静止电磁场的角动量

并不是每个学生都会爽快地相信《费曼物理学》第二卷17.4[1]中的佯谬可以用初始就存在的静止电磁场角动量来解释.这里p是动量密度,它正比于坡印廷矢量[2],这一角动量在电流切断后变为0。为保证电磁角动量与机械角动量构成的总角动量守恒,塑料圆盘将开始旋转. 角动量概念是与绕轴运动相联系的.圆偏振波具有角动量似乎易于理解,因为电场与磁场均绕波传播的方向旋转,静止场具有角动量则难于理解. 下面我们来考虑一个可以绕轴自由旋转的力学系,它可借其上所带的电荷同时激发电场和磁场.它与费曼盘不是雷同的模型,但具有某些相同特性而且优点在于…     

两个同心带电球壳体系电磁场角动量的研究

电磁场的动量和角动量是经典电磁学有趣而又基本的问题。根据标准电磁理论,电磁场角动量可表示为L_(em)=ε_0∫_vr×(E×B)dV,对恒定电磁场,角动量还可以用L_(em)=∫r×ρAdV来计算,文章对此进行了分析。通过一个简单的模型,即一个固定的绝缘带电球壳处在另一个旋转的绝缘带电球壳中心,分析了该体系电磁场从建立到稳定,再到撤去的过程中,整个系统角动量的变化情况。验证了在建立稳定电磁场的过程中,外力提供的冲量矩一部分转化为电磁场角动量储存在电磁场中,一部分提供体系的转动机械角动量,当撤掉磁场后,电磁场角动量转换成机械角动量,整个过程角动量守恒。     

光子飓风——具有光子横向轨道角动量的时空涡旋

众所周知,光子具有沿传播方向的线性动量。光子的线性动量在光与物质的相互作用中具有重要的作用。光子与物质的动量交换可以产生光压。2018年获得诺贝尔物理学奖的光镊技术基于微纳米颗粒对光子动量改变产生的光场梯度力。光子也具有沿传播方向的角动量,包括与圆偏振相关的自旋角动量和与涡旋相位关联的轨道角动量。近年来,研究表明在高数值孔径聚焦光场和隐失波光场中存在垂直于光束传播方向的光子横向自旋角动量[1]。那么是否存在垂直于光束传播方向的光子横向轨道角动量呢(图1)?我们最新的研究给出了肯定的答案,成果发表于《自然—光子学》[2]。     

基于哈密顿函数的永磁同步电机混沌系统鲁棒控制

哈密顿系统理论是研究非线性系统的一种重要工具, 近年来在电机调速、控制等方面得到广泛应用. 本文针对永磁同步电机运行中存在的混沌现象, 提出一种基于哈密顿函数的永磁同步电机混沌系统鲁棒控制器设计方法. 将永磁同步电机动态模型变换为类Lorenz混沌方程, 在特定参数下, 通过Lyapunov指数和Lyapunov维数的计算可知系统是混沌的. 令电机转速跟踪给定值得误差方程. 由于误差方程并不具有标准哈密顿函数形式, 将其转化为具有扰动不确定项的哈密顿系统, 并与负载扰动一起作为系统的总扰动量, 设计了一种鲁棒控制器. 控制器由两部分组成, 一部分基于互联与阻尼配置法, 实现任意转速的有效跟踪, 另一部分实现扰动补偿. 仿真表明, 控制器使电机迅速脱离混沌状态, 并能实现转速趋近跟踪, 验证了控制器的可行性与有效性. 该方法扩展了哈密顿函数的适用范围, 具有一定的优越性.     

关于“质点角动量”学习中的几个问题

角动量是描述物体运动状态的重要物理量，初学者常会感到困难．本文就如何理解学习质点角动量知识，谈几点意见． 什么是角动量．在描述质点平动运动时，“线动量”是很有用的物理量．那么，在转动运动中与线动量相类似的物理量就是角动量．若质量为ｍ、线动量为ｍｖ的质点，相对于惯性系原点的位矢为ｖ，我们定义这质点对原点Ｏ的角动量为：Ｌ＝ｖ×ｍｖ． 角动量是矢量．从定义式中可看出，角动量是涉及矢积的一个物理量，其方向由矢积右手法则给定，即方向垂直于位失和动量决定的平面，如图１（ａ）、（ｂ）中角动量入方向的标示． 角动…     

