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---
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.
|
