泻流速率分布函数及其应用举例

气体分子从容器壁上的小孔逃逸时,如果小孔直径远小于平均自由程,则容器内处于平衡的气体所受到的扰动可以忽略,这种分子从容器壁上小孔逸出的现象称为污流[1].泻流现象在分子速率分布的测定,同位素分离等方面有许多应用.泻流分子的速率分布与麦克斯韦速率分布有区别,初学者容易忽视以致产生混淆.本文简单介绍泻流速率分布函数,并以斯特恩-盖拉赫实验的分析为例说明其应用,供普通物理教学的参考. 如所周知,按麦克斯韦速率分布律,分子的速率在区间υ～υ dυ内的几率为式中dn为单位体积内该速率区间中的分子数,n为单位体积分子数.但泻流气体…     

复物体的恢复

为了恢复一个复值物函数,本文提出了利用透镜后1f距离,2f距离及3f距离处的强度信息来恢复物函数的振幅分布和位相分布.计算表明,此算法具有广泛的实用性;并能给出很好的收敛结果.     

力学一题

一个边长为a的立方体,其密度p大于水的密度p水,将其放入一盛水容器内,使它正好被水浸没,如图一所示.松手后使该物体的一面沿容器些向下运动,初速度为零.试描述该物体的运动情况. 按照物体在水中的浮沉条件,只要 P>P水,物体就下沉.但值得注意的是,本题中物体的一面与容器壁相接触,两者之间无水渗入,故当物体在下沉过程中,在竖直方向除了受到向下的重力G和向上的浮力F以外,还受到物体与容器壁之间的摩擦力f,方向向上,如图二所示,而f是随着水对物体的水平压力N的增大而增大的,N又是随着深度的增加而增大的,故当物体运动到某一深度时,将停止…     

颗粒固体应力转向比的光弹法探测

利用压敏双折射光学特性材料,实验测量了在自然堆积和密堆积两种制样方式下颗粒仓轴向荷载在仓壁上随深度的分布.发现在填充颗粒总质量相同、容器不变的情况下,颗粒仓轴向荷载在仓壁上的分布不随颗粒深度单调变化,而是随深度呈单峰结构,数值模拟与实验观察定性符合,并且发现峰值依赖于荷载大小和样品的制备方式.另外,我们也测量了在不同填充高度下的颗粒底部平均应力随轴向荷载的变化,将其与边壁应力对比,得到颗粒体系不同深度处的转向比.     

相衬显微镜与泽尔尼克

一、相衬显微镜的基本原理及应用1.基本原理 相衬显微技术的关键是把人眼无法直接观察到的位相变化转变为可以直接观察到的强度变化.设位相物体是透明度很高的物体,光波通过该物体后,只改变波的位相,波振幅不变.把该物体放在透镜的物平面上,由于位相物体各部分的光学厚度不同,其复振幅透射系数可写为 f(x,y)=exp[1](1)式中(x,y)为该位相物体的位相分布函数.在透镜的后焦平面(即频谱面)上,f(x,y)的傅里叶变换为(2)式中u=x/λf,υ=y/λf是频谱面上的坐标,也是二维空间函数f(x,y)沿x方向和y方向的空间频率;f为透镜焦距.λ为入射光波K.在应用相…     

基于Hilbert-Huang变换的数字全息图中的零级项抑制

针对数字全息图中的零级项问题,提出一种用Hilbert-Huang变换对零级项进行抑制的方法.首先将二维数字全息图数据展开为一维数据,然后用Hilbert-Huang变换进行经验模型分解,得到第一层的本征模函数,该函数包含了全息图的高频分量,继续分解,得到第二层的本征模函数,该函数包含了全息图的直流分量.实验结果证明,将第一层的本征模函数进行再现,即为零级项得到有效抑制的共轭像,第二层的本征模函数为零级项.该方法只需在空域中进行一次经验模型分解就可有效抑制零级项,较好地保存了原物波信息,在计算速度上具有明显优势.     

量子点系统的热力学稳定性及其单粒子分布函数的单调性质：零温时的情况

利用一个量子点体系中基态的稳定性和变分原理,证明了在零温时,该体系的单粒子分布函数对于量子点上单粒子能级而言是非增的.这一结果表明,一个量子多体体系中谱函数的行为在很大程度上是由其热力学稳定性条件决定的.     

