diff --git a/docs/1b/ece140.md b/docs/1b/ece140.md
index 9decd5d..20e056a 100644
--- a/docs/1b/ece140.md
+++ b/docs/1b/ece140.md
@@ -160,3 +160,11 @@ i=\sum i_x
 $$
 
 To do so, each unused independent source should be replaced with a short circuit (voltage) or an open circuit (current).
+
+### Source transformation
+
+In linear circuits, a voltage source in series with a resistor can be replaced by a current source in parallel to that resistor (or vice versa), so long as Ohm's law is followed for the replacement:
+
+$$v_1=i_2R$$
+
+The arrow of the current source must point in the positive direction of the voltage source. This can also be used with dependent sources.