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A circuit node is a set of electrically connected points treated as having a common electric potential in a lumped-element model. Ideal connecting wires belong to the same circuit node even if the drawing places them far apart.

Conservation of electric charge leads to Kirchhoff's current law: the algebraic sum of currents entering an ideal node is zero when unmodelled charge accumulation is negligible. For example, a current entering a junction equals the sum of two outgoing branch currents. A capacitor connected at that node does not invalidate the law: its charging current must be included as another branch current.

An electrical circuit is a network of components and connections used to transport electric charge and exchange energy. Resistors dissipate energy, capacitors store energy in an electric field, and inductors store energy in a magnetic field.

A lumped-element model represents the circuit by branch electric currents and terminal electric potential differences. Kirchhoff's current law expresses charge balance at connections, while Kirchhoff's voltage law relates the branch voltages around loops. ^[ref] For constant ideal components, with current entering the positive-voltage terminal,Combining these relations produces algebraic equations or time-dependent differential equations.

For example, an RC circuit charging an initially uncharged capacitor from a fixed voltage source $V_0$ obeysThe product $RC$ is its time constant. This transient connects energy storage to the rate of the circuit's response. ^[ref] When propagation along connections matters, a transmission line description can replace a single lumped model.

A lumped-element model represents an electrical circuit by discrete ideal elements such as resistors, capacitors and inductors, linked by equipotential circuit nodes. Each element is assigned terminal variables, rather than a fully resolved spatial electromagnetic field.

This approximation is effective when signal-propagation times across the circuit are short compared with the timescales of interest, and the relevant dimensions are small compared with the wavelength. A long cable carrying a rapid pulse may instead require a transmission line model: different points on the cable can have different voltages at the same instant.