7.5: A Point Magnetic Dipole
- Page ID
- 22836
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Consider an oscillating magnetic dipole moment, mz, oriented along the z-axis and located at the origin of co-ordinates similar to the case of the oscillating electric dipole of Figure (7.4.3). If the dipole were static it would generate a vector potential having only a \(\phi\)-component:
\[\text{A}_{\phi}=\frac{\mu_{0}}{4 \pi} \frac{\text{m}_{\text{z}} \sin \theta}{\text{R}^{2}}. \label{7.34}\]
This follows from the general expression for the vector potential generated by a point dipole, Equation (4.3.4)
\[\vec{\text{A}}=\frac{\mu_{0}}{4 \pi} \frac{(\vec{\text{m}} \times \vec{\text{R}})}{\text{R}^{3}}. \nonumber \]
However, it can be shown that due to the effects of time retardation the equation for the vector potential,(\ref{7.34}) must be modified to read
\[\text{A}_{\phi}=\frac{\mu_{0}}{4 \pi} \sin \theta\left[\frac{\text{m}_{\text{z}}}{\text{R}^{2}}+\frac{\text{m}_{\text{z}}}{\text{cR}}\right]. \label{7.35}\]
The fields derived from this expression for the vector potential, \(\vec{\text{B}}=\operatorname{curl}(\vec{\text{A}})\), are
\[\text{B}_{\text{R}}=\frac{\mu_{0}}{4 \pi} 2 \cos \theta\left[\frac{\text{m}_{\text{z}}}{\text{R}^{3}}+\frac{\dot{\text{m}}_{\text{z}}}{\text{cR}^{2}}\right]_{\text{t}_{\text{R}}}, \label{7.36}\]
\[\text{B}_{\theta}=\frac{\mu_{0}}{4 \pi} \sin \theta\left[\frac{\text{m}_{z}}{\text{R}^{3}}+\frac{\dot{\text{m}}_{z}}{\text{c} \text{R}^{2}}+\frac{\ddot{\text{m}}_{z}}{\text{c}^{2} \text{R}}\right]_{\text{t}_{\text{R}}}, \nonumber \]
\[ \text{B}_{\phi}=0=\text{E}_{\text{R}}=\text{E}_{\theta}, \nonumber \]
\[\text{E}_{\phi}=-\frac{\mu_{0}}{4 \pi} \sin \theta\left[\frac{\dot{\text{m}}_{\text{z}}}{\text{R}^{2}}+\frac{\ddot{\text{m}}_{\text{z}}}{\text{cR}}\right]_{\text{t}_{\text{R}}}. \nonumber \]
where tR = t − R/c. Far from the dipole the radiation fields that decrease with distance like (1/R) are given by
\[\text{B}_{\theta}=\frac{\mu_{0}}{4 \pi} \frac{\ddot{\text{m}}_{z}}{\text{c}^{2} \text{R}} \sin \theta, \label{7.37}\]
\[\text{E}_{\phi}=-\frac{\mu_{0}}{4 \pi} \frac{\ddot{\text{m}}_{z}}{\text{cR}} \sin \theta=\text{cB}_{\theta}, \nonumber\]
both evaluated at the retarded time tR. Just as for the electric dipole far fields \(|\vec{\text{E}}|=\text{c}|\vec{\text{B}}|\), and \(\vec E\) and \(\vec B\) are orthogonal to each other and to the line joining the position of the observer to the dipole.