# What is the Difference Between an Ideal and Practical Voltage Source?

Level 2 (without higher mathematics)
An ideal voltage source has no internal resistance, that is it supplies a voltage $$U_0$$ (called source voltage) which is independent of which load resistor $$R$$ is connected to the terminals of the voltage source. That means the voltage at the resistor $$R$$ is always the voltage $$U_0$$. Look at the following Ohm's law:
To keep the voltage $$U_0$$ constant, the current $$I$$ may become arbitrary - depending on the choice of resistor. This can result in very high currents that damage the circuit.
A practical voltage source, on the other hand, has an internal resistance $$R_{\text i}$$, which limits the current $$I$$ through the load resistor. The application of the Kirchhoff's voltage law results in a voltage $$U$$ at the resistor $$R$$, which does not necessarily have to correspond to the source voltage $$U_0$$ anymore:
So that a real voltage source corresponds as far as possible to the ideal voltage source, the internal resistance $$R_{\text i}$$ must be chosen as small as possible, so that the second term in 2 is almost zero: $$U \approx U_0$$.