电子信息类专业英语第1章辩析.ppt

Text　　The primary purpose of an electric circuit is to move or transfer charges along specified paths. This motion of charges constitutes an electric current. Current flow is represented by the letter symbol i or I, and it is the time rate of change of charge, given by i=dq/dt. The basic unit in which current is measured is the ampere(A). An ampere of current is defined as the movement of one coulomb(6.28×1018electrons) past any point of a conductor during one second of time. The milliampere(mA) and the microampere(mA) units are also used to express a magnitude of current much smaller than the ampere. One milliampere is equivalent to one-thousandth of an ampere, and one microampere is equivalent to one-millionth of an ampere. 　　We shall define voltage “across” an element as the work done in moving a unit charge (+1C) through the element from one terminal to the other. The term voltage (represented by the letter symbol U) is commonly used to indicate both a potential difference and an electromotive force. The unit in which voltage is measured is the volt(V). One volt is defined as that magnitude of electromotive force required cause a current of one ampere to pass through a conductor having a resistance of one ohm. Besides the volt, smaller or larger magnitude of voltage are expressed in millivolts(mV), microvolts(μV) or kilovolts(kV). 　　Resistors restrict the flow of electric current, for example, a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED. The value of resistor is called resistance and is represented by the letter symbol R. Resistance is measured in ohms; the symbol for ohm is an omega Ω. One ohm is defined as that amount of resistance that will limit the current in a conductor is one ampere when the voltage applied to the conductor is one volt. 1?Ω is quite small so resistor values are often given in kiloohm(kΩ) or megohm (MΩ) . 　　“Ohm’s Law”is one of the fundamental laws of electronics, and pert