Direct current circuits consist of batteries, resistors, and meters (ammeters and voltmeters). The batteries are characterized by the values of their electromotive forces (emf) ε and their internal resistance r. Ohm's law plays an important role in the functioning of direct current circuit: U = I(R + r) - ε Complex circuits with junctions are calculated using Kirchhoff's rules.

Junction Rule: The algebraic sum of the currents into any junction is equal to zero: ΣI = 0

Loop Rule: The algebraic sum of the potential drops on the resisting elements in any closed loop must equal to the algebraic sum of the emfs

ΣR_{i}I_{i} = Σε_{i}

Kirchhoff's rules allow to determine the currents and potential differences for any circuit.