**Ohm’s Law can be applied to a piece of a circuit with a source, and we can draw a Volt-Ampere characteristic for this piece of a circuit.**

Let’s consider piece of a circuit with a resistor *$r$* and voltage source *$E$* in parallel are shown. Current *i* flows from terminal 1 to terminal 2. Basically the current direction depends not only on the voltage source, but from the other part of a circuit, but we will assume that current direction is considered. The potentials difference in this way is:

The potentials difference in the direction from terminal 2 to 1 is:

${u}_{21}=E\u2013ri$Voltage between terminals 1 and 2 is the following:

$u=E\u2013ri$If current has a negative sign (it flows in the opposite direction):

$u=E+ri$There is a piece of a circuit with resistance $r$ and current source $I$ in parallel between terminals 1 and 2 (see figures in the previous post). As before, the current direction *$i$* depends not only on the considered currents source, but also on other current sources in the circuit. With the mentioned current directions, current from terminal 2 to terminal 1 is* $i\u2013I$*, then voltage:

Potentials difference in the direction from terminal 2 to terminal 1 is:

${u}_{21}=rI\u2013ri$Volt-ampere characteristics of this piece of circuit are shown on the figure below.

Comparing these volt-ampere-characteristics we can say that resistor circuits with voltage source in series or current source in parallel are interchangable.