基于轨道角动量的多自由度W态纠缠系统

提出了一种制备三光子纠缠W态的方案, 该方案利用携带轨道角动量为lħ的光子(其中l可取(-∞, +∞)的任意整数)可构成无穷维向量空间的特性, 采用两种类型的参量下转换, 产生轨道角动量-自旋角动量纠缠的两对光子和一对偏振纠缠光子, 通过纠缠交换制备三光子多自由度的W态, 实现三光子体系纠缠的高维度、大容量量子信息处理. 方案采用q-plate相位光学器件和单模光纤等器件制备两个不同自由度(轨道角动量与偏振)混合的W态, 并利用计算机全息相位图改进方案制备三个不同自由度(轨道角动量、线动量和偏振)混合的W态. 本方案可稳定产生两种等概率互为对称的W态, 具有高维度、强纠缠特性与抗比特丢失能力, 信息量达log₂^m+2比特(m为l的可取值个数), 有望实现可扩容量子比特的安全通信.     

20MeV到180MeV/u 16O+197Au系统的BUU计算:线动量与角动量转移依赖碰撞参数与能量研究

用考虑了角动量守恒的BUU模型计算了20MeV≤E/u≤180MeV能区¹⁶O+¹⁹⁷Au系统的反应线性动量转移(LMT)及余核角动量,着重讨论了反应线性动量转移及余核角动量对反应碰撞参量、入射能E/u的依赖关系,比较了计算LMT与Viola系统性给出的结果间的偏差.计算结果揭示了当E/u≥90MeV时,余核角动量对E/u增长出现的饱和现象,主要来源于靶核对弹核捕获能力的持续丢失.     

力学中的守恒定律与时空对称性

本文按照近代物理观点，应用Ｈ函数对时空变换的不变性，导出动量守恒、角动量守恒和能量守恒定律，说明三个守恒定律是时空对称性的表现，从而阐明了三个守恒定律的物理根源和本质．     

刚体力学中守恒定律的教学难点研究

我们从事工科物理教学多年,在教学过程及历年的考试、测验中,我们发现,相当多的学生在学习刚体力学的守恒定律时掌握不好.特别是在处理物体与转动刚体碰撞的问题时,常发生一些严重的错误.主要表现为:混淆角动量(动量矩)与动量的概念;混淆角动量定理、角动量守恒定律与动量定理、动量守恒定律;混淆刚体的转动动能公式 E_k=     

声子角动量与手性声子

在经典的物理学理论中,声子广泛地被认为是线极化的、不具有角动量的.最近的理论研究发现,在具有自旋声子相互作用的磁性体系(时间反演对称性破缺)中,声子可以携带非零的角动量,在零温时声子除了具有零点能以外还带有零点角动量;非零的声子角动量将会修正通过爱因斯坦-德哈斯效应测量的回磁比.在非磁性材料中,总的声子角动量为零,但是在空间反演对称性破缺的六角晶格体系中,其倒格子空间的高对称点上声子具有角动量,并具有确定的手性;三重旋转对称操作给予声子量子化的赝角动量,赝角动量的守恒将决定电子谷间散射的选择定则;此外还理论预测了谷声子霍尔效应.     

Investigation of the dynamics of angular motion and construction of algorithms for controlling the angular momentum of spacecraft using a magnetic attitude control system

The problem of controlling the angular momentum of spacecraft using magnetic attitude control systems interacting with the Earth’s magnetic field is considered. A mathematical model for the angular motion dynamics of a spacecraft has been constructed. An approach to determining the parameters of the control law for a spacecraft attitude control and stabilization system that ensures angular momentum dissipation is proposed.     

Mechanical equivalence of spin and orbital angular momentum of light: an optical spanner

We use a Laguerre-Gaussian laser mode within an optical tweezers arrangement to demonstrate the transfer of the orbital angular momentum of a laser mode to a trapped particle. The particle is optically confined in three dimensions and can be made to rotate; thus the apparatus is an optical spanner. We show that the spin angular momentum of +/-?per photon associated with circularly polarized light can add to, or subtract from, the orbital angular momentum to give a total angular momentum. The observed cancellation of the spin and orbital angular momentum shows that, as predicted, a Laguerre-Gaussian mode with an azimuthal mode index l=1 has a well-defined orbital angular momentum corresponding to ? per photon.     

