Physics Part 42

Comparison between Magnetism and Static Electricity.

Substances are: { magnetic, { conductors, { non-magnetic. { insulators.

Produced by: induction. friction, or induction.

Theory: molecular. electron. (fluid)

{ attraction, { attraction, Fields of Force { repulsion, { repulsion, Explain: { induction, { induction.

{ action of compa.s.s.

{ magnetoscope, dip, { electroscope, electron, { declination, pole, { positive, negative, { retentivity, { potential, capacity, Terms: { permeability, { condenser, electrophorus, { lodestone, { oscillatory discharge, { magnetic meridian. { lightning.

Likeness: { _a_--produced by induction, _b_--attract both are: { and repel, _c_--have fields of force.

{ _a_--electricity can be _conducted_, { magnetism cannot.

{ Differences: { _b_--electricity in _all substances_, { magnetism in few.

{ { _c_--magnetism with the compa.s.s indicates { direction.

CHAPTER XI

CURRENT ELECTRICITY

(1) ELECTRICAL CURRENTS AND CIRCUITS

=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 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 k.n.o.b of an electroscope and a similar plate, coated with sh.e.l.lac 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 acc.u.mulate 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_.

[Ill.u.s.tration: FIG. 215.--Cross-section of a simple voltaic cell.]

[Ill.u.s.tration: 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 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.

[Ill.u.s.tration: FIG. 217.--The magnetic needle is deflected by the current.]

[Ill.u.s.tration: 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-b.u.t.ton_ 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 voltaic cell, the two lines representing the plates of the cell. A cross-section view of the push-b.u.t.ton (_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-b.u.t.ton. 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 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.

[Ill.u.s.tration: 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 SO_{4} or _sulphion_, combines with the zinc forming zinc sulphate, the other, or hydrogen (H) ion, pa.s.ses over to the copper plate, and acc.u.mulates on the surface of this plate giving it a positive charge. It is therefore called the _positive ion_. The sulphion ion, or SO_{4} 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_.

[K] Many scientists consider that current in a conductor consists of _negative electrons_ flowing in a direction _opposite_ to that described in Art. 243. This is called the _electron current_, as distinguished from the _electric current_ described above.

[Ill.u.s.tration: 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 ill.u.s.trated 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 ill.u.s.tration, 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 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 acc.u.mulation 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 pa.s.sing through the main outside circuit, and the zinc may be consumed even when no outside current is flowing.

[Ill.u.s.tration: FIG. 221.--Local action.]