Question 4: While analyzing a circuit, the internal resistance of emf sources is ignored. Why?

ANSWER

Any source of emf has an internal resistance r. No emf source without any internal resistance has been found.

“It is the opposition offered by the source to the flow of current”. It is represented by a box as shown in figure. However, while analyzing circuit problems, this resistance is usually ignored. The reason is this resistance is always very small.

We know that an emf source provides voltage to the circuit. All the resistors in the circuit have a potential drop so that the sum of the individual potential drops is equal to the voltage of the emf source provided to the circuit. The internal resistance behaves as it is connected in series with the source therefore; it also has a potential drop. This obviously affects the voltage strength of the battery, i-e, the voltage provided to the circuit is less than the total voltage produced by the emf source. However, this effect (of voltage drop across the internal resistance) is so small that it is practically insignificant. The reason for less voltage drop is that the internal resistance of the emf source is always very small. For this reason, the resistance of the emf source is neglected in circuit analysis.