Alternating Current
An alternating current is one which varies in the circuit with time. The value of the current at any given instant of time is the instantaneous value of current, symbolized by . The instantaneous value of current is deemed positive for one direction of flow though the cross-section of a conductor, and negative for the opposite direction. The direction of current flow in which instantaneous values are positive is called positive. A current is fully specified, if one knows its instantaneous value as a function of time, =F(t),and its positive direction.
Current whose value recur for equal increments of time are called periodic, and the least increment of time for which this recurrent takes place is called the period . For a periodic current = F(t)= F(t+T)
Fig.4.1 shows an example of the relationship = F(t) for a periodic current. The arrow in the diagram indicates the positive direction of current flow. The dotted arrows show the actual direction of current flow at the instants of time when >0 and <0. The segments of the curve between points a and b or O and c cover a complete cycle of current alternations over one period.


交流電
交流電在電路中是一個隨時間變化的值。電流值在任何給定的時間上都是一個瞬態(tài)值，用符號 表示。當(dāng)瞬態(tài)電流值 從用正號標(biāo)記的導(dǎo)體的端流進(jìn)時為正值，而從相反的方向流動時為負(fù)值。當(dāng)電流的任一瞬態(tài)值為正時，它流進(jìn)導(dǎo)體的方向就為正極。假如知道瞬態(tài)值與時間的關(guān)系式： 以及它的正方向，那么瞬時電流值就是一個定值。
電流值隨時間T進(jìn)行周期性變化的這種現(xiàn)象叫做交流電的周期性，能滿足這個條件的最小T值我們把它叫做周期。周期電流的公式為：
= F(t)= F(t+T)
圖4.1是 = F(t)周期電流的一個例子。正如圖表中箭頭所示，指出的方向表示電流的方向。打點的箭頭表示瞬時電流的正方向。在曲線上a與b或者o與c之間的部分就表示交流電一個完整的周期。