A few lines below equation 7.8 D. Tong writes
The final fact is the Lorentz transformation of the electric field: as electron moving with velocity $\vec{v}$ in an electric field E will experience a magnetic field $\vec{B}=\frac{\gamma}{c^2}(\vec{v}\times\vec{E})$.
The note says that it was derived in another note but I couldn't find this expression.
Is this coefficient $\gamma/c^{2}$ correct? Griffiths derives this to be $-1/c^2$ and I did not find anything wrong there. See Griffiths electrodynamics, third edition, equation 12.109.
Then I looked at this book which uses Griffiths' expression in Sec. 20.5, but uses $\vec{p}=m\vec{v}$ instead to $\vec{p}=\gamma m \vec{v}$ to derive the same result. Which one is correct and why?
Answer
In above Figure-01 an inertial system $\:\mathrm S'\:$ is translated with respect to the inertial system $\:\mathrm S\:$ with constant velocity
\begin{align} \boldsymbol{\upsilon} & \boldsymbol{=}\left(\upsilon_{1},\upsilon_{2},\upsilon_{3}\right) \tag{02a}\label{02a}\\ \upsilon & \boldsymbol{=}\Vert \boldsymbol{\upsilon} \Vert \boldsymbol{=} \sqrt{ \upsilon^2_{1}\boldsymbol{+}\upsilon^2_{2}\boldsymbol{+}\upsilon^2_{3}}\:\in \left(0,c\right) \tag{02b}\label{02b} \end{align}
The Lorentz transformation is \begin{align} \mathbf{x}^{\boldsymbol{\prime}} & \boldsymbol{=} \mathbf{x}\boldsymbol{+} \dfrac{\gamma^2}{c^2 \left(\gamma\boldsymbol{+}1\right)}\left(\boldsymbol{\upsilon}\boldsymbol{\cdot} \mathbf{x}\right)\boldsymbol{\upsilon}\boldsymbol{-}\dfrac{\gamma\boldsymbol{\upsilon}}{c}c\,t \tag{03a}\label{03a}\\ c\,t^{\boldsymbol{\prime}} & \boldsymbol{=} \gamma\left(c\,t\boldsymbol{-} \dfrac{\boldsymbol{\upsilon}\boldsymbol{\cdot} \mathbf{x}}{c}\right) \tag{03b}\label{03b}\\ \gamma & \boldsymbol{=} \left(1\boldsymbol{-}\dfrac{\upsilon^2}{c^2}\right)^{\boldsymbol{-}\frac12} \tag{03c}\label{03c} \end{align}
For the Lorentz transformation \eqref{03a}-\eqref{03b}, the vectors $\:\mathbf{E}\:$ and $\:\mathbf{B}\:$ of the electromagnetic field are transformed as follows \begin{align} \mathbf{E}' & \boldsymbol{=}\gamma \mathbf{E}\boldsymbol{-}\dfrac{\gamma^2}{c^2 \left(\gamma\boldsymbol{+}1\right)}\left(\mathbf{E}\boldsymbol{\cdot} \boldsymbol{\upsilon}\right)\boldsymbol{\upsilon}\,\boldsymbol{+}\,\gamma\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{B}\right) \tag{04a}\label{04a}\\ \mathbf{B}' & \boldsymbol{=} \gamma \mathbf{B}\boldsymbol{-}\dfrac{\gamma^2}{c^2 \left(\gamma\boldsymbol{+}1\right)}\left(\mathbf{B}\boldsymbol{\cdot} \boldsymbol{\upsilon}\right)\boldsymbol{\upsilon}\boldsymbol{-}\!\dfrac{\gamma}{c^2}\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{E}\right) \tag{04b}\label{04b} \end{align} Now, if in system $\:\mathrm S\:$ we have $\:\mathbf{B}\boldsymbol{=0}$, then from \eqref{04a}-\eqref{04b} \begin{align} \mathbf{E}' & \boldsymbol{=}\gamma \mathbf{E}\boldsymbol{-}\dfrac{\gamma^2}{c^2 \left(\gamma\boldsymbol{+}1\right)}\left(\mathbf{E}\boldsymbol{\cdot} \boldsymbol{\upsilon}\right)\boldsymbol{\upsilon} \tag{05a}\label{05a}\\ \mathbf{B}' & \boldsymbol{=} \boldsymbol{-}\dfrac{\gamma}{c^2}\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{E}\right) \tag{05b}\label{05b} \end{align} Equation \eqref{05b} corresponds to Tong's equation (it remains to explain the minus sign).
