### 物理代写|统计力学代写Statistical mechanics代考|PHYS3034

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• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|统计力学代写Statistical mechanics代考|THE MAXWELL SPEED DISTRIBUTION

The Hamiltonian of a gas of $N$ noninteracting particles is $H=\sum_{i=1}^N \boldsymbol{p}i^2 /(2 m)$. The partition function for this system (volume $V$, temperature $T$ ) is found from Eqs. (4.47) and (4.53), $$Z{\operatorname{can}}(N, V, T)=\frac{1}{N !}\left(\frac{V}{\lambda_T^3}\right)^N \equiv \frac{1}{N !} Z(N, V, T),$$
where $\lambda_T$ is the thermal wavelength, Eq. (1.65), which results from integrating over the momentum variables. With $Z_{\mathrm{can}}$ one can calculate the equation of state and the entropy using Eq. (4.58) (Exercise 5.1). The phase-space probability density is, from Eq. (4.54),
$$\rho(p, q)=\frac{1}{Z} \exp \left(-\beta \sum_{i=1}^N \boldsymbol{p}i^2 /(2 m)\right)=\prod{i=1}^N\left(\frac{\lambda_T^3}{V} \mathrm{e}^{-\beta \boldsymbol{p}i^2 /(2 m)}\right) \equiv \prod{i=1}^N \rho_i,$$
where $\rho_i$ is a one-particle distribution function. Because the Hamiltonian is separable, the $N$ particle distribution occurs as the product of $N$, single-particle distributions, i.e., the particles are independently distributed. ${ }^2$ Note that $\rho_i$ is normalized on a one-particle phase space:
$$\int \rho_i \mathrm{~d} \Gamma_i \equiv \frac{\lambda_T^3}{h^3 V} \int_V \mathrm{~d} x \mathrm{~d} y \mathrm{~d} z \int_{-\infty}^{\infty} \mathrm{d} p_x \mathrm{~d} p_y \mathrm{~d} p_z \mathrm{e}^{-\beta\left(p_x^2+p_y^2+p_z^3\right) /(2 m)}=1 .$$
Another way to calculate the entropy is through the distribution function, Eq. (4.60). One can show that Eq. (4.60) yields the Sackur-Tetrode formula when combined with Eq. (5.2) (see Exercise 5.3).

## 物理代写|统计力学代写Statistical mechanics代考|PARAMAGNETS

Some of the most successful applications of statistical mechanics involve the magnetic properties of materials. Under the general banner of magnetism there are different types of magnetic phenomena: ferromagnetism, antiferromagnetism, paramagnetism, diamagnetism, and others. In the limited space of this book we can only offer a cursory treatment of the subject. Ferro- and antiferromagnetism are cooperative effects produced by interactions among the magnetic dipoles of the atoms in a solid. Paramagnetism is the “ideal gas” of magnetism, in which magnetic moments interact only with an applied magnetic field and not with each other.

For a collection of magnetic moments $\left{\boldsymbol{\mu}i\right}$ that interact only with the external field, we need treat only the statistical mechanics of a single magnetic moment. The partition function for $N$ identical, noninteracting particles $Z_N=\left(Z_1\right)^N$, where $Z_1$ is the single-particle partition function. The energy of interaction between a magnetic dipole moment $\mu$ and a magnetic field ${ }^9 \boldsymbol{B}$ is $E=-\boldsymbol{\mu} \cdot \boldsymbol{B}$. Should we adopt a classical or a quantum treatment of this problem? It turns out that a quantum treatment leads to excellent agreement with experimental results. Thus, we consider the energy of interaction between $\mu$ and $B$ as the Hamiltonian operator, $$\hat{H}=-\boldsymbol{B} \cdot \hat{\boldsymbol{\mu}}=\frac{g \mu_B}{\hbar} \boldsymbol{B} \cdot \hat{\boldsymbol{J}}=\frac{g B \mu_B}{\hbar} \hat{J}_z,$$ where we’ve used Eq. (E.4), $\boldsymbol{\mu}=-g \mu_B \boldsymbol{J} / \hbar$, where $\mu_B \equiv e \hbar /(2 m)$ is the Bohr magneton, $g$ is the Landé g-factor (see Appendix E), and the operator $\hat{J}_z$ is the $z$-component of the total angular momentum (the $B$-field defines the $z$-direction). To use Eqs. (4.123) or (4.125) (quantum statistical mechanics in the canonical ensemble), we require the eigenfunctions and eigenvalues of the Hamiltonian operator, which in this case is proportional to $\hat{J}_z$ (Eq. (5.9)). As is well known, $\hat{J}^2$ and $\hat{J}_z$ have a common set of eigenfunctions $|J, m\rangle$ (a complete orthonormal set), such that \begin{aligned} &\hat{J}^2|J, m\rangle=J(J+1) \hbar^2|J, m\rangle \ &\hat{J}_z|J, m\rangle=m \hbar|J, m\rangle \end{aligned} where the quantum number $J$ has the values $J=0,1,2, \cdots$ or $J=\frac{1}{2}, \frac{3}{2}, \frac{5}{2}, \cdots$, and $m=$ $-J,-J+1, \cdots, J-1, J$ so that there are $(2 J+1)$ values of $m$. The energy eigenvalues are therefore $E_m=g \mu_B m B$. From Eq. (4.123), ${ }^{10}$ $$Z_1=\sum{m=-J}^J \mathrm{e}^{-\beta m \mu_B g B}=\frac{\sinh \left(y\left(J+\frac{1}{2}\right)\right)}{\sinh (y / 2)},$$
where $y \equiv \beta \mu_B g B$. The summation in Eq. (5.10) is simple because it’s a finite geometric series.

## 物理代写|统计力学代写Statistical mechanics代考|THE MAXWELL SPEED DISTRIBUTION

$$Z \operatorname{can}(N, V, T)=\frac{1}{N !}\left(\frac{V}{\lambda_T^3}\right)^N \equiv \frac{1}{N !} Z(N, V, T),$$

$$\rho(p, q)=\frac{1}{Z} \exp \left(-\beta \sum_{i=1}^N \boldsymbol{p} i^2 /(2 m)\right)=\prod i=1^N\left(\frac{\lambda_T^3}{V} \mathrm{e}^{-\beta p i^2 /(2 m)}\right) \equiv \prod i=1^N \rho_i,$$

$$\int \rho_i \mathrm{~d} \Gamma_i \equiv \frac{\lambda_T^3}{h^3 V} \int_V \mathrm{~d} x \mathrm{~d} y \mathrm{~d} z \int_{-\infty}^{\infty} \mathrm{d} p_x \mathrm{~d} p_y \mathrm{~d} p_z \mathrm{e}^{-\beta\left(p_x^2+p_y^2+p_z^3\right) /(2 m)}=1 .$$

## 物理代写|统计力学代写Statistical mechanics代考|PARAMAGNETS

$$\hat{H}=-\boldsymbol{B} \cdot \hat{\boldsymbol{\mu}}=\frac{g \mu_B}{\hbar} \boldsymbol{B} \cdot \hat{\boldsymbol{J}}=\frac{g B \mu_B}{\hbar} \hat{J}_z,$$

$$\hat{J}^2|J, m\rangle=J(J+1) \hbar^2|J, m\rangle \quad \hat{J}_z|J, m\rangle=m \hbar|J, m\rangle$$

$$Z_1=\sum m=-J^J \mathrm{e}^{-\beta m \mu_B g B}=\frac{\sinh \left(y\left(J+\frac{1}{2}\right)\right)}{\sinh (y / 2)}$$

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