ece140: add power

docs/1b/ece140.md

@@ -23,3 +23,18 @@ $$i=\frac{dw}{dq}=\frac{i}{R}$$
 
 $$i=\frac{dq}{dt}\approx \frac{\Delta q}{\Delta t}$$
 
+### Power
+
+Power represents the rate of doing work, in unit watts ($\pu W$, J/s)
+
+$$P=\frac{dw}{dt}$$
+
+It is also directly related to voltage and current:
+
+$$P=vi$$
+
+Much like relative velocity, power is directional and relative, with a positive sign indicating the direction of conventional current.
+
+$$P_{CB}=-P_{BC}$$
+
+The **ground** is the "absolute zero" voltage with a maximum potential difference. It is also known as the "reference voltage".