Internal Impedance of a Source:

 All electrical energy sources have some internal impedance (or resistance). It is due to this internal impedance that the source does not behave ideally. When a voltage source supplies power to a load, its terminal voltage (voltage available at its terminals) drops. A cell used in a torch has a voltage of 1.5 V across its two electrodes when nothing is connected to it. However, when connected to a bulb, its voltage becomes less than 1.5 V. Such a reduction in the terminal voltage of the cell may be explained as follows. Figure 1(a) shows a cell of 1.5 V connected to a bulb. When we say "cell of 1.5 V", we mean a cell whose open-circuit voltage is 1.5 V. In the equivalent circuit of Fig. 1(b), the bulb is replaced by a load resistor RL, (of, say, 0.9 𝛺 ), and the cell is replaced by a constant voltage source of 1.5 V in series with the internal resistance Rs (of, say, 0.1 𝛺. The total resistance in the circuit is now 0.1 + 0.9 = 1.0 𝛺. Since the net voltage that sends current into the circuit is 1.5 V, the current in the circuit is

 𝐼=𝑉/𝑅=1.5/1.0=1.5 𝐴

The terminal voltage (the voltage across the terminals AB) of the cell is same as the voltage across the load resistor RL. Therefore,

 𝑉𝐴𝐵=𝐼×𝑅𝐿=1.5×0.9=1.35 𝑉

The voltage that drops because of the internal resistance is 

1.5−1.35=0.15 𝑉

Note this, if the internal resistance of the cell were smaller (compared to the load resistance), voltage drop would also have been smaller than 0.15 V. The internal resistance (or impedance in case case of ac source) of a source may be due to one or more of the following reasons:

(i) The resistance of the electrolyte between the electrodes, in case of a cell.

(ii) The resistance of the armature winding in case of an alternator or a dc generator.

(iii) The output impedance of the active device like a transistor or vacuum tube in case of an oscillator (or signal generator), and rectification-type dc supply.