傅里叶变换光学基本原理讲座第三讲 正弦光栅的衍射

一、空间频率概念 衍射屏可以是各式各样的,其屏函数的具体形式也各不相同.最基本的屏函数是具有空间周期性的函数.描述空间周期函数的重要概念是“空间频率”,鉴于它在变换光学中的重要性,我们在此作些说明. 读者对时间周期性函数,如简谐交流电,是较为熟悉的.同样,对空间周期性函数也应不难理解.下面我们对二者各举一例,且将对应的概念并列在一起(图3.1).简谐交流电压两光束干涉的强度分布 时间周期T　　空间周期d 时间频率　　　　　空间频率f—1/d,时间圆频率　　　　　　空间圆频率p=2πf. 对干涉场来说,空间周期d就是千涉条纹的间隔,…     

正常金属/绝缘层/正常金属/f波超导体量子点接触的微分电导（英文）

在Blonder–Tinkham–Klapwijk理论的框架下,计算了正常金属/绝缘层/正常金属(N/I/N)/f波超导体量子点接触的隧道谱.结果表明:在单模情况下,当f22 x-y波超导体的a轴与正常金属/f22 x-y波超导体间的界面夹角α为零度时,零偏压电导随绝缘层到界面的距离(L)变化,这导致了隧道谱中零偏压电导峰的出现或消失;当α=π/4,零偏压附近的电导随绝缘层势垒的增加,从零偏压电导峰逐渐变成了V型结构,能隙内的谱线出现尖谷,并随L的变化发生谷裂.N/I/N/f波超导体量子点接触隧道谱的这些特征与N/I/N/d波和N/I/N/p波超导体量子点接触隧道谱的特征不同,利用这些特征,我们能把f波超导体与d波和p波超导体区别开.     

纳米多晶铜微观结构的分子动力学模拟

基于EAM镶嵌原子势函数,采用分子动力学模拟了零温下纳米多晶铜的微观结构.首先用Voronoi几何方法构造了5个纳米多晶铜数值模型,在300K弛豫50ps并退火至0K.然后分析零温下弛豫模型的径向分布函数、原子能量、配位数、原子Voronoi体积、以及本征应力分布 关键词： 纳米多晶铜 微观结构 分子动力学     

The Boundary Condition in the Classical Derivation of the BBR

The classical derivation of the black body radiation (BBR) spectrum by Boyer was based on an equilibrium mechanism such that in the absence of thermal radiation particles in a container can gain kinetic energy from the random electromagnetic zero point field (ZPF) radiation. Their loss of that energy was to be by means of their collisions with the walls of the container. Theoretically, energy dissipation through collisions with the walls might lead to a divergent kinetic energy value for the particles. This is because the box can be taken large enough to minimize the collisions probability, and that can lead to a particle’s indefinite growth in energy. Furthermore, a derivation of a general phenomenon such as the BBR should be possible without relying on the walls boundary of a box. Therefore, a new boundary condition is proposed here which is related to relativistic effects. It is shown that with the new boundary condition one may still recover the BBR spectrum. A discussion is presented that shows how the new boundary condition is indeed responsible for energy dissipations.     

水在氢气、氧气转变时的自由能的变化及水的汽化潜热分析

水和氢气、氧气在同等条件下(一个大气压,温度为25度)互相转变时自由能是相同的;同等条件下水变水蒸气时的汽化潜热和水蒸气变为水时的凝结热是相同的。利用一个刚性的绝热的容器,在海拔零米处电解水,利用氢气和氧气的混合密度小于空气密度的特点,携带一定质量的物体到数千米的高空。点燃混合气体,生成高温水蒸气。通过这个过程分析,将发现两部分能量多出。     

Relaxation of ideal classical particles in a one‐dimensional box

We study the deterministic dynamics of non‐interacting classical gas particles confined to a one‐dimensional box as a pedagogical toy model for the relaxation of the Boltzmann distribution towards equilibrium. Hard container walls alone induce a uniform distribution of the gas particles at large times. For the relaxation of the velocity distribution we model the dynamical walls by independent scatterers. The Markov property guarantees a stationary but not necessarily thermal velocity distribution for the gas particles at large times. We identify the conditions for physical walls where the stationary velocity distribution is the Maxwell distribution. For our numerical simulation we represent the wall particles by independent harmonic oscillators. The corresponding dynamical map for oscillators with a fixed phase (Fermi–Ulam accelerator) is chaotic for mesoscopic box dimensions.     

