ece140: add kirchoff intro, resistance

docs/1b/ece140.md

@@ -77,3 +77,32 @@ $$
 A **cathode ray tube** produces an electron beam of variable intensity depending on the input signal. Electrons are deflected by the screen to produce imagery.
 
 <img src="/resources/images/crt.png" />
+
+### Resistance
+
+A **resistor** *always absorbs power*, so must be oriented such that current goes into the positive sign.
+
+According to Ohm's law, the voltage, current, and resistance are related:
+
+$$v=iR$$
+
+The **conductance** of a resistor is the inverse of its resistance, and is expressed in siemens ($\pu{S}$)
+
+$$G=\frac 1 R = \frac I V$$
+
+Therefore, power can be expressed by manipulating the equations:
+
+$$
+\begin{align*}
+P &= IR^2 \\
+&= V^2G \\
+&= \frac{V^2}{R}
+\end{align*}
+$$
+
+## Kirchoff's laws
+
+!!! definition
+    - A **node** is any point in the circuit to which 3+ elements are *directly* connected (i.e., all junctions).
+    - A **supernode** is any connected group in the circuit to which 3+ elements are *directly* connected.
+    - A **loop** is any closed path of elements.