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diff --git a/source/know/concept/lorentz-force/index.md b/source/know/concept/lorentz-force/index.md new file mode 100644 index 0000000..293cdbc --- /dev/null +++ b/source/know/concept/lorentz-force/index.md @@ -0,0 +1,190 @@ +--- +title: "Lorentz force" +date: 2021-09-08 +categories: +- Physics +- Electromagnetism +- Plasma physics +layout: "concept" +--- + +The **Lorentz force** is an empirical force used to define +the [electric field](/know/concept/electric-field/) $\vb{E}$ +and [magnetic field](/know/concept/magnetic-field/) $\vb{B}$. +For a particle with charge $q$ moving with velocity $\vb{u}$, +the Lorentz force $\vb{F}$ is given by: + +$$\begin{aligned} + \boxed{ + \vb{F} + = q (\vb{E} + \vb{u} \cross \vb{B}) + } +\end{aligned}$$ + + +## Uniform electric field + +Consider the simple case of an electric field $\vb{E}$ +that is uniform in all of space. +In the absence of a magnetic field $\vb{B} = 0$ +and any other forces, +Newton's second law states: + +$$\begin{aligned} + \vb{F} + = m \dv{\vb{u}}{t} + = q \vb{E} +\end{aligned}$$ + +This is straightforward to integrate in time, +for a given initial velocity vector $\vb{u}_0$: + +$$\begin{aligned} + \vb{u}(t) + = \frac{q}{m} \vb{E} t + \vb{u}_0 +\end{aligned}$$ + +And then the particle's position $\vb{x}(t)$ +is found be integrating once more, +with $\vb{x}(0) = \vb{x}_0$: + +$$\begin{aligned} + \boxed{ + \vb{x}(t) + = \frac{q}{2 m} \vb{E} t^2 + \vb{u}_0 t + \vb{x}_0 + } +\end{aligned}$$ + +In summary, unsurprisingly, a uniform electric field $\vb{E}$ +accelerates the particle with a constant force $\vb{F} = q \vb{E}$. +Note that the direction depends on the sign of $q$. + + +## Uniform magnetic field + +Consider the simple case of a uniform magnetic field +$\vb{B} = (0, 0, B)$ in the $z$-direction, +without an electric field $\vb{E} = 0$. +If there are no other forces, +Newton's second law states: + +$$\begin{aligned} + \vb{F} + = m \dv{\vb{u}}{t} + = q \vb{u} \cross \vb{B} +\end{aligned}$$ + +Evaluating the cross product yields +three coupled equations for the components of $\vb{u}$: + +$$\begin{aligned} + \dv{u_x}{t} + = \frac{q B}{m} u_y + \qquad \quad + \dv{u_y}{t} + = - \frac{q B}{m} u_x + \qquad \quad + \dv{u_z}{t} + = 0 +\end{aligned}$$ + +Differentiating the first equation with respect to $t$, +and substituting $\idv{u_y}{t}$ from the second, +we arrive at the following harmonic oscillator: + +$$\begin{aligned} + \dvn{2}{u_x}{t} = - \omega_c^2 u_x +\end{aligned}$$ + +Where we have defined the **cyclotron frequency** $\omega_c$ as follows, +which may be negative: + +$$\begin{aligned} + \boxed{ + \omega_c + \equiv \frac{q B}{m} + } +\end{aligned}$$ + +Suppose we choose our initial conditions so that +the solution for $u_x(t)$ is given by: + +$$\begin{aligned} + u_x(t) + = u_\perp \cos(\omega_c t) +\end{aligned}$$ + +Where $u_\perp \equiv \sqrt{u_x^2 + u_y^2}$ is the constant total transverse velocity. +Then $u_y(t)$ is found to be: + +$$\begin{aligned} + u_y(t) + = \frac{m}{q B} \dv{u_x}{t} + = - \frac{m \omega_c}{q B} u_\perp \sin(\omega_c t) + = - u_\perp \sin(\omega_c t) +\end{aligned}$$ + +This means that the particle moves in a circle, +in a direction determined by the sign of $\omega_c$. + +Integrating the velocity yields the position, +where we refer to the integration constants $x_{gc}$ and $y_{gc}$ +as the **guiding center**, around which the particle orbits or **gyrates**: + +$$\begin{aligned} + x(t) + = \frac{u_\perp}{\omega_c} \sin(\omega_c t) + x_{gc} + \qquad \quad + y(t) + = \frac{u_\perp}{\omega_c} \cos(\omega_c t) + y_{gc} +\end{aligned}$$ + +The radius of this orbit is known as the **Larmor radius** or **gyroradius** $r_L$, given by: + +$$\begin{aligned} + \boxed{ + r_L + \equiv \frac{u_\perp}{|\omega_c|} + = \frac{m u_\perp}{|q| B} + } +\end{aligned}$$ + +Finally, it is easy to integrate the equation +for the $z$-axis velocity $u_z$, which is conserved: + +$$\begin{aligned} + z(t) + = z_{gc} + = u_z t + z_0 +\end{aligned}$$ + +In conclusion, the particle's motion parallel to $\vb{B}$ +is not affected by the magnetic field, +while its motion perpendicular to $\vb{B}$ +is circular around an imaginary guiding center. +The end result is that particles follow a helical path +when moving through a uniform magnetic field: + +$$\begin{aligned} + \boxed{ + \vb{x}(t) + = \frac{u_\perp}{\omega_c} + \begin{pmatrix} + \sin(\omega_c t) \\ \cos(\omega_c t) \\ 0 + \end{pmatrix} + + \vb{x}_{gc}(t) + } +\end{aligned}$$ + +Where $\vb{x}_{gc}(t) \equiv (x_{gc}, y_{gc}, z_{gc})$ +is the position of the guiding center. +For a detailed look at how $\vb{B}$ and $\vb{E}$ +can affect the guiding center's motion, +see [guiding center theory](/know/concept/guiding-center-theory/). + + + +## References +1. F.F. Chen, + *Introduction to plasma physics and controlled fusion*, + 3rd edition, Springer. |