Main concepts. Ohm's law(simple circuits):

Ohm's law (simple circuits):

a) for homogeneous subcircuit:

,

where I – a current through the subcircuit; U – voltage on the subcircuit connection terminal; R – the resistance of the subcircuit;

Voltage on homogeneous subcircuit:

U = IR.

b) for subcircuit containing EMF source:

,

where Dj_{2, 1} = j₂–j₁ = U – potential difference (or voltage) on subcircuit connection terminal, e – value of Electro Motive Force (EMF) of source, which contains in the subcircuit; r – internal source resistance;

Voltage on branch of circuit containing EMF source:

U = e – IR–Ir.

c) For the closed circuit (when it is possible to reduce to one equivalent source and one resistor):

,

where R – external resistance of circuit, r – internal resistance of source.

Kirchoff’s rules (branched circuits):

For a branched circuits solution use the two Kirchoff’s rules (see example 3):

1^st – junction rule: .

where – algebraic sum of a current into the junction (which is positive when a current flows in the junction, and negative when a current flows out of the junction);

2^nd – closed loop rule: ,

– algebraic sum of voltage drop on external resistors around any closed loop, and – algebraic sum of the voltage drop on internal resistance of sources (which are positive, when the direction of a current coincides with chosen one in advance direction of path-tracing);

– algebraic sum of the source electromotive force of the closed loop (which are positive, when the direction of extraneous force work (from – to + inside the source) coincides with chosen one in advance direction of path-tracing).