使用滑动变阻器易疏漏的一点

滑动变阻器是中学物理实验中常用的仪器。使用时,有些人经常让电阻值处于最大状态,觉得这样对用电器可起到安全保护作用,其实不然。现举两例加以说明:     

读者来信

编辑老师:在为参加自学考试的学员辅导时,我有幸使用了丁俊华、祁有龙二位老师主编的教材《物理(Ⅰ)》[1] .这本书作为全国高等教育自学考试(公共课程)的指定教材,发行量很大,在全国参加高教自考的学生中很有影响.总的说来,这本书是适合自考学生的好教材,但也有一些不足之处.如在第十章电磁振荡和电磁波§1.1无阻尼自由振荡(LC电路)小节中还存在一些错误.具体有以下3点.1)该小节所用电路图如图1所示.电流I(t)的图1参考方向从电容正极板流出,I(t)与电容器电量q之间的关系应为I(t) =- dqdt而该书在下面的推导:“q(t) =C Ua-Ub电感上有电动…     

用能量观点解决纯电阻电路中电流和电压的分配问题

1引言 在直流电路的教学中,对串联电路的电压分配、并联电路的电流分配,以往是根据欧姆定律和电流、电压实验结果得到的.一个直流电路由电阻R1、R2和电源组成.     

误差理论教学中常被忽视的问题

引言本文指出了目前误差教学中常被忽视,但相当重要的问题:误差点的概念和同构方程组中的变量消去问题。另外,还提出了较粗略但很实用的误差分析方法——有效数字位数跟踪法,可建议学生将该方法用于实验误差分析和表达之中。     

阻尼和无阻尼振荡全真波形演示仪的研制

用合适的磁电式自动转换开关和电子式自动转换开关控制LC振荡回路在示波器荧屏上产生稳定的阻尼振荡中无阻尼振荡波形，配合电磁振荡教学演示．     

为何见不到B灯先亮？

1问题的提出 在电路的自感现象教学中，我们遇到了这样一个问题。     

对一个电路学习的例子的看法

<物理通报>2001年第10期刊登了美国莉莲·麦克德莫特的文章<改造学生学习的关键>一文,文中的一些观点,对指导我们的物理教学和学习以及物理教育研究,都具有十分重要的意义.但笔者认为,文中关于一个电路学习的例子的答案是值得商榷的.     

一个出人意料的实验结论

这是最近我在一个物理实验中得到的结论.因为有一类物理习题的答案和对答案的解释与这个实验结论不统一,我们必须重新讨论、认识这个问题.那么,这是一个什么样的结论呢?     

[Russian Text Ignored]

The stability of the electromagnetic plasma confinement by powerful external s-polarized pump waves is considered. The parametric excitation of standing electromagnetic waves along the plasma boundary with frequencies close to the frequency of the pump wave leads to a periodic density modulation of the plasma boundary. The density disturbances along the direction of the external wave field are connected to the excitation of transverse p-polarized surface waves while the modulation in the direction perpendicular to the pump field are created by the parametric interaction between the external wave and s-polarized trapped leaking oscillations. Only when the leaking waves are excited the scale length of the modulation is larger than half the free space wave length of the incident radiation.     

[Russian Text Ignored]

The parametric excitation of longitudinal waves in an infinite homogeneous plasma by a pump field E (t) = E ₀(t) sin (ω₀t + φ(t)) is studied on the basis of the Vlasov equation, where the amplitude E ₀(t) and the phase φ(t) are slowly varying compared with ω₀ periodic functions. Firstly it is assumed that ω₀ is much larger than the electron plasma frequency ω_Le. In the second part the parametric instabilities are considered under the resonance condition ω₀ ≈ ω_Le. In both parts the threshold fields for the excitation of the longitudinal waves and their growth rates are calculated. As an example these values are analysed for both a sequence of pump impulses and a phase-moduated pump field. They are compared with the results received for a monochromatic pump field.     

[Russian Text Ignored]

The plasmon turbulence parametrically excited by two strong external waves with a frequency difference close to twice the electron plasma frequency is investigated in underdense plasmas. The nonlinear stabilization of the growing plasmons due to scattering by ions is described by a nonlinear integro-differential equation for the spectral plasmon energy density. The dependence of the turbulent relaxation of the plasmon energy and its stationary level on the parameters of the plasma and the pump waves is given by analytical and numerical solutions of this equation.     

[Russian Text Ignored]

A nonlinear theory of the interaction of an external pump field with ion-acoustic and h.f. surface waves in a plasma layer with inhomogeneous boundary regions is presented for the case, when the pump field frequency is less than the electron plasma frequency. The evolution of the parametrically excited turbulence and its stationary state are both analytically treated and numerically demonstrated. Dependent on the pump field strength, two mechanisms are to be distinguished: (i) nonlinear generation of damped ion-acoustic harmonics, (ii) cascade excitation of secondary eigenmodes by growing surface oscillations. At only slight overthreshold values, mechanism (i) is responsible for the occurence of the steady state, whereas, for higher pump field strength, the wave amplitudes as well as their saturated levels are mainly determined by mechanism (ii).     

[Russian Text Ignored]

A theory of the magneto-acoustic radiation from an upper-hybrid soliton moving in an inhomogeneous magnetoactive plasma is presented. The derived expression for the spatial distribution of the radiation field is analysed for a linear and two different parabolic density profiles. The energy flux density connected with the transversal part of the magneto-acoustic waves is investigated.     

[Russian Text Ignored]

Some aspects of the theory of fluctuating microfield in a system of charged particles are considered. The question concerning regions of validity for Holtsmark-like and Gauss-like microfield distributions in a rarefied and a dense plasma is discussed. The influence of finite dimensions of particles (test particles as well as field particles) on the microfield distribution is investigated. Expressions for a “dynamic” (two-time) generalization of the Holtsmark distribution are obtained for limiting cases. By methods of microfield theory certain formulas for the fluctuations of the electric charges within a finite volume of an electrolyte are derived too.     

[Russian Text Ignored.]

An investigation of the heterogeneous low temperature plasma ionization is stimulated by practical needs. Earlier theoretical studies of the ionization process are connected with many difficulties and realized by simple models. In this paper a solution of the kinetic equations of the electron-ion process and the Poisson equation for a self-consistent electrostatic potential is presented. A pecularity of the volume density determination of the electron-ion atmosphere around a solid particle, the connection between the integral and local density of charged particles and the own particle volume are taken into consideration. The influence of characteristic parameters of the condensed phase on the equilibrium electron density is received.     

[Russian Text Ignored].

Using the drift kinetic equation the influence of the ballooning component of the perturbed transit ion pressure on the low-frequency Alfvéen waves in a toroidal plasma is considered. It is shown that this contribution to the plasma pressure leads in a relatively cold plasma (plasma edge) to a stabilization of the Alfvéen modes. Taking into account drift effects the modified Alfvéen wave becomes unstable if the temperature gradient is large enough. All the perturbations are strongly localized around the corresponding rational magnetic surface.