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
《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》由会员分享,可在线阅读,更多相关《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(132页珍藏版)》请在麦多课文档分享上搜索。
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)
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