IS IT TRUE THAT TO GET THE MAXIMUM CURRENT IN SERIES COMBINATION THE LOAD RESISTANCE MUST BE GRATER THAN THE INTERNAL RESISTANCES OF IDENTICAL CELLS CONNECTED IN SERIES?IF YES WHAT IS THE REASON FOR THAT?WHAT HAPPENS IF THE CELLS WERE NON IDENTICAL?

Asked by vishnuraj | 24th May, 2012, 08:02: AM

Expert Answer:

Emf and Internal Resistance

Now, real batteries are constructed from materials which possess non-zero resistivities. It follows that real batteries are not just pure voltage sources. They also possessinternal resistances. Incidentally, a pure voltage source is usually referred to as an emf (which stands for electromotive force). Of course, emf is measured in units of volts. A battery can be modeled as an emf  connected in series with a resistor , which represents its internal resistance. Suppose that such a battery is used to drive a current through an external load resistor , as shown in Fig. 1. Note that in circuit diagrams an emf  is represented as two closely spaced parallel lines of unequal length. The electric potential of the longer line is greater than that of the shorter one by  volts. A resistor is represented as a zig-zag line. 

Figure 1: A battery of emf  and internal resistance  connected to a load resistor of resistance .

Consider the battery in the figure. The voltage  of the battery is defined as the difference in electric potential between its positive and negative terminals: i.e., the points and , respectively. As we move from  to , the electric potential increases by  volts as we cross the emf, but then decreases by  volts as we cross the internal resistor. The voltage drop across the resistor follows from Ohm's law, which implies that the drop in voltage across a resistor , carrying a current , is  in the direction in which the current flows. Thus, the voltage  of the battery is related to its emf  and internal resistance  via 

 
So current will be maximum when external (or load) resistance will be very much higher than internal resistance.

Answered by  | 24th May, 2012, 10:58: AM

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