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--- formulas:lorentz_force_law [2017/10/27 13:12]
jakobadmin ↷ Page moved from classical_theories:electrodynamics:lorentz_force_law to theories:classical_theories:electrodynamics:lorentz_force_law
+++ formulas:lorentz_force_law [2018/04/14 10:27]
aresmarrero [Concrete]
@@ Line 1: / Line 1: @@
-====== Lorentz Force Law ======
+<WRAP lag>$ \vec  F= q \vec E + q\vec v \times \vec B$</WRAP>
-<tabbox Why is it interesting?>
-<tabbox Layman>
+====== Lorentz Force Law ======
-<note tip>
-Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party.
-</note>
-<tabbox Student>
-<note tip>
+<tabbox Intuitive>
-In this section things should be explained by analogy and with pictures and, if necessary, some formulas.
-</note>
-<tabbox Researcher>
-For a derivation, using the Ehrenfest theorem, see http://www.physics.drexel.edu/~bob/PHYS517/Ehrenfest.pdf
+The Lorentz force law completes classical electromagnetic and describes the effect of electric and magnetic fields on a point charge.
---> Common Question 1#
+In addition, the [[equations:maxwell_equations|Maxwell equations]] tells us how charges give rise to electric and magnetic fields.
-<--
---> Common Question 2#
-<--
-<tabbox Examples>
+<tabbox Concrete>
+**Derivation**
---> Example1#
+The [[formalisms:lagrangian_formalism|Lagrangian]] for a charge $e$ with mass $m$ in an electromagnetic potential $A$ is
+\begin{equation}
+\label{eq:Lagrangian-relativistic-EM}
+ L(q,\dot{q}) = m|{\dot{q}}| + eA_i\dot{q}^i
+\end{equation}
+so we can work out the Euler--Lagrange equations:
+\begin{align*}
+ p_i = \frac{\partial L}{\partial \dot{q}^i} &= m\frac{\dot{q}_i}{|{\dot{q}}|} + eA_i\\
+ &= m v_i + e \,A_i
+\end{align*}
+where $v$ is the velocity, which we normalize such that $|v|=1$.  An important observation is that here momentum is no longer simply mass times velocity! Continuing the analysis, we find the force
+\begin{align*}
+ F_i = \frac{\partial L}{\partial q^i} &= \frac{\partial}{\partial q^i}\Bigl(e\,A_j\dot{q}^j\Bigr)\\
+ &= e\frac{\partial A_j}{\partial q^i} \dot{q}^j
+\end{align*}
+So the [[equations:euler_lagrange_equations|Euler-Lagrange equations]] give us (using $A_i=A_j\Bigl(q(t)\Bigr)$:
+\begin{align*}
+ \dot{p} &= F \\
+ \frac{d}{dt}\Bigl(mv_i+eA_i\Bigr) &= e\frac{\partial A_j}{\partial q^i}\dot{q}^j\\
+ m\frac{d v_i}{dt} &= e\frac{\partial A_j}{\partial q^i}\dot{q}^j - e\frac{d A_i}{dt}\\
+ m\frac{d v_i}{dt} &= e\frac{\partial A_j}{\partial q^i}\dot{q}^j
+ - e\frac{\partial A_i}{\partial q^j}\dot{q}^j\\
+ &= e\left(\frac{\partial A_j}{\partial q^i} - \frac{\partial A_i}{\partial q^j}\right)\dot{q}^j .
+\end{align*}
+Here, term in parentheses is $F_{ij}=$ the electromagnetic field, $F=dA$.  Therefore, the equations of motion are
+\begin{equation}
+ m\frac{d v_i}{dt} = e F_{ij}\dot{q}^j,
+\end{equation}
+which we call the Lorentz law.
-<--
+<tabbox Abstract>
---> Example2:#
+<blockquote>The classical mechanics of an electron propagating in an electromagnetic field on a spacetime X is all encoded
+in a differential 2-form on X, called the Faraday tensor F, which encodes the classical Lorentz force that the
+electromagnetic field exerts on the electron.<cite>https://arxiv.org/abs/1601.05956</cite></blockquote>
+For a derivation, using the Ehrenfest theorem, see http://www.physics.drexel.edu/~bob/PHYS517/Ehrenfest.pdf
+<tabbox Why is it interesting?>
-<--
-<tabbox History>
 </tabbox>

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