Three Essential Laws for Working with Circuits
At the most basic level, analyzing circuits involves calculating the current and voltage for a particular device. That’s where device and connection equations come in. Device equations describe the relationship between voltage and current for a specific device. One of the most important device equations is Ohm’s law, which relates current (I) and voltage (V) using resistance (R), where R is a constant: V = IR or I = V/R or R = V/I.
The two connection equations you need to know are Kirchhoff’s current law (KCL) and Kirchhoff’s voltage law (KVL):

Kirchhoff’s current law: Sum of incoming currents = Sum of outgoing currents at a node

Kirchhoff’s voltage law: Sum of voltage rises = Sum of voltage drops around a closed loop