# Pre-Quantum Electrodynamics
Author: Jean-Sébastien Caux

##### Spherical Coordinates
c.m.cs.sph

\((r, \theta, \phi)\). \(\theta\) is the polar angle, \(\phi\) the azimuthal angle.

\[ x = r \sin \theta \cos \phi, y = r \sin \theta \sin \phi, z = r \cos \theta. \label{Gr(1.62)} \]

Unit vectors: \(\hat{\boldsymbol r}, \hat{\boldsymbol \theta}, \hat{\boldsymbol \phi}\).

\[ {\bf A} = A_r \hat{\bf r} + A_{\theta} \hat{\bf \theta} + A_{\phi} \hat{\boldsymbol \phi} \label{Gr(1.63)} \]

In terms of Cartesian unit vectors:

\begin{align} \hat{\boldsymbol r} &= \sin \theta \cos \phi \hat{\bf x} + \sin \theta \sin \phi \hat{\bf y} + \cos \theta \hat{\bf z}, \nonumber \\ \hat{\boldsymbol \theta} &= \cos \theta \cos \phi \hat{\bf x} + \cos \theta \sin \phi \hat{\bf y} - \sin \theta \hat{\bf z}, \nonumber \\ \hat{\boldsymbol \phi} &= -\sin \phi \hat{\bf x} + \cos \phi \hat{\bf y}. \label{Gr(1.64)} \end{align}

Careful: these unit vectors are direction dependent, i.e. we should really write \(\hat{\boldsymbol r} (\theta, \phi), \hat{\boldsymbol \theta} (\theta, \phi), \hat{\boldsymbol \phi} (\theta, \phi)\).

Infinitesimal displacement \(d{\bf l}\):

\[ d{\bf l} = dr \hat{\boldsymbol r} + r d\theta \hat{\boldsymbol \theta} + r\sin \theta d\phi \hat{\boldsymbol \phi}. \label{Gr(1.68)} \]

Infinitesimal volume element:

\[ d\tau = dl_r dl_{\theta} dl_{\phi} = r^2 \sin \theta dr d\theta d\phi \label{Gr(1.69)} \]

Infinitesimal surface element: depends on situation.

###### Gradient

\[ {\boldsymbol ∇} T = \frac{\partial T}{\partial r} \hat{\boldsymbol r} + \frac{1}{r} \frac{\partial T}{\partial \theta} \hat{\boldsymbol \theta}

- \frac{1}{r\sin \theta} \frac{\partial T}{\partial \phi} \hat{\boldsymbol \phi}.

\label{Gr(1.70)} \]

###### Divergence

\[ {\boldsymbol ∇} ⋅ {\bf v} = \frac{1}{r^2} \frac{\partial}{\partial r} (r^2 v_r) + \frac{1}{r\sin \theta} \frac{\partial}{\partial \theta} (sinθ v_{θ})

- \frac{1}{r \sin \theta} \frac{∂ v_{φ}}{∂ φ}

\label{Gr(1.71)} \]

###### Curl

\[ {\boldsymbol ∇} × {\bf v} = \frac{1}{r\sin \theta} \left[ \frac{\partial}{\partial \theta} (sin θ v_{φ}) - \frac{∂ v_{θ}}{∂ φ} \right] \hat{\bf r}

- \frac{1}{r} \left[ \frac{1}{\sin \theta} \frac{\partial v_r}{\partial \phi} - \frac{\partial}{\partial r} (r v_{φ}) \right] \hat{\boldsymbol \theta}
- \frac{1}{r} \left[ \frac{\partial}{\partial r} (r v_{θ}) - \frac{\partial v_r}{\partial \theta} \right] \hat{\boldsymbol \phi}

\label{Gr(1.72)} \]

###### Laplacian

\[ {\boldsymbol ∇}^2 T = \frac{1}{r^2} \frac{\partial}{\partial r} \left(r^2 \frac{\partial T}{\partial r}\right)

- \frac{1}{r^2 \sin \theta} \frac{\partial}{\partial \theta} \left( sin θ \frac{\partial T}{\partial \theta}\right)
- \frac{1}{r^2 \sin^2 \theta} \frac{\partial^2 T}{\partial \phi^2}

\label{Gr(1.73)} \]

Author: Jean-Sébastien Caux

Created: 2022-02-08 Tue 06:55

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