从一道上海高考题谈起——通过单摆测万有引力常量G

从2021年上海市一道物理高考题说起,简单叙述通过单摆测量万有引力常量G的历史.从最早17世纪牛顿提出想法,接着18世纪的榭赫伦实验,到最近的双单摆F-P腔法,简略向读者介绍单摆测G的发展历程.     

万有引力常数G精确测量实验进展

万有引力常数G是人类历史上引入的第一个基本物理学常数,其在理论物理、天体物理和地球物理等许多领域中扮演着重要角色.两百多年来,人们共测量出了200多个G值,但G的测量精度仍然是所有物理学常数中最差的,这一现象反映了测G工作本身的复杂性和困难性.本文简要概述了G值测量的意义和测G的历史,并结合自2010年以来国际上新出现的三个高精度测G实验介绍这一领域的研究进展,以及华中科技大学引力实验中心测G工作的最新动态.     

物理奥林匹克竞赛实验专题

第 7期试题解答注意 ,这里切不可将重力加速度当成常量 .相反 ,它是一个待测的物理量 .由于地球表面物体 (质量为 m)所受的重力近似等于它与地球之间的引力mg=Gm MR2式中 G为万有引力常量 ,M为地球质量 ,R为所要测定的距离 .显然 ,重力加速度 g不能视为常量 ,而是要通过测量予以确定 .质量为零的弹簧在砝码作用下沿竖直方向作简谐振动的公式为T=2 π mk式中 k为弹簧的劲度系数 .改变砝码质量 ,并测量振动周期 ,通过 T2 -m图的斜率可以求出 k.由胡克定律可得f =mg=kx式中 x为弹簧在砝码作用下的位移 .改变砝码质量并测量相应的位移 ,因 k…     

物理学中的常量

 物理学中有很多重要的常量,如万有引力常量G、基本电荷量e、普朗克常量h、光速c等,它们都是具有特征性的普适常量,当方程中出现某一常量时,该方程一定与某一特定的物理学内容相联系。例如出现G时表示方程与万有引力有关;出现e时表示与电学有关;出现c时表示与相对论有关;出现h时表示与量子论有关。这些常量的出现以及测定它们数值的过程,往往是漫长而曲折的,与此相应的是物理学理论的发展和完善。本文试图从历史的角度考察这一过程,重温物理学先驱们的伟大思想历程,并从中受到启发。万有引力常量G万有引力定律是牛顿对人类最伟大的贡献之一。     

激光干涉法测引力常量

在扭摆法测量引力常量的基础上,提出利用激光干涉法测扭转偏角,并采用磁感应力来消减周围环境的噪声干扰,能使引力常量G的测量精度有很大提高.     

利用三次样条函数绘制光电管的I-U特性曲线

在光电效应法测普朗克常量实验中需要人工绘制不同频率下光电管的I-U特性曲线,并从曲线上确定截止电压,获取有关截止电压与入射频率的关系,计算出普朗克常量.针对该实验中手动绘制I-U特性曲线过程繁琐、精确度低、容易出错的问题,阐述了在计算机上如何利用三次样条函数数值模拟特性曲线的原理和方法.     

改进测电阻方法

1测量电阻器的电阻Rx 常用电流表的内接法或外接法伏安法测Rx.由于忽略RV、RA的影响,实验测量存在系统误差.减小误差测法如图1、图2所示.     

角度传感器在分光计实验中的应用

用精密导电塑料电位器代替分光计的游标刻度盘,可以实现用电测法来测量角度.定标曲线的相关系数r=0.999 9,说明精密导电塑料电位器的线性度很好,输出电压和角度变化有很好的线性关系.从实验测量结果来看,由刻度盘测量的三棱镜顶角、最小偏向角和光栅的光栅常量与用精密导电塑料电位器测出的对应值,两者的相对偏差小于1%,可满足实验要求.     

