diff --git a/education/physics/PHYS2220/Gauss's Law.md b/education/physics/PHYS2220/Gauss's Law.md
index 971158d..3789d67 100644
--- a/education/physics/PHYS2220/Gauss's Law.md	
+++ b/education/physics/PHYS2220/Gauss's Law.md	
@@ -20,5 +20,6 @@ $$ \Phi = \oint \vec{E} \cdot d\vec{A} = \oint EdA \cos\theta$$
 For a closed sphere, the equation becomes:
 $$ \Phi = E(4\pi r^2) $$
 
+Before proceeding, we introduce the so-called permittivity constant $\in_0$, defined as $\in_0 = 1/4 \pi k$, where $k$ is the Coulomb constant. 
 
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\ No newline at end of file
+---