Formula: Practical (Real) Voltage Source Terminal Voltage Source voltage Internal resistance Load resistance
$$U = \frac{\class{blue}{U_0}}{1 + \frac{R_{\text i}}{R}}$$
$$U = \frac{\class{blue}{U_0}}{1 + \frac{R_{\text i}}{R}}$$
$$\class{blue}{U_0} ~=~ \left( 1 + \frac{ R_{\text i} }{ R } \right) \, U$$
$$R_{\text i} = \left( \frac{\class{blue}{U_0}}{U} - 1 \right) \, R$$
$$R = \left( \frac{\class{blue}{U_0}}{U} - 1 \right)^{-1} \, R_{\text i}$$
Terminal Voltage
$$ U $$ Unit $$ \mathrm{V} $$
Electrical voltage of an ideal voltage source MINUS the voltage across the internal resistance \(R_{\text i}\).
It can be seen from the formula that the terminal voltage depends on both the internal resistance of the voltage source and the load resistance.
Source voltage
$$ \class{blue}{U_0} $$ Unit $$ \mathrm{V} $$
Voltage of an ideal voltage source - by definition, it has infinite resistance.
Internal resistance
$$ R_{\text i} $$ Unit $$ \mathrm{\Omega} $$
Resistance of a practical (real) voltage source.
Load resistance
$$ R $$ Unit $$ \mathrm{\Omega} $$
Electrical resistance connected to the terminals (to which the terminal voltage is applied). The terminal voltage and source voltage are equal when the internal resistance is zero.