From 16555851b6514a736c5c9d8e73de7da7fc9b6288 Mon Sep 17 00:00:00 2001 From: Prefetch Date: Thu, 20 Oct 2022 18:25:31 +0200 Subject: Migrate from 'jekyll-katex' to 'kramdown-math-sskatex' --- .../maxwell-boltzmann-distribution/index.md | 60 +++++++++++----------- 1 file changed, 30 insertions(+), 30 deletions(-) (limited to 'source/know/concept/maxwell-boltzmann-distribution') diff --git a/source/know/concept/maxwell-boltzmann-distribution/index.md b/source/know/concept/maxwell-boltzmann-distribution/index.md index 65e983c..ebb2460 100644 --- a/source/know/concept/maxwell-boltzmann-distribution/index.md +++ b/source/know/concept/maxwell-boltzmann-distribution/index.md @@ -17,23 +17,23 @@ probability distributions with applications in classical statistical physics. In the [canonical ensemble](/know/concept/canonical-ensemble/) (where a fixed-size system can exchange energy with its environment), -the probability of a microstate with energy $E$ is given by the Boltzmann distribution: +the probability of a microstate with energy $$E$$ is given by the Boltzmann distribution: $$\begin{aligned} f(E) \:\propto\: \exp\!\big(\!-\! \beta E\big) \end{aligned}$$ -Where $\beta = 1 / k_B T$. -We split $E = K + U$, -with $K$ and $U$ the total kinetic and potential energy contributions. -If there are $N$ particles in the system, -with positions $\tilde{r} = (\vec{r}_1, ..., \vec{r}_N)$ -and momenta $\tilde{p} = (\vec{p}_1, ..., \vec{p}_N)$, -then $K$ only depends on $\tilde{p}$, -and $U$ only depends on $\tilde{r}$, +Where $$\beta = 1 / k_B T$$. +We split $$E = K + U$$, +with $$K$$ and $$U$$ the total kinetic and potential energy contributions. +If there are $$N$$ particles in the system, +with positions $$\tilde{r} = (\vec{r}_1, ..., \vec{r}_N)$$ +and momenta $$\tilde{p} = (\vec{p}_1, ..., \vec{p}_N)$$, +then $$K$$ only depends on $$\tilde{p}$$, +and $$U$$ only depends on $$\tilde{r}$$, so the probability of a specific microstate -$(\tilde{r}, \tilde{p})$ is as follows: +$$(\tilde{r}, \tilde{p})$$ is as follows: $$\begin{aligned} f(\tilde{r}, \tilde{p}) @@ -62,10 +62,10 @@ $$\begin{aligned} \:\propto\: \exp\!\big(\!-\! \beta U(\tilde{r}) \big) \end{aligned}$$ -We cannot evaluate $f_U(\tilde{r})$ further without knowing $U(\tilde{r})$ for a system. -We thus turn to $f_K(\tilde{p})$, and see that the total kinetic -energy $K(\tilde{p})$ is simply the sum of the particles' individual -kinetic energies $K_n(\vec{p}_n)$, which are well-known: +We cannot evaluate $$f_U(\tilde{r})$$ further without knowing $$U(\tilde{r})$$ for a system. +We thus turn to $$f_K(\tilde{p})$$, and see that the total kinetic +energy $$K(\tilde{p})$$ is simply the sum of the particles' individual +kinetic energies $$K_n(\vec{p}_n)$$, which are well-known: $$\begin{aligned} K(\tilde{p}) @@ -75,7 +75,7 @@ $$\begin{aligned} = \frac{|\vec{p}_n|^2}{2 m} \end{aligned}$$ -Consequently, the probability distribution $f(p_x, p_y, p_z)$ for the +Consequently, the probability distribution $$f(p_x, p_y, p_z)$$ for the momentum vector of a single particle is as follows, after normalization: @@ -84,7 +84,7 @@ $$\begin{aligned} = \Big( \frac{1}{2 \pi m k_B T} \Big)^{3/2} \exp\!\Big( \!-\!\frac{(p_x^2 + p_y^2 + p_z^2)}{2 m k_B T} \Big) \end{aligned}$$ -We now rewrite this using the velocities $v_x = p_x / m$, +We now rewrite this using the velocities $$v_x = p_x / m$$, and update the normalization, giving: $$\begin{aligned} @@ -114,10 +114,10 @@ $$\begin{aligned} ## Speed distribution We know the distribution of the velocities along each axis, -but what about the speed $v = |\vec{v}|$? -Because we do not care about the direction of $\vec{v}$, only its magnitude, -the [density of states](/know/concept/density-of-states/) $g(v)$ is not constant: -it is the rate-of-change of the volume of a sphere of radius $v$: +but what about the speed $$v = |\vec{v}|$$? +Because we do not care about the direction of $$\vec{v}$$, only its magnitude, +the [density of states](/know/concept/density-of-states/) $$g(v)$$ is not constant: +it is the rate-of-change of the volume of a sphere of radius $$v$$: $$\begin{aligned} g(v) @@ -125,8 +125,8 @@ $$\begin{aligned} = 4 \pi v^2 \end{aligned}$$ -Multiplying the velocity vector distribution by $g(v)$ -and substituting $v^2 = v_x^2 + v_y^2 + v_z^2$ +Multiplying the velocity vector distribution by $$g(v)$$ +and substituting $$v^2 = v_x^2 + v_y^2 + v_z^2$$ then gives us the **Maxwell-Boltzmann speed distribution**: $$\begin{aligned} @@ -137,9 +137,9 @@ $$\begin{aligned} \end{aligned}$$ Some notable points on this distribution are -the most probable speed $v_{\mathrm{mode}}$, -the mean average speed $v_{\mathrm{mean}}$, -and the root-mean-square speed $v_{\mathrm{rms}}$: +the most probable speed $$v_{\mathrm{mode}}$$, +the mean average speed $$v_{\mathrm{mean}}$$, +and the root-mean-square speed $$v_{\mathrm{rms}}$$: $$\begin{aligned} f'(v_\mathrm{mode}) @@ -176,7 +176,7 @@ $$\begin{aligned} Using the speed distribution, we can work out the kinetic energy distribution. -Because $K$ is not proportional to $v$, +Because $$K$$ is not proportional to $$v$$, we must do this by demanding that: $$\begin{aligned} @@ -187,9 +187,9 @@ $$\begin{aligned} = f(v) \dv{v}{K} \end{aligned}$$ -We know that $K = m v^2 / 2$, -meaning $\dd{K} = m v \dd{v}$ -so the energy distribution $f(K)$ is: +We know that $$K = m v^2 / 2$$, +meaning $$\dd{K} = m v \dd{v}$$ +so the energy distribution $$f(K)$$ is: $$\begin{aligned} f(K) @@ -197,7 +197,7 @@ $$\begin{aligned} = \sqrt{\frac{2 m}{\pi}} \: \bigg( \frac{1}{k_B T} \bigg)^{3/2} v \exp\!\Big( \!-\!\frac{m v^2}{2 k_B T} \Big) \end{aligned}$$ -Substituting $v = \sqrt{2 K/m}$ leads to +Substituting $$v = \sqrt{2 K/m}$$ leads to the **Maxwell-Boltzmann kinetic energy distribution**: $$\begin{aligned} -- cgit v1.2.3