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Gauss's Law for Magnetism is one of Maxwell's four fundamental equations of electromagnetism. It expresses the experimental fact that magnetic monopoles do not exist (unlike electric charges). Magnetic field lines always form closed loops—they never begin or end.
Integral Form (Flux through a closed surface)
∮SB⋅dA=0\oint_S \mathbf{B} \cdot d\mathbf{A} = 0
The total magnetic flux through any closed surface S S is exactly zero.
Differential Form (Divergence Form)
In differential (pointwise) form this becomes:
∇⋅B=0\nabla \cdot \mathbf{B} = 0
Partial Derivative Form (Cartesian Coordinates) — Expanded Terms
Let the magnetic field vector be
B=Bx(x,y,z) i^+By(x,y,z) j^+Bz(x,y,z) k^.\mathbf{B} = B_x(x,y,z),\hat{i} + B_y(x,y,z),\hat{j} + B_z(x,y,z),\hat{k}.
The divergence operator in Cartesian coordinates expands to:
∇⋅B=∂Bx∂x+∂By∂y+∂Bz∂z=0.\nabla \cdot \mathbf{B} = \frac{\partial B_x}{\partial x} + \frac{\partial B_y}{\partial y} + \frac{\partial B_z}{\partial z} = 0.
This is the partial differential form you asked for. It must hold at every point in space (in the absence of magnetic monopoles).
Physical Meaning

The net “source” or “sink” of magnetic field lines is zero everywhere.
Magnetic field lines that enter a volume must also leave it (they form continuous closed loops).
This is why we can always define a magnetic vector potential A \mathbf{A} such that B=∇×A \mathbf{B} = \nabla \times \mathbf{A} . Taking the divergence of both sides automatically satisfies Gauss’s law because ∇⋅(∇×A)=0 \nabla \cdot (\nabla \times \mathbf{A}) = 0 .

Key Applications

Quick Note on Units
In both SI and cgs units the differential form is simply ∇⋅B=0 \nabla \cdot \mathbf{B} = 0 (no extra constants appear on the right-hand side, unlike Gauss’s law for electricity).
This law is exact in classical electromagnetism and remains one of the pillars that any unified theory (including those that might allow magnetic monopoles at extremely high energies) must reduce to at everyday scales.

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