What is the Internal Resistance of cells & how it affects the current flow in a circuit?
All electric cells are made of materials that have resistance. The resistance of the cell is called the internal resistance.
If a cell with internal resistance is connected to a resistor, the current will flow from the cell.
As current flows through the internal resistance, some energy is converted from electrical to heat inside the cell (so the cell gets hot). That results in a potential drop across the internal resistance of the cell.
This means that there is less energy to be converted to heat in the resistor. The potential difference (p.d.) across the resistor is therefore less than the emf of the cell.
Applying Ohm’s law to the internal resistance, the p.d. across it will be Ir.
From the law of conservation of energy, when a certain charge flows, the amount of energy converted from chemical to electrical equals the amount converted from electrical to heat.
e = IR + Ir
Rearranging this formula, we can get an equation for the current from the battery.
I = e/ (R + r)