# Maxwell's equations | 🗖 nosplit | ↑ parent "Particle physics" | 428, 12, 954

Unified all previous electro-magnetism theories into one equation.

Explains the propagation of light as a wave, and matches the previously known relationship between the speed of light and electromagnetic constants.

The equations are a limit case of the more complete quantum electrodynamics, and unlike that more general theory account for the quantization of photon.

TODO: what does it mean that it is coherent with special relativity?

The equations are a system of partial differential equation.

The system consists of 6 unknown functions that map 4 variables: time t and the x, y and z positions in space, to a real number:and two known input functions:

- $E_{x}(t,x,y,z)$, $E_{y}(t,x,y,z)$, $E_{z}(t,x,y,z)$: directions of the electric field $E:R_{4}→R_{3}$
- $B_{x}(t,x,y,z)$, $B_{y}(t,x,y,z)$, $B_{z}(t,x,y,z)$: directions of the magnetic field $B:R_{4}→R_{3}$

- $ρ:R_{3}→R$: density of charges in space
- $J:R_{3}→R_{3}$: current vector in space. This represents the strength of moving charges in space.

Due to the conservation of charge however, those input functions have the following restriction:

$∂t∂ρ +∇⋅J=0$

Also consider the following cases:

- if a spherical charge is moving, then this of course means that $ρ$ is changing with time, and at the same time that a current exists
- in an ideal infinite cylindrical wire however, we can have constant $ρ$ in the wire, but there can still be a current because those charges are movingSuch infinite cylindrical wire is of course an ideal case, but one which is a good approximation to the huge number of electrons that travel in a actual wire.

The goal of finding $E$ and $B$ is that those fields allow us to determine the force that gets applied to a charge via the Equation 6. "Lorentz force".

and then to find the force we just need to integrate over the entire body.

Finally, now that we have defined all terms involved in the Maxwell equations, let's see the equations:

$divE=ε_{0}ρ $

$divB=0$

$∇×E=−∂t∂B $

$∇×B=μ_{0}(J+ε_{0}∂t∂E )$

and you should review the interpretation of divergence $div$ and curl $∇×$:

- divergence: how much fluid goes out or into a point
- curl: points in the direction in which a wind spinner spins fastest