Passive vibration isolation of reaction wheel disturbances using a low frequency flexible space platform

Reaction wheel assemblies (RWAs) are momentum exchange devices used in fine pointing control of spacecrafts. Even though the spinning rotor of the reaction wheel is precisely balanced to minimize emitted vibration due to static and dynamic imbalances, precision instrument payloads placed in the neighborhood can always be severely impacted by residual vibration forces emitted by reaction wheel assemblies. The reduction of the vibration level at sensitive payloads can be achieved by placing the RWA on appropriate mountings. A low frequency flexible space platform consisting of folded continuous beams has been designed to serve as a mount for isolating a disturbance source in precision payloads equipped spacecrafts.Analytical and experimental investigations have been carried out to test the usefulness of the low frequency flexible platform as a vibration isolator for RWAs. Measurements and tests have been conducted at varying wheel speeds, to quantify and characterize the amount of isolation obtained from the reaction wheel generated vibration. These tests are further extended to other variants of similar design in order to bring out the best isolation for given disturbance loads. Both time and frequency domain analysis of test data show that the flexible beam platform as a mount for reaction wheels is quite effective and can be used in spacecrafts for passive vibration control.     

Rotation of transparent, nonbirefringent objects by transfer of the spin angular momentum of light

The transfer of the spin angular momentum of light to small objects has been discussed only for absorbing or birefringent objects. However, we show that it can also be transferred to objects that are nonabsorbing and nonbirefringent. Our estimate of the magnitude of the spin angular momentum transferred to a transparent, nonbirefringent sodium bromate crystal predicts rotational speeds in reasonable agreement with experimental results.     

Intrinsic and extrinsic nature of the orbital angular momentum of a light beam

We explain that, unlike the spin angular momentum of a light beam which is always intrinsic, the orbital angular momentum may be either extrinsic or intrinsic. Numerical calculations of both spin and orbital angular momentum are confirmed by means of experiments with particles trapped off axis in optical tweezers, where the size of the particle means it interacts with only a fraction of the beam profile. Orbital angular momentum is intrinsic only when the interaction with matter is about an axis where there is no net transverse momentum.     

Calculation of gluon contribution to the proton spin by using the non-perturbative quantization à la Heisenberg

The contribution of crossed gluon fields in flux tubes connecting quarks to the proton spin is calculated. The calculations are performed following non-perturbative Heisenberg’s quantization technique. In our approach a proton is considered as consisting of three quarks connected by three flux tubes. The flux tubes contain colour longitudinal electric and transversal electric and magnetic fields. The transversal fields causes the appearance of the angular momentum density. The dimensionless relation between the angular momentum and the mass of the gluon fields is obtained. The contribution to proton spin from rotating quarks and flux tubes connecting quarks is estimated. Simple numerical relation between the proton mass, the speed of light and the proton radius, which is of the same order as the Planck constant, is discussed.     

Modeling the Angular Momentum Control of Small Spacecraft with a Magnetic Attitude Control System

Doklady Physics - The problem of modeling the angular momentum control modes of small spacecraft using electromagnetic systems interacting with the Earth’s magnetic field is considered. The...     

Spin-to-orbital angular momentum conversion in spin Hall effect of light

From the viewpoint of classical electrodynamics, we identify the role of spin-to-orbital angular momentum conversion in spin Hall effect (SHE) of light. We introduce a distinct separation between spin and orbital angular momenta to clarify the spin–orbital interaction in conventional beam refraction. We demonstrate that the refractive index gradient can enhance or suppress the spin-to-orbital angular momentum conversion, and thus can control the SHE of light. We suggest that the metamaterial whose refractive index can be tailored arbitrarily may become a good candidate for amplifying or eliminating the SHE of light, and by properly facilitating the spin-to-orbital angular momentum conversion the SHE may be enhanced dramatically. The transverse spatial shifts governed by the spin-to-orbital angular momentum conversion, provide us a clear physical picture to clarify the role of refractive index gradient in the SHE of light. These findings provide a pathway for modulating the SHE of light and can be extrapolated to other physical systems.     

杠杆式回转仪定量演示方法研究

在对杠杆式回转仪运动现象进行理论分析的基础上,通过测量出圆轮自转角速度,旋转轴进动角速度、进动角度的参数,定量研究了力矩与进动速度间关系、角速度与进动速度间关系、角动量定理的实验验证,最后将测量值与理论计算值进行了比较。将杠杆式回转仪运动状态的定量分析引入大学物理实验,培养了学生实际动手能力,加深了学生对角动量定理,力...