All electrical circuit fall into one of three classes; series circuits,parallel circuits,and series-parallel circuits. 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 (figure 1).In a parallel circuit there are two or more current paths bwtween two points in the circuit ,as shown in figure 2.Here the current dividws at A,part going through R1,and part through R2,and combines at B to return to the battery. Figure 3.shows a series-parallel circuit. There are two paths bwtween pounts A and B as in the parallel circuit, and in addition there are two resistances in series in each branch og the parallel combination .two other example of series-parallel arrangements appear in figure 4.the way in which the current splits to flow through the paralel branches is shown by the arrows. In every circuit,eath 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 througt it, there will be a voltage drop across it. 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 ig is called the IR drop. 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. thus ,if the terminal voltage of the source could be measured in a way that would cuause no current to flos ,it would be found to be more than the voltage measured when a current flows by the amout of the IR drop in the source.The voltage measured with ni current flowing is termed the no-load voltage. it is apparedt that a volage source having a low internal a low internal resistance is more desirable.
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