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Direct Current Circuits
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

ΣRiIi = Σεi

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