AGA FUN-E-2002 Fundamentals of Electricity - Combined Manual and Workbook (Second Edition XH0206)《电学的基本原理.综合手册和业务手册.第二版 XH0206》.pdf

1、FUNDAMENTALS OF ELECTRICITYSecond Edition Manual and Workbook COMBINEDPrepared byOperations the voltage produced by the ther-mocouple falls off to zero; the electromag-net becomes deenergized; and the spring-loaded automatic pilot valve closes, shut-ting off the gas supply to the appliance.Thermoele

2、ctric voltage output can besubstantially increased by using a num-ber of pairs of thermocouples in a singledevice, called a thermopile, Figure 15.The increased voltage can be used to cre-ate a more powerful electromagnet thanwith a single thermocouple.Thermostatically controlled automaticgas valves

3、can be powered by thermo-electrically generated voltages and notrequire AC voltage from the house cir-cuit. Thermocouples and thermopiles willbe discussed further in later sections.PIEZOELECTRIC SOURCES OF VOLTAGEIf certain materials, such as quartz andRochelle crystals or barium titanate, arecompre

4、ssed in a certain direction, elec-trons in the material shift, creating a volt-age across two surfaces of the material,Figure 16. This is called the piezoelectriceffect and substances that exhibit it are13Fig. 14 Use of thermocouple in automatic pilotdeviceFig. 15 A thermopile is composed of many th

5、ermo-couples connected together so that the electricaleffect is additiveFundementals11 1/9/03 8:20 PM Page 13called piezoelectric materials. T h eamount of the voltage created depends onthe rate of change of pressure applied andon how high a pressure is applied. A volt-age is not generated by a stea

6、dy pressureon the material, but rather by a rapidsqueezing or striking. Higher voltages aregenerated by rapidly applying greaterpressures to the crystal.Piezoelectric materials have beenused to provide spark ignition, such as incamping equipment, and barbeque grills.14Fig. 16 When a force is applied

7、 as shown by thearrows, a voltage is generated across the piezoelec-tric crystalFundementals11 1/9/03 8:20 PM Page 1410. As stated earlier, electricity is the movement of electrons froma body which contains an 1._ of electrons toa body which has a deficiency of 2._. (p. 5)11. The amount of electron

8、flow between “A” and some nickel-chromi-um alloys (nichrome), have resistancesranging from 60 to 70 times that of cop-per. Other resistor materials such as car-bon offer higher resistances to electricalcurrent flow (400 to 2,400 times that ofcopper).Commercially made resistors are of twogeneral type

9、s, depending on the materialsthey are made from: wire-wound resis-tors for carrying relatively high electriccurrents, and composition resistors forcarrying relatively low currents.INSULATORSIn an appliance it is necessary to containthe electrical flow within proper pathswithin the circuit itself. In

10、 other words, itis necessary to insulate the circuit. If thisprecaution is not taken, the circuit maynot perform its function properly or anunsafe condition may exist.For example, in a millivolt controlcircuit, if one of the conductors isallowed to touch the other (a so-calledshort circuit or short)

11、 the proper amountof current will not flow to the electro-magnet of the automatic pilot safetydevice and the latter will act to shut offgas supply to the appliance. Uninsulatedconductors carrying line voltage (120 or240 volts) are hazardous. If these con-ductors touch, shorting out the normalresista

12、nce of the circuit, excessive currentcan flow in the circuit. Either some com-ponent(s) in the system will burn out, orthe safety fuse will blow. The humanbody also conducts electricity. If contactis made with uninsulated parts of the cir-cuit, an electrical shock may be sus-22Fundementals11 1/9/03

13、8:21 PM Page 22tained, perhaps even a fatal one. For all ofthese reasons, insulators are necessary inelectrical circuits.An insulator is a material that con-tains few free electrons and hence per-mits relatively little current to flowthrough the material. Glass, plastics andrubber are examples of in

14、sulators.Perhaps the most familiar application ofan insulator is the coating or tubing thatsurrounds a copper wire conductor.Lacquer is commonly used as an insula-tor to separate individual wires in thewindings of a generator armature.Insulation is intended to prevent shortcircuits in conductors (wi

15、res). However,provisions are made to protect insulatedwires carrying line voltages from physi-cal damage that might destroy the effec-tiveness of the insulation. Two commonapproaches, the use of electrical steelc o n d u i t and steel-armored c abl e a r eillustrated in figure 20.Figure 20(a) shows

16、insulated wiresdrawn through a metal tube (or pipe),while taking care not to scrape the insula-tion. A junction box, shown at the left, isprovided to allow ready access to theenclosed wires for splicing and/or attach-ment of electrical outlets at these points.In armored cable systems (such as BX),in

17、sulated conductors are assembledinside a protective armor made of spiral-ly wound steel strip. This arrangementprovides a flexible cable that is easier toinstall than rigid tubes.CapacitanceElectrical resistance is a property of a cir-cuit that opposes current flow in that cir-cuit. A second propert

18、y of electrical cir-cuits, capacitance, creates certain effectswhich can be used to the advantage in DCand AC electrical and electronic circuits.A capacitor basically consists of twoplates of conducting materials (which canreadily accept or give up free electrons)separated by an insulation material,

19、called the dielectric. A capacitor acts tostore electrical energy in an electric field.Figure 21 illustrates a simple capaci-tor that is being supplied electrical ener-gy from a DC voltage source.Figure 21(a) shows the capacitor in anuncharged state. When a DC source isconnected to the capacitor, Fi

20、gure 21(b),current flows in the circuit. Free electrons23Fig. 20 Protection of insulation by conduits andBXFundementals11 1/9/03 8:21 PM Page 23collect on Plate A to make it negativelycharged and electrons are removed fromPlate B to make it positively charged. Asfree electrons collect on Plate A they actto build up a voltage, resisting the furtherflow of free electrons from the source toPlate A. Flow of electrons continues untilthe voltage on Plate A equals the voltage24Fig. 21 Illustration of charging and discharging a capacitor with a DC sourceFundementals11 1/9/03 8:21 PM Page 24