Physics Part 49

2. Electric energy, unit value, how computed?

3. Electric power, three units, value, how computed, how sold?

Exercises

1. In what three respects are voltaic and storage cells alike? In what two ways different?

2. Name the four advantages of storage cells in the order of their importance. Give your reasons for choosing this order.

3. Why are dry cells more suitable for operating a door-bell circuit, than a storage battery? Give two reasons.

4. The current for a city telephone system is provided by a storage battery. Why is this better than dry cells at each telephone?

5. An incandescent lamp takes 0.5 ampere at 110 volts. What power is required to operate it? How much _energy_ will it transform in 1 minute?

6. How long would it take for this lamp to use a kilowatt hour of energy?

7. A street car used 100 amperes at 600 volts pressure. What power was delivered to it? Express also in kilowatts and horse-power.

8. An electric toaster takes 5 amperes at 110 volts. If it toasts a slice of bread in 2 minutes, what is the cost at 10 cents a kilowatt hour?

9. An electric flat iron takes 5 amperes at 110 volts. Find the cost of using it for 2 hours at 12 cents a kilowatt hour.

10. A 1/4 kilowatt motor is used to run a was.h.i.+ng-machine for 5 hours.

What is the expense for this power at 10 cents a kilowatt hour?

11. What is the efficiency of a motor that takes 7390 watts and develops 9 horse-power?

12. How many horse-power are there in a water-fall 212 ft. high over which flows 800 cu. ft. of water per second? Express this power in kilowatts.

13. What horse-power must be applied to a dynamo having an efficiency of go per cent. if it is to light 20 arc lamps in series, each taking 10 amperes at 60 volts?

(3) THE HEAT EFFECT OF ELECTRIC CURRENTS

=292. The Production of Heat by an Electric Current.=--When no chemical or mechanical work is done by an electric current its energy is employed in overcoming the resistance of the conducting circuit and is transformed into _heat_. This effect has many practical applications and some disadvantages. Many devices employ the heating effect of electric currents, (a) the electric furnace, (b) electric lights, (c) heating coils for street cars, (d) devices about the home, as flat irons, toasters, etc. Sometimes the heat produced by an electric current in the wires of a device such as a transformer is so large in amount that especial means of cooling are employed. Unusually heavy currents have been known to melt the conducting wires of circuits and electrical devices. Hence all circuits for electric power as well as many others that ordinarily carry small currents are protected by _fuses_. An _electric fuse_ is a short piece of wire that will melt and break the circuit if the current exceeds a determined value. The fuse wire is usually enclosed in an incombustible holder. Fuse wire is frequently made of lead or of an alloy of lead and other easily fusible metals.

(See Figs. 271 and 272.)

[Ill.u.s.tration: FIG. 271.--A type of enclosed fuse.]

[Ill.u.s.tration: FIG. 272.--A link fuse (above); plug fuses (below).]

=293. Heat Developed in a Conductor.=--A rule for computing the amount of heat produced in an electric circuit by a given current has been accurately determined by experiment. It has been found that 1 _calorie_ of heat (Art. 142), is produced by an expenditure of 4.2 joules of electrical (or other) energy. In other words, 1 joule will produce 1/4.2 or 0.24 calorie. Now the number of joules of electrical energy in an electric circuit is expressed by the following formula:

Joules = volts amperes seconds, or since 1 joule = 0.24 calorie,

Calories = volts amperes seconds 0.24 or _H_ = _EI_ _t_ 0.24 (1)

By Ohm's law, _I_ = _E_/_R_ or _E_ = _I_ _R_, subst.i.tuting in equation (1) _IR_ for its equal _E_ we have

_H_ = _IR_ _t_ 0.24 (2)

Also since _I_ = _E_/_R_ subst.i.tute _E_/_R_ for _I_ in equation (1) and we have

_H_ = _E_/_R_ _t_ 0.24 (3)

To ill.u.s.trate the use of these formulas by a problem suppose that a current of 10 amperes is flowing in a circuit having a resistance of 11 ohms, for 1 minute. The heat produced will be by formula (2) = (10) 11 60 0.24 equals 15,840 calories.

[Ill.u.s.tration: FIG. 273.--A carbon filament incandescent lamp.]

[Ill.u.s.tration: FIG. 274.--A tungsten lamp.]