Tellurium recrystallization under microgravity conditions and the resulting properties of samples

Three experiments on the tellurium recrystallization by a modified Bridgman method were performed under microgravity conditions on board the Mir orbital space laboratory using a ChSK-1 Kristallizator furnace. The physical properties of samples were studied, including the final crystal structure, the distribution of impurities and defects, and the charge carrier concentration and mobility. The results were compared to the analogous parameters of crystals remelted using the same method under the normal gravity conditions. It is established that the samples recrystallized in a close volume under the on-board microgravity conditions “break off” from the container walls and touch the walls only in a few points. This circumstance gives rise to special effects, such as the growth of crystals with a free surface and deep supercooling. Study of the distribution of electrically active impurities over the length of ingots shows evidence of the presence of thermocapillary convective flows in the melt under the microgravity conditions. The flows tend to increase upon separation of the melt from the container walls. The contributions due to impurities and electrically active structural defects to the charge carrier distribution are taken into account. The single-crystal sample obtained upon the partial recrystallization of tellurium in a close container volume under the on-board microgravity conditions exhibits the electrical characteristics comparable to those of a crystal grown by the Czochralski technique under the normal gravity conditions.     

Speed-Dependent Weighting of the Maxwellian Distribution in Rarefied Gases: A Second-Law Paradox?

We show that the velocity distribution in rarefied (i.e., Knudsen) gases is spontaneously weighted in favor of small speeds away from the Maxwellian distribution corresponding to the temperature of the container walls—despite thermodynamic equilibrium with the walls. The consequent paradox concerning the second law of thermodynamics is discussed.     

A joint acceptance function for structural-acoustical coupling problems

In estimating the extent of coupling between the normal modes of an acoustic cavity and the mechanical modes of vibration of the enclosing walls, a joint acceptance function is introduced which enables the modal geometries of the cavity and the structure to be compared. On a scale between zero and unity this function gives a measure of the coupling for any given surface velocity distribution, measurement position and frequency.     

MRI Comparative Study of Container Geometry Impact on the PMMA Spheres Sedimentation

Nuclear magnetic resonance (NMR) imaging results are presented for the comparative study of sedimentation of the polymethylmethacrylate colloidal suspensions with spherical particle diameters of 475, 350 and 255 nm. The time evolution of the particle volume fraction in the sedimenting system, velocity of the fluid/suspension interface, interface broadening, and sediment growth velocity are measured against the system concentration as well as the container geometry. Using the experimental data the hindered settling function is evaluated. The influence of the container geometry on the parameters defining the hindered settling function in different approaches is evaluated for the first time. The limiting value of the particle diameter is estimated at which the container geometry has no impact any longer. This effect can be explained by the microscale structure persisting at low Peclet numbers. In addition, the influence of the container geometry on interface broadening and sediment formation for different particle diameters and volume fractions is studied. Spontaneous sediment packing induced by a modified container geometry has been found. NMR imaging has proven to be a highly efficient research tool for studying sedimentation at low Peclet numbers.     

圆芯边孔光纤的应力区和双折射

报道了圆芯边孔光纤(CSF)中应力分布和双折射的研究结果。用有限元法建立了求解圆芯边孔光纤横截面应力和双折射的计算模型,分析了圆芯边孔光纤横截面应力和应力双折射的分布形态,给出了详尽的物理解释。计算结果表明圆芯边孔光纤的横截面上出现应力分量的拉应力区和压应力区,应力区在纤芯附近对称分布,且不同应力区中应力双折射取向不同。应力分量和应力双折射在圆芯边孔光纤横截面上积分为零。圆芯边孔光纤的几何双折射随波长增加而增大,短波长处几何双折射为零,模式双折射等于应力双折射,随着波长增加,模式双折射逐渐偏离纤芯中心处应力双折射而靠近几何双折射,到长波长处模式双折射主要是几何双折射。对于不同材料组成和结构尺寸的圆芯边孔光纤,模式双折射的数值在10-5量级,随波长增加模式双折射先减小后增加。     

球差透镜聚焦部分相干光束获得局域空心光束

基于Collins公式,研究了空间相干度为零阶贝塞耳函数的部分相干光通过球差透镜后在几何焦平面附近的光强分布情况.研究表明,通过改变部分相干光的光谱相干度,球差系数可以得到平顶光束和局域空心光束.球差系数决定了最佳聚焦点的位置.当球差系数为正时,最佳聚焦点随着球差系数的增大向远离透镜的方向运动,甚至超过几何焦点;当球差系数为负时,最佳聚焦点随着球差系数绝对值的增大向靠近透镜方向移动.另外,在正负球差绝对值相等时,光强分布以几何焦点相对称.     

The stability of a quantized vortex in a dilute Bose gas confined to a toroidal trap

We investigate the stability of a quantized vortex in a weakly interacting Bose gas, trapped in a toroidal container with hard walls. Calculating the excitation spectrum numerically and determining the stability condition by the Landau criterion, we examine the effect of reducing the confinement region of the condensate on the vortex stability. We find that tight confinement of the condensate increases the stabilization of the quantized vortex because an increase in the zero sound velocity due to tight confinement prevents the emergence of the elementary excitation which breaks superfluidity of the Bose system. We also discuss the experimental setup to observe such an effect.