed circuits? Have you seen them? what kind are they?

➨ Chapter XII. Magnetic Effects Of Electric Currents,And Electrical Measurements

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