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# 物理代写|原子物理代考Atomic and Molecular Physics代考|PHYS5620 The atomistic structure of matter

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## 物理代写|原子物理代考Atomic and Molecular Physics代考|Do atoms exist?

Matter offers to our senses as a continuum, categorised as a solid, liquid or gas by simply observing whether it has, respectively, a definite volume and shape, or just a definite volume, or none of them. According to this continuum picture, we can split (in the case of solids) or separate (in the case of liquids and gases) any given matter sample into two (or more) parts and, in principle, we can repeat this operation an arbitrary number of times, still obtaining continuum matter specimens (eventually very small).

This macroscopic description is contrasted by a more accurate observation of the structure of matter, as first elaborated at the dawn of the nineteenth century through the combined efforts of a number of scientists ${ }^{1}$. On the basis of chemical evidence, it was observed that matter can be formed either by pure substances (or elements) and by compounds mixing two or more elements. Next, a law of multiple definite proportions for any compound was formulated: the fixed amount of a given element and the corresponding amounts of any other element needed to form a compound are in the ratio of small integer numbers. For instance: for any given ‘quantity’ of, say, hydrogen there are needed two ‘quantities’ of oxygen to form that compound we name water. These empirical observations suggest that matter in fact cannot be arbitrarily divided into increasingly smaller parts, still maintaining its original chemical properties. Rather, a new picture was emerging: matter in whatever state of aggregation is made by elements or compounds; each element is made by elementary constituents, hereafter referred to as atoms; and compounds are formed by combining numbers of different atoms in simple ratios. It was a natural guess assuming that atoms of the same element are identical in nature and have just the same weight. Since the atomistic structure of matter cannot be addressed by our senses, it was further assumed that atoms are really very small.

## 物理代写|原子物理代考Atomic and Molecular Physics代考|What are atoms made of?

None of the experimental evidence discussed in the previous section provides information about the inner structure of an atom, nor do the phenomenological theories based on the early atomistic hypothesis indeed require any knowledge about this issue (for instance, the kinetic theory of gases assumes structureless atoms).
There is, however, direct and indirect evidence that matter-made of atomsdoes contain electrically charged particles. To name just a few examples: (i) by applying an electric field to a polar liquid, an electrolytic current is observed and explained, as originally proposed by M Faraday, in terms of dissociation of molecules into positive and negative constituents, hereafter referred to as ions, drifting in opposite directions; (ii) particles like $\alpha$ or $\beta$ ones (which we nowadays recognise as helium nuclei and electrons, respectively) emitted by radioactive substances are differently deflected by an external magnetic field due to the Lorentz force, thus proving that they carry a charge; (iii) an electrical current is observed in metals under bias, as proved by electrical measurements. We must eventually conclude not only that is matter made from atoms, but also that atoms are made by substructures with either positive or negative electric charge. The differently charged constituents of atoms have different masses as well.

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。