=294. The Incandescent Lamp.=--One of the most common devices employing the heat effect of an electric current is the _incandescent lamp_. (See Fig. 273.) In this lamp the current is sent through a carbon filament, which is heated to incandescence. In order to keep the filament from burning as well as to prevent loss of heat by convection, it is placed in a gla.s.s bulb from which the air is exhausted. Two platinum wires fused in the gla.s.s connect the carbon filament with the grooved rim and the end piece of the base. The end piece and rim connect with the socket so that an electric current may flow through the filament of the lamp.

The carbon incandescent lamp has a low efficiency. It takes 0.5 ampere of current at 110 volts or in other words it requires 55 watts to cause a 16-candle-power lamp to glow brightly, hence 1 candle power in this lamp takes 55/16 = 3.43 watts.

The _efficiency of electric lamps_ is measured by the _number of watts per candle power_. This is a peculiar use of the term efficiency, as the larger the number the less efficient is the lamp. More efficient lamps have been devised with filaments of the metals _tantalum_ and _tungsten_ (Fig. 274). These give a whiter light than do carbon lamps, and consume but about 1.25 watts per candle power.

COMPARATIVE "EFFICIENCY" OF ELECTRIC LAMPS

------------------+----------+----------------+------------ |Watts per | | Watts per Name of lamp | candle | Name of lamp | candle | power | | power ------------------+----------+----------------+------------ Carbon filament | 3 to 4 |Arc lamp | 0.5 to 0.8 Metallized carbon | 2.5 |Mercury arc | 0.6 Tantalum | 2.0 |Flaming arc | 0.4 Tungsten | 1.0 to |Nitrogen-filled | 0.6 to 0.7 | 1.5 | tungsten | ------------------+----------+----------------+------------

Incandescent lamps are connected in parallel (see Fig. 254) to wires that are kept at a constant difference of potential of 110 or 115 volts.

It is customary to place not more than twelve lamps upon one circuit, each circuit being protected by a fuse and controlled by one or more switches.

=295. The Arc Light.=--The electric _arc_ light (see Fig. 275) is extensively used for lighting large rooms, also in stereopticons and motion picture machines. The light is intense, varying from 500 to 1700 candle power. The so-called mean spherical candle power of the arc light is about 510. The candle power in the direction of greatest intensity is about 1200. It is produced at an expenditure of about 500 watts. It is therefore more efficient than the incandescent lamp, often taking less than 0.5 watt per candle power produced. The arc light was first devised by Sir Humphrey Davy in 1809, who used two pieces of charcoal connected to 2000 voltaic cells. The arc light requires so much power that its production by voltaic cells is very expensive. Consequently it did not come into common use until the dynamo had been perfected. Fig. 276 shows the appearance of the two carbons in an arc light. If a direct current is used the positive carbon is heated more intensely, and gives out the greater part of the light. The positive carbon is consumed about twice as fast as the negative and its end is concave, the negative remaining pointed.

[Ill.u.s.tration: FIG. 275.--An electric arc light.]

[Ill.u.s.tration: FIG. 276.--The appearance of a pair of used carbons.]

With alternating currents, the rods are equally consumed and produce equal amounts of light. In the stereopticon, the carbons are usually placed at right angles as in Fig. 277. In the stereopticon as well as in outdoor lighting the direct current is more effective, although the alternating current is often used, since the latter can be produced and distributed more cheaply than can direct currents. In arc lamps, placing an inner gla.s.s globe (Fig. 278) about the carbons, decreases the consumption of the carbons materially. The carbon rods of _enclosed_ arc lamps often last 60 to 100 hours.

[Ill.u.s.tration: FIG. 277.--A right-angle electric arc lamp for a stereopticon.]

[Ill.u.s.tration: FIG. 278.--An enclosed arc lamp.]

The reason why an _open_ arc lamp needs to be "retrimmed" oftener than the _enclosed_ lamp, that is, have new carbons placed in it, is because the carbons "burn" freely, that is unite with the oxygen of the air. In the enclosed arc lamp, the supply of oxygen in the inner globe is limited and is soon consumed, therefore the carbons last many times longer in such lamps.

Some carbon rods have soft cores containing calcium salts. These vaporize in the arc producing the _flaming arc light_ of a bright yellow color, and give more light than the ordinary lamp.

Important Topics

1. Heat effects of electric currents, uses and applications.

2. Computation of the heat developed in a circuit. Three formulas.