如何调节灯的亮度？

Which segment decides the brightness of the light  in the circuit, as shown in Fig 2-1? We can see the brightness adjustable  light anywhere in the market or in our home. Rotate the knob, the light will  turn brilliant or turn dark. So when you rotate the knob, what has changed in  the circuit?

如图2-1所示，电路中哪部分决定灯的亮度？不论是家里还是在市场，我们都可以看见亮度可调的灯。当你旋转旋钮的时候，灯就会变亮或者变暗。那么当你旋转旋钮的时候，究竟改变了电路中的什么参数呢？

图2-1 如何调节电路中灯的亮度

All electrical circuits fall into one of three  classes: series circuits, parallel circuits, and series parallel circuits.

所有的电路都可以归为三类：串联电路、并联电路以及串并联电路。

A series circuit is one in which the current flows  in a single continous path and is of the same value at every point in circuit  (Fig2-2).

串联电路是指这样一种电路：在这个电路里，电流在单一的连续的路径里流动，并且在这个电路的任何一个点，电流值都是一样的，如图2-2所示。

图2-2 串联电路

In a parallel circuit there are two or more current  paths between two points in the circuit, as shown in Fig 2-3. Here the  current divides at A, one part going through R1, and the other part through  R2, and combines at B to return to the battery.

而并联电路是指：在这个电路里，某两个节点之间，存在两个或者两个以上的回路，如图2-3，在这个电路里，电流在A点分流，一部分流经R1，一部分流经R2，最后在B点汇合，再回到电池中。

图2-3 并联电路

Fig2-4 shows series-parallel  circuits. There are two paths between points A and B as in the parallel  circuit, and in addition there are two resistances in series in each branch  of the parallel combination.

图2-4所示的是串并联电路，并联部分在A点和B点之间有两个回路，此外，在每一个并联回路中，还串联了一个电阻。

图2-4 串并联电路

The other example of series parallel arrangements  appears in Fig2-5. The way in which the current splits to flow through the  parallel branches is shown by the arrows.

图2-5是串并联电路的另一个例子，图中已用箭头标出了电流的流动方向。

图2-5 串并联电路

In every circuit, each of the parts has some  resistance: the batteries or generator, the connecting conductors, and the  apparatus itself, thus, if each part has some resistance, no matter how little,  and a current is flowing through it, there will be a voltage drop across it.

在电路系统中，每一个部件都有一定的电阻，如电池、发电机、连接导体，甚至仪器本身。所以，既然每个部件都有电阻，不管多小，  只要有电流流过，这个部件的两端就会产生电压降。

In  other words, there will be a potential difference between the two ends of the  circuit element in question. The drop in voltage is equal to the product of  the current and the resistance，hence it is  called the IR drop.

换句话说，在这个电路元件的两端就会有电位差。电压降的值等于电阻和电流值的乘积，因此被称为电阻压降。

The source of voltage has an internal resistance, and  when connected into a circuit so that current flows, there will be an IR drop  in the source just as in every other part of the circuit.

电压源存在内部电阻，当把它接入电路中，并且有电流经过，电压源内部就会有电压降，这和电路其他元件的情况是一样的。

Thus,  if the terminal voltage of the source could be measured in a way that would  cause no current to flow, it would be found to be more than the voltage measured  when a current flows by the amount of the IR drop in the source. The voltage  measured with no current flowing is termed the no-load voltage. It is  apparent that a voltage source having a low internal resistance is more  desirable.

因此，如果我们可以找到一种不产生电流而测量出电源两端的电压的方法，那么这个值就会大于在有电流流过且电压降为IR的情况下测出的电源两端的电压值。我们把没有电流流过时测量出来的电压值称为空载电压，很显然，理想电路中期望电源的内部电阻越小越好。