When the resistance connected in series with a cell is halved, the current is not exactly doubled but slightly less, why?

Why are resistance connected in series ?

When the resistance connected in series with a cell is halved, the current is equal to or slightly less ro slighty greater than double. Why?

Why is a resistance connected in series with a zener diode when used in a circuit?

If the resistance is halved the current gets ____

n cells of e.m.f E and internal resistance r are connected in series with an external resistance R . the current will increase n times that of a single cell, if

A resistance of 1 Omega is connected to a battery of emf 2 V and internal resistance 0.4 Omega what current is flowing through the circuit ? If another battery of double the emf is connected in series with the earlier one then current flowing in the circuit becomes double . what should be the internal resistance of battery ?