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# Kirchoff's Laws

The conservation of energy and conservation of charge when applied to electrical circuits are known as Kirchoff's laws.

Conservation of energy - zero algebraic sum of the voltage drops around a closed circuit loop (imaginary loop)

Conservation of charge - zero algebraic sum of the currents flowing into a point (total charge in, equals total charge out)

When applying these laws to solve for circuit unknowns we will find the following definitions useful:

• an element is an impedance (resistance) or EMF (ideal voltage source or ideal current source),
• a node is a point where three or more current-carrying elements are connected,
• a branch is one element or several in series connecting two adjacent nodes, and
• an interior loop is a circuit loop not subdivided by a branch.

Using these definitions we can apply Kirchoff's laws to a circuit to solve for the unknown quantities. The general procedure is:

1. define the currents and voltages on a diagram,
2. apply Kirchoff's laws to loops and nodes,
3. write down a set of linear algebraic equations, and
4. solve for the unknowns.

But before we look at general circuits lets consider how simple resistors add.

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999