From equations \eqref{05a}-\eqref{05b} we have \begin{align} \mathbf{B}' & \boldsymbol{=} \boldsymbol{-}\dfrac{\gamma}{c^2}\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{E}\right) \boldsymbol{=}\boldsymbol{-}\dfrac{1}{c^2}\left(\boldsymbol{\upsilon}\boldsymbol{\times}\gamma\mathbf{E}\right) \nonumber\\ & \boldsymbol{=} \boldsymbol{-}\dfrac{1}{c^2}\Biggl(\boldsymbol{\upsilon}\boldsymbol{\times}\left[\gamma \mathbf{E}\boldsymbol{-}\dfrac{\gamma^2}{c^2 \left(\gamma\boldsymbol{+}1\right)}\left(\mathbf{E}\boldsymbol{\cdot} \boldsymbol{\upsilon}\right)\boldsymbol{\upsilon}\right]\Biggr) \boldsymbol{=}\boldsymbol{-}\dfrac{1}{c^2}\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{E}'\right) \nonumber \end{align} that is \begin{equation} \mathbf{B}' \boldsymbol{=}\boldsymbol{-}\dfrac{1}{c^2}\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{E}'\right) \tag{06}\label{06} \end{equation} Equation \eqref{06} corresponds to Griffiths' equation.
$\boldsymbol{=\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!==\!=\!=\!=}$
$\textbf{ADDENDUM}$
If in system $\:\mathrm S\:$ we have $\:\mathbf{E}\boldsymbol{=0}$, then from \eqref{04a}-\eqref{04b} \begin{align} \mathbf{E}' & \boldsymbol{=}\gamma\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{B}\right) \tag{07a}\label{07a}\\ \mathbf{B}' & \boldsymbol{=} \gamma \mathbf{B}\boldsymbol{-}\dfrac{\gamma^2}{c^2 \left(\gamma\boldsymbol{+}1\right)}\left(\mathbf{B}\boldsymbol{\cdot} \boldsymbol{\upsilon}\right)\boldsymbol{\upsilon} \tag{07b}\label{07b} \end{align} so that \begin{align} \mathbf{E}' & \boldsymbol{=} \gamma\left(\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{B}\right)\boldsymbol{=} \left(\boldsymbol{\upsilon}\boldsymbol{\times}\gamma\mathbf{B}\right) \nonumber\\ & \boldsymbol{=} \boldsymbol{\upsilon}\boldsymbol{\times}\left[\gamma \mathbf{B}\boldsymbol{-}\dfrac{\gamma^2}{c^2 \left(\gamma\boldsymbol{+}1\right)}\left(\mathbf{B}\boldsymbol{\cdot} \boldsymbol{\upsilon}\right)\boldsymbol{\upsilon}\right] \boldsymbol{=}\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{B}' \nonumber \end{align} that is \begin{equation} \mathbf{E}' \boldsymbol{=}\boldsymbol{\upsilon}\boldsymbol{\times}\mathbf{B}' \tag{08}\label{08} \end{equation} Equations \eqref{06} and \eqref{08} are the following equations \eqref{12.109} and \eqref{12.110} respectively \begin{equation} \boxed{\:\:\overset{\boldsymbol{-\!\!\!\!\!-}}{\mathbf{B}} \boldsymbol{=}\boldsymbol{-}\dfrac{1}{c^2}\left(\mathbf{v}\boldsymbol{\times}\overset{\boldsymbol{-\!\!\!\!\!-}}{\mathbf{E}}\right)\boldsymbol{.}\:\:\vphantom{\dfrac{\tfrac{a}{b}}{\tfrac{a}{b}}}} \tag{12.109}\label{12.109} \end{equation}
\begin{equation} \boxed{\:\:\overset{\boldsymbol{-\!\!\!\!\!-}}{\mathbf{E}} \boldsymbol{=}\mathbf{v}\boldsymbol{\times}\overset{\boldsymbol{-\!\!\!\!\!-}}{\mathbf{B}}\,\boldsymbol{.}\:\:\vphantom{\dfrac{\tfrac{a}{b}}{\tfrac{a}{b}}}} \tag{12.110}\label{12.110} \end{equation} as shown in ''Introduction to Electrodynamics'' by David J.Griffiths, 3rd Edition 1999.
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