ece106: add tut

docs/1b/ece106.md

@@ -333,7 +333,7 @@ Where $\vec dl$ is the path of the test charge from infinity to the point, and $
 
 $$\vec E\bullet\vec{dl}=Edr$$
 
-Therefore, the potential due to a point charge is equal to:
+Therefore, the potential due to a point charge is equal to (the latter is true only if distance from charge is always constant, regardless of distribution):
 
 $$V_p=-\int^p_\infty\frac{kQ}{r^2}dr=\frac{kQ}{r}$$
 
@@ -356,3 +356,7 @@ $$\vec E=-\nabla V$$
 If $\vec E$ is constant:
 
 $$\vec E=\frac{Q_{enc\ net}}{\epsilon_0\oint dS}$$
+
+The **superposition** principle allows potential due to different charges to be calculated separately and summed together to achieve the same result.
+
+