驻波法测声速时两种数据处理方法的对比研究

驻波法是常用的声速测量方法. 驻波法测声速时常用逐差法进行数据处理, 也有采用最小二乘法的, 为 了比较两种方法的优劣, 本文对驻波法测声速分别采用逐差法和最小二乘法处理数据, 并对测量结果的不确定度进 行了分析. 通过数据处理结果发现, 逐差法和最小二乘法在驻波法测声速的数据处理上是等效的, 都能得到良好的 效果     

测量物质密度的两种特殊方法

测量物质密度的常规方法有:一是对规则固体而言,用测长工具测体积,用天平称质量,再根据密度的定义求得;二是对不规则的固体用流体静力法进行测量,流体静力法对液体的密度测量同样适用;三是用比重瓶法测液体和固体的密度.而根据液体内部压强特点利用U型连通器或W型连通器测液体的密度和根据浮力的原理利用悬浮法测固体的密度的方法则不多见,而这种构思新颖、设计巧妙的方法能解决一些特殊物质的密度的测量.下面详细介绍这两种测密度的方法.     

A modified moiré interferometer for three-dimensional displacement measurement

This paper presents a new optical interferometric system, MMI-T/G, composed of a modified four-beam moire interferometer and a Twyman/Green interferometer. The MMI-T/G system can measure threedimensional displacement fringe patterns with a single loading on the specimen, and the in-plane and out-of-plane displacement fields can be measured independently and defined clearly. The optical setup has the advantages of structural novelty, flexibility, and high fringe contrast. Moreover, the in-plane displacement sensitivity is twice of that of the normal moire interferometer. The measuring techniques to obtain the fringe patterns and displacement fields using the MMI-T/G system axe described. The experimental results of thermal displacement of an electronic device are shown.     

Experimental growth law for bubbles in a moderately "wet" 3D liquid foam

We used x-ray tomography to characterize the geometry of all bubbles in a liquid foam of average liquid fraction phi(l) approximately 17% and to follow their evolution, measuring the normalized growth rate G=V(-1/3) dV/dt for 7000 bubbles. While G does not depend only on the number of faces of a bubble, its average over f-faced bubbles scales as G(f) approximately f - f(0) for large f's at all times. We discuss the dispersion of G and the influence of V and phi(l) on G.     

G(E(p))/G(M(p)) ratio by polarization transfer in e-->p --> ep-->

The ratio of the proton's elastic electromagnetic form factors, G(E(p))/G(M(p)), was obtained by measuring P(t) and P(l), the transverse and the longitudinal recoil proton polarization, respectively. For elastic e-->p-->ep-->, G(E(p))/G(M(p)) is proportional to P(t)/P(l). Simultaneous measurement of P(t) and P(l) in a polarimeter provides good control of the systematic uncertainty. The results for the ratio G(E(p))/G(M(p)) show a systematic decrease as Q2 increases from 0.5 to 3.5 GeV2, indicating for the first time a definite difference in the spatial distribution of charge and magnetization currents in the proton.     

光束波前畸变的契伯格-山克斯顿测量方法探讨

利用数值模拟对契伯格－山克斯顿（Gerchberg-Saxton）方法用于湍流大气中激光传输波前的探测进行了研究,分析了其误差特性,并将该方法与哈特曼－夏克波前传感器的测量特征进行了比较。     

Upgrade of the magnetic diagnostic system for restart of HT-6M operation

The HT-6M tokamak at the Thailand Institute of Nuclear Technology has been restarted. In order to ensure the smooth breakdown of plasma and obtain plasma discharge parameters, optimization of the poloidal field coils and upgrade of the magnetic diagnostics are described in this article. A perfect null field (stray field in the main chamber < 10 G) is obtained using an ohmic heating field. To obtain important information about the plasma, an external magnetic diagnostics system is designed and calibrated, including a Rogowski coil (measuring plasma current), a magnetic probe (measuring external field), diamagnetic loops (measuring β_p) and so on. In order to realize high-frequency signal measurement and transmission, a series of frequency responses with the magnetic probe and transmission line are tested. Later, to verify the null field, a fitting code is developed to reconstruct the stray field in the vacuum chamber based on magnetic probe measurements and flux loops. The results show that the error is within 1.5%. This indicates the accuracy of the magnetic measurement system and ensures the preparation for the breakdown of plasma.     

