Any source of emf has an internal resistance r.  However, while analyzing circuit problems, this resistance is usually ignored. The reason is this resistance is always very small.
Explanation:
•	An emf source provides voltage to the circuit. The internal resistance behaves as it is connected in series with the source therefore; it 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. For this reason, the resistance of the emf source is ignored.

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