A practical (real-world) voltage source is characterized by the fact that the voltage $$U_0$$ given by the voltage source does not correspond to the voltage $$U$$ at the load resistor $$R$$, because the voltage source has an internal resistance $$R_{\text i}$$. Using the Kirchoff's and Ohm's laws, the voltage at the load resistor is given by the following equation:$$U = U_0 - R_{\text i} \, I$$