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

## Answer #1

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:

**Formula for 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:

**Formula: Voltage of a practical voltage source**

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: \( U \approx U_0 \).