Resistor circuits are the starting point for learning about circuits. Resistance is a natural feature of all everyday circuits, and is purposely used to regulate how much current is in a circuit. Generally, there are two main ways of combining multiple resistors in a circuit, series and parallel. One feature of circuit analysis is to try and find the total, or effective, resistance, in order to find total currents.
Two fundamental rules we will use over and over in circuit analysis are Kirchhoff's rules. For series, the rule is whatever voltage is put into a circuit, it is all used up by the time you make it around. FOr parallel, whatever your total current is coming into a junction, the sum of the currents in individual branches must equal the total current that came in (i.e. you don't lose current). The resistor rules for series and parallel follow directly from Kirchhoff's rules.
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