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V=iR

Cell is a device that is used to create a potential difference in the circuit and thus there is flow of charges in the circuit. We define emf for a cell.

When the circuit is open and there is no current, the potential difference across the cell’s terminal is equal to emf E.

When the cell is connected across a resistance the cell creates a potential difference equal to E across the resistance (R) and due the potential difference, there is current in the circuit. The value of the current is $i=\dfrac{E}{R}$.

However, every cell has a self resistance called internal resistance (r). Therefore, when a cell is connected across the resistance, the internal resistance contributes to the net resistance and thus the current in the circuit is less than then $\dfrac{E}{R}$.

It is given that E=2.2V.

When the cell is connected across the resistance of 5 ohms, The potential difference across 5 ohms is 1.8V.

Let us make the diagram of the above circuit. The current in the circuit be i.

Let the potential difference across 5 ohms resistance be V. Then according to Ohm’s law, V=iR.

This implies, $1.8=i(5)$

$\Rightarrow i=\dfrac{1.8}{5}=0.36A$.

Therefore current in the circuit is 0.36 A.

The potential difference that should have been created across the resistance of 5 ohms is 2.2V. However, due to the internal resistance, the potential difference across the resistance is 1.8V. That means the potential difference across r is 2.2-1.8=0.4V.

The current flowing through r is 0.36A.

Again use Ohm’s law.

Hence, we get

$0.4=0.36r$

$\Rightarrow r=\dfrac{0.4}{0.36}=1.11ohms$

Therefore, the internal resistance of the cell is 1.11 ohms.

Note that internal resistance of a cell is inside the cell though in the diagram it is shown outside the cell. The internal resistance is the same as a normal resistance and takes up some potential difference. It is the same as connecting a resistance r in series to an ideal cell.