diff --git a/docs/physics/electromagnetism/electromagnetic-dynamics.md b/docs/physics/electromagnetism/electromagnetic-dynamics.md index ac1a0f2..e75a42f 100644 --- a/docs/physics/electromagnetism/electromagnetic-dynamics.md +++ b/docs/physics/electromagnetism/electromagnetic-dynamics.md @@ -221,6 +221,47 @@ then it follows from Newton's second law that: Should be rewritten/reconsidered in Lagrangian/Hamiltonian formalism. +### Waves + +We may decouple the posed equations in *Axiom 1*, obtaining second order equations describing the electric and magnetic field. + +> *Theorem 5*: In regions of space where $\rho = 0$ and $\mathbf{J} = \mathbf{0}$, the electric $\mathbf{E}: (\mathbf{r},t) \mapsto \mathbf{E}(\mathbf{r},t)$ and magnetic field $\mathbf{B}: (\mathbf{r},t) \mapsto \mathbf{B}(\mathbf{r},t)$ adhere to +> +> $$ +> \begin{align*} +> \nabla^2 \mathbf{E} &= \frac{1}{c^2} \partial_t^2 \mathbf{E},\\ +> \nabla^2 \mathbf{B} &= \frac{1}{c^2} \partial_t^2 \mathbf{B}, +> \end{align*} +> $$ +> +> with a wave speed $c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$. + +??? note "Proof:" + + For the electric field $\mathbf{E}$ in a region for which $\rho=0$ and $\mathbf{J}=\mathbf{0}$ we have + + $$ + \begin{align*} + \nabla^2 \mathbf{E} &= - \nabla \times (\nabla \times \mathbf{E}),\\ + &= \nabla \times \partial_t \mathbf{B},\\ + &= \partial_t (\nabla \times \mathbf{B}),\\ + &= \mu_0 \epsilon_0 \partial^2_t \mathbf{E}. + \end{align*} + $$ + + For the magnetic field $\mathbf{B}$ in a region for which $\rho=0$ and $\mathbf{J}=\mathbf{0}$ we have + + $$ + \begin{align*} + \nabla^2 \mathbf{B} &= - \nabla \times (\nabla \times \mathbf{B}),\\ + &= -\nabla \times (\mu_0 \epsilon_0 \partial_t \mathbf{E}),\\ + &= -\mu_0 \epsilon_0 \partial_t (\nabla \times \mathbf{E}),\\ + &= \mu_0 \epsilon_0 \partial_t^2 \mathbf{B}. + \end{align*} + $$ + +The second order equations posed in *Theorem 5* satisfy exactly the three-dimensional wave equation. Thus, vacuum supports the propagation of electromagnetic waves traveling at a constant speed $c$. In my opinion one of the most mesmerizing results of the theory of electromagnetism. + ## Electromagnetic dynamics in matter Let $\mathbf{J}_p = \partial_t \mathbf{P}$ denote the polarisation volume current density such that @@ -236,7 +277,7 @@ $$ complying to the conservation of charge. -> *Theorem 5*: The electric field $\mathbf{E}: (\mathbf{r},t) \mapsto \mathbf{E}(\mathbf{r},t)$, electric displacement $\mathbf{D}: (\mathbf{r},t) \mapsto \mathbf{D}(\mathbf{r},t)$, magnetic field $\mathbf{B}: (\mathbf{r},t) \mapsto \mathbf{B}(\mathbf{r},t)$ and auxiliary field $\mathbf{H}: (\mathbf{r},t) \mapsto \mathbf{H}(\mathbf{r},t)$ in a medium are described by +> *Theorem 6*: The electric field $\mathbf{E}: (\mathbf{r},t) \mapsto \mathbf{E}(\mathbf{r},t)$, electric displacement $\mathbf{D}: (\mathbf{r},t) \mapsto \mathbf{D}(\mathbf{r},t)$, magnetic field $\mathbf{B}: (\mathbf{r},t) \mapsto \mathbf{B}(\mathbf{r},t)$ and auxiliary field $\mathbf{H}: (\mathbf{r},t) \mapsto \mathbf{H}(\mathbf{r},t)$ in a medium are described by > > $$ > \begin{align*} @@ -272,3 +313,58 @@ complying to the conservation of charge. or $\nabla \times \mathbf{H} = \mathbf{J}_f + \partial_t \mathbf{D}$. Which require constitutive relations for closure. + +### Waves + +The posed equations in *Theorem 6* may de decoupled as well, obtaining, as expected, three-dimensional wave equations describing the electric and magnetic field. + +> *Theorem 7*: In a linear, homogeneous, dielectric medium where $\rho_f=0$ and $\mathbf{J}_f=\mathbf{0}$, the electric displacement $\mathbf{D}: (\mathbf{r},t) \mapsto \mathbf{D}(\mathbf{r},t)$ and the auxiliary field $\mathbf{H}: (\mathbf{r},t) \mapsto \mathbf{H}(\mathbf{r},t)$ adhere to +> +> $$ +> \begin{align*} +> \nabla^2 \mathbf{D} &= \frac{1}{v^2} \partial_t^2 \mathbf{D},\\ +> \nabla^2 \mathbf{H} &= \frac{1}{v^2} \partial_t^2 \mathbf{H}, +> \end{align*} +> $$ +> +> with a wave speed $v = \frac{1}{\sqrt{\mu \epsilon}}$. + +??? note "Proof:" + + For the electric displacement in a linear, homogeneous, dielectric medium where $\rho_f=0$ and $\mathbf{J}_f=\mathbf{0}$ we have + + $$ + \begin{align*} + \nabla^2 \mathbf{D} &= \epsilon \nabla^2 \mathbf{E},\\ + &= - \epsilon \nabla \times (\nabla \times \mathbf{E}),\\ + &= \epsilon \nabla \times \partial_t \mathbf{B},\\ + &= \epsilon \partial_t (\nabla \times \mathbf{B}),\\ + &= \mu \epsilon \partial_t (\nabla \times \mathbf{H},\\ + &= \mu \epsilon \partial_t^2 \mathbf{D}. + \end{align*} + $$ + + For the auxiliary field in a linear, homogeneous, dielectric medium where $\rho_f=0$ and $\mathbf{J}_f=\mathbf{0}$ we have + + $$ + \begin{align*} + \nabla^2 \mathbf{H} &= \nabla^2 \mathbf{H},\\ + &= - \nabla \times (\nabla \times \mathbf{H}),\\ + &= - \nabla \times \partial_t \mathbf{D},\\ + &= - \partial_t (\nabla \times \mathbf{D}),\\ + &= - \epsilon \partial_t (\nabla \times \mathbf{E}),\\ + &= \epsilon \partial_t^2 \mathbf{B},\\ + &= \mu \epsilon \partial_t^2 \mathbf{H}. + \end{align*} + $$ + +Equivalently we may write *Theorem 7* in terms of the electric and magnetic fields in the linear, homogeneous, dielectric medium where $\rho_f = 0$ and $\mathbf{J}_f = \mathbf{0}$ + +$$ + \begin{align*} + \nabla^2 \mathbf{E} = \frac{1}{v^2} \partial_t^2 \mathbf{E},\\ + \nabla^2 \mathbf{B} = \frac{1}{v^2} \partial_t^2 \mathbf{B}, + \end{align*} +$$ + +since $\mathbf{D} = \epsilon \mathbf{E}$ and $\mathbf{B} = \mu \mathbf{H}$.