Raman spectroscopic evidence for hot-phonon generation in electrically biased carbon nanotubes

We provide spectroscopic evidence for hot-phonon generation in biased single-walled carbon nanotubes by measuring simultaneously the Stokes and anti-Stokes Raman lines of the G mode and of the radial-breathing mode as a function of current bias. Using Bose-Einstein statistics we can directly calculate the phonon temperature from the intensity ratio of the anti-Stokes to Stokes lines. Upon nanotube biasing we observe (i) an increase of the G mode phonon temperature in contrast to the radial-breathing mode phonons that remain thermalized at room temperature, and (ii) no softening of the G mode. Based on these observations, we exclude current-induced thermal heating of the nanotube.     

Surface-enhanced resonance Raman scattering spectroscopy of single R6G molecules

Surface-enhanced resonance Raman scattering (SERRS) of Rhodamine 6G (R6G) adsorbed on colloidal silver clusters has been studied. Based on the great enhancement of the Raman signal and the quench of the fluorescence, the SERRS spectra of R6G were recorded for the samples of dye colloidal solution with different concentrations. Spectral inhomogeneity behaviours from single molecules in the dried sample films were observed with complementary evidences, such as spectral polarization, spectral diffusion, intensity fluctuation of vibrational lines and even ``breathing' of the molecules. Sequential spectra observed from a liquid sample with an average of 0.3 dye molecules in the probed volume exhibited the expected Poisson distribution for actually measuring 0, 1 or 2 molecules. Difference between the SERRS spectra of R6G excited by linearly and circularly polarized light were experimentally measured.     

Feasibility of diffusion-NMR surface-to-volume measurements tested by calculations and computer simulations

It has been demonstrated previously that the surface-to-volume ratio S/V can be determined from the derivative of the time-dependent diffusion coefficient D(t), in the limit t --> 0. Several questions arise concerning the practicality of determining S/V by NMR. In particular, how large are the errors generated by (1) working outside the t --> 0 limit and (2) measuring D outside the b --> 0 limit, both for narrow and full-width gradient pulses? Here b is gamma2G2delta2Delta for narrow pulses and gamma2G2t3/12 for broad pulses. These questions are addressed by random-walk computer simulations and numerical calculations in geometries relevant to small-airways of lung. The results demonstrate that one can work well outside the t --> 0 and b --> 0 limits, provided 10-20% accuracy in the measured S/V is sufficient. Emphasis is placed on the useful range of times t for which NMR determinations of lung S/V are feasible.     

MEASUREMENT OF LOW MAGNETIC FIELD DISTRIBUTION OF A β SPECTROMETER

The low magnetic field distribution of an iron-core double-focusing low-energy β spectrometer is measured with a second harmonic magnetic flux gate magnetometer, the relative measuring precision is better than ±0.05%. The field strength of the spectrometer is about 10G. This spectrometer is used to measure the β spectrum from tritium β decay for estimating the rest mass of electron antineutrion.     

Permeability of R6G across Cx43 hemichannels through a novel combination of patch clamp and surface enhanced Raman spectroscopy

We have measured the permeability of rhodamine-6G across Cx43 hemichannels reconstituted on a pipette tip. Cx43 hemichannels were overexpressed in Sf9 cells, and affinity-purified. The hemichannels were reconstituted in a lipid bilayer on a pipette tip by the tip-dip method. R6G in the pipette permeated across the channels into the bath. The permeability of R6G was quantified by measuring R6G concentration in the bath after several hours by surface enhanced Raman spectroscopy (SERS) with 100 nm silver colloid particles. The ratio of the permeability of dye to salt, as extracted by this combined electrical-SERS technique, is compatible with similar ratios for other dyes across whole gap junction channels. The results for the permeability ratio were further compared to fluorescence measurements. The novel combination of patch and SERS techniques can be extended to quantifying the transport of biologically significant non-fluorescent molecules, such as cAMP and IP3, across 1 nm sized pores, such as the gap junction channel.