Physics Part 50

3. Electric lamps; incandescent and arc; construction, uses, efficiency.

Exercises

1. Sketch a circuit containing 10 incandescent lamps in parallel. If each lamp when hot has a resistance of 220 ohms, and the E.M.F. is 100 volts, what current will flow?

2. What will it cost to use these lights for 3 hours a day for 30 days at 10 cents a kilowatt hour?

3. How much heat will these lamps produce per minute?

4. How could you connect 110-volt lamps to a street car circuit of 660 volts? Explain this arrangement and draw a diagram.

5. A certain arc lamp required 10 amperes of current at 45 volts pressure. What would it cost at 10 cents per kilowatt hour if used 3 hours a day for 30 days?

6. Show a diagram of 3 arc lamps in series. If each takes 45 volts and 10 amperes, how much E.M.F. and current will they require?

7. If an electric toaster uses 5 amperes at 115 volts, how much heat will this develop in half an hour?

9. How much heat is developed in an electric toaster in 2 minutes, if it uses 5 amperes at 100 volts?

10. How many B.t.u.'s are given off in an electric oven that takes 10 amperes at 110 volts for 1 hour? (1 B.t.u. equals 252 calories.)

11. An electric heater supplies heat at the rate of 700 B.t.u.'s an hour. How much power does it require?

12. How many watts are required to operate 120 incandescent lamps in parallel if each takes 0.5 amperes at 110 volts?

13. An electric lamp takes 12 amperes at a P.D. of 110 volts. How many B.t.u.'s are radiated from it each second? How many calories?

14. If a 110-volt incandescent lamp is submerged for 10 minutes in 400 gr. of cold water while a current of 0.5 amperes is flowing, how many degrees centigrade will the water be warmed?

15. In an electric furnace a current of 3000 amperes is used at a P.D.

of 10 volts. Find the heat developed in 1 minute.

16. How many candle power should a 20-watt tungsten lamp give if its efficiency is one watt per candle power?

17. What is the "efficiency" of a 40-watt tungsten lamp if it gives 34 candle power?

Review Outline: Current Electricity

Produced by--Chemical action; voltaic and storage cells.

Three { Magnetic, electromagnet, uses and applications.

Princ.i.p.al { Chemical, electrolysis, applications.

Effects: { Heat, lighting and heating devices.

Theories: (a) of voltaic cells, (b) of electrolysis.

Units: Ampere, ohm, volt, watt, joule, kilowatt, horse power.

Measurement--(a) magnetic effect; galvanometer, ammeter, voltmeter, wattmeter, Wheatstone bridge, construction and use.

(b) chemical effect; voltameter.

Laws: (a) Right hand rules, for conductor and helix.

(b) Resistance, Conductors in series and parallel.

(c) Ohm's law, heat law, power law, 3 forms for each.

(d) Cells in parallel and series.

Problems: Upon applications of the laws and formulas studied.

Devices. { Voltaic cells; wet, dry, and Daniell.

and { Electrolysis and the storage battery.

Instruments: { Measuring instruments, electric bell, sounder, { heating and lighting devices.

Terms: Anode, cathode, electrolyte, ion, circuit switch, current, e.m.f., resistance, potential.

CHAPTER XIV

INDUCED CURRENTS

(1) ELECTROMAGNETIC INDUCTION

=296. Current Induced by a Magnet.=--The discovery in 1819 that a current in a conductor can deflect a magnetic needle or that it has a magnetic effect, led to many attempts _to produce an electric current by means of a magnet_. It was not until about 1831, however, that _Joseph Henry_ in America and _Michael Faraday_ in England, independently discovered how to accomplish this important result.

At the present time, voltaic cells produce but a very small part of the current electricity used. Practically all that is employed for _power, light, heat, and electrolysis is produced by the use of magnetic fields, or by electromagnetic induction_.

=297. Laws of Induced Currents.=[M]--To ill.u.s.trate how a current can be produced by electromagnetic induction:

[M] An induced current is one produced by changing the number of magnetic lines of force pa.s.sing through a coil.

Connect a coil of 400 or more turns of No. 22 insulated copper wire to a sensitive galvanometer. (See Fig. 279.) Now insert a bar magnet in the coil. A sudden movement of the galvanometer will be noticed, indicating the _production of a current_. When the magnet stops moving, however, the current stops, and the coil of the galvanometer returns to its first position. If now the magnet is removed, a movement of the galvanometer coil _in the opposite direction is_ noticed. This action may be repeated as often as desired with similar results.

Careful experiments have shown that it is the _magnetic field_ of the magnet that produces the action, and that only when the _number of lines of force in the coil is changing_ do we find a current produced in the coil. These facts lead to _Law I_. _Any change in the number of magnetic lines of force pa.s.sing through or cut by a coil will produce an electromotive force in the coil._ In the account of the experiment just given, _electric currents_ are produced, while in Law I, _electromotive forces_ are mentioned. This difference is due to the fact that an E.M.F.

is _always_ produced in a coil when the magnetic field within it is changed, while a current is found only when the coil is part of a _closed circuit_. The inductive action of the earth's magnetic field (see Fig. 280), may be shown by means of a coil of 400 to 500 turns a foot in diameter.

[Ill.u.s.tration: FIG. 279.--The moving magnet induces a current in the coil.]

[Ill.u.s.tration: FIG. 280.--A current may be induced by turning the coil in the earth's magnetic field.]

Connect its ends to a sensitive galvanometer and hold it at right angles to the earth's field. Then quickly revolve the coil through 180 degrees and note the movement of the galvanometer. Reverse the coil and the galvanometer swings in the opposite direction.

If the magnet in Fig. 279 is moved _in_ and _out_ of the coil at first _slowly_ and _later swiftly_, _small and large_ deflections of the galvanometer coil are noticed. The quicker the movement of the magnetic field the greater are the galvanometer deflections produced. This leads to _Law II_. _The electromotive forces produced are proportional to the number of lines of force cut per second._

=298. The magneto= is a device that ill.u.s.trates the laws of induced currents stated in Art. 297. The magneto (see Fig. 281), consists of several permanent, "U"-shaped magnets placed side by side. Between the poles of these magnets is placed a slotted iron cylinder having a coil of many turns of fine insulated copper wire wound in the slot as in Fig.