电路电子电工基础知识 (4).ppt

Chapter 1.Basic Concepts1.3 Charge and CurrentCharge is an electrical property of the atomic particles of which matter consists,measured in coulombs(C).1.The coulomb is a large unit for charges.Thus realistic or laboratory values of charges are on the order of pC,nC,or C.2.According to experimental observations,the only charges that occur in nature are integral multiples of the electronic charge e=-1.60210-19 C.3.The law of conservation of charge states that charge can neither be created nor destroyed,only transferred.Thus the algebraic sum of the electric charges in a system does not change.Chapter 1.Basic Concepts1.3 Charge and Current Electric currentis the time rate of change of charge,measured in amperes(A).Convention:current is the net flow of positive charges.1 ampere=1 coulomb/secondThe charge transferred between time and tChapter 1.Basic Concepts1.3 Charge and Current Direct current(dc)is a current that remains constant with time.Convention:the symbol I is used to represent a dc current.Alternating current(ac)is a current that varies sinusoidally with time.