College Physics

Connective problems of the wave equation at interfaces

YANG Shi-jie

College Physics. 2023, 42(11): 1. doi:10.16854/j.cnki.1000-0712.230051

Abstract ( 474 )

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The paper investigates eigen modes of the wave equation in discontinuous media and explores the underlying physical mechanism. We also study the reflection and transmission of wave at the interface and reveal a full transmission effect under specific physical conditions. This effect may find applications in wave-absorptive materials.

Complex relationships of angular momentum of rotating rigid-bodies ——Discussing the significance of choosing the right reference frame

DONG Ke, ZHOU Yu-qing,

College Physics. 2023, 42(11): 5. doi:10.16854/j.cnki.1000-0712.230023

Abstract ( 520 )

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A seemingly simple rigid-gyro-motion problem involves an overlooked aspect in university physics teaching, namely, the calculation of angular momentum when rigid bodies rotate simultaneously around mutually perpendicular axes. Starting from the solution of question, this paper elaborates the complex relationship of angular momentum of rigid-body (gyro) rotation, and derives the correct angular momentum transformation relationship of simple and constrained gyroscope. At the same time, it shows that, this kind of problems can be greatly simplified when selecting the appropriate reference frame.

The comprehensive and unified analysis of the general Doppler effect

ZHOU Qiang, LIU Jing, LI Peng

College Physics. 2023, 42(11): 8. doi:10.16854/j.cnki.1000-0712.230047

Abstract ( 332 )

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The Doppler effect formulae of sound wave and light wave are given respectively in physics textbooks, but the two expressions are different. Some literatures analyze the connection between the two, but it does not really unify the two; although other literatures are uniformly analyzed, but they are all discussed based on the special Lorentz transformation, and the results are not general. In this article, without distinguishing between sound wave and light wave, based on the common essence of wave propagation, the expressions of the general Doppler effect are derived by multiple methods to systematically demonstrate the unity of the two. Though analysis, three equivalent formulae of the general Doppler effect are obtained, and the Doppler effect formulae of sound wave and light wave in physics textbooks are special cases of different situations.

Sublimation from point charge to field—reflections on the teaching of electromagnetism in college physics

QIN Ji-hong, QIU Hong-mei, XU Jun-jun, LIANG Ying

College Physics. 2023, 42(11): 14. doi:10.16854/j.cnki.1000-0712.230021

Abstract ( 524 )

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Electromagnetics is one of the important contents of physics courses in engineering colleges. Many abstract formulas such as Maxwell’s equations in electromagnetism are difficult for freshmen who are new to college physics. In this paper, the core content of “field” is revolved around and the entire electromagnetism content runs through via the clue of “experimental laws → properties of the field → interaction between the field and matter”. It is discussed how to realize the sublimation from point charge to field, and finally reach a unified theory of the field. Taking a moving point charge with variable speed as an example, it is illustrated how the solution of Maxwell’s equations under the field description exceeds Gauss theorem of electrostatic field. Guided by problems, students are directed to gradually understand the core content of electromagnetism by exploration and thinking.

Mapping the electrostatic field based on the pen mouse and the microcrystalline conductive plate

CAO Yan-peng, ZHANG Zhi-lan, BAO Zhi-gang, MA Qiao-yun, ZHOU Yan

College Physics. 2023, 42(11): 18. doi:10.16854/j.cnki.1000-0712.230038

Abstract ( 292 )

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Electrostatic field mapping is an important part of university physics experiment, but the electrostatic field mapping instrument used has disadvantages such as large inherent error, low efficiency of manual point finding and vulnerable data processing. The mouse and microcrystal conductive plate were designed and developed to map the electrostatic field. During the experiment, the pen mouse and Geogebra software are used to locate and record the coordinates of the medium potential point of the electrostatic field, and origin data is used to process and visualize the electrostatic field. This design reduces the system error, improves the efficiency of finding points, and improves the operability of the experiment; uses software to process the experimental data, avoids the error caused by hand-drawn isopotential lines, and increases the rigor of the experiment.

Device design for measuring the concentration of transparent liquid based on total reflection

CUI Jing-ying, HAN Wen-qi, XIN Shuo, XIE Zhen, LI Hua-yu, WANG Kai-ge

College Physics. 2023, 42(11): 23. doi:10.16854/j.cnki.1000-0712.230153

Abstract ( 453 )

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An experimental device for measuring the concentration of transparent liquids is designed based on optical principles and total reflection. The liquid to be measured is contained in an acrylic plate container, which is used to study the optical path parameters when total reflection generates in the liquid. The refractive index of the transparent liquid is calculated by measuring two length parameters. Its concentration is obtained by using linear relationship between the refractive index and the concentration of transparent liquid. The measured data of sucrose solution and NaCl solution show that the relative error of refractive index is within 0.4%, and the absolute error of concentration is within 0.7%. This experimental device has the advantages of simple structure, simple operation, high measurement accuracy, low cost and obvious phenomenon.

Study on thermal conductivity of ice by steady-state method

HUANG Jia-hao, WANG Ying-lian, LIAO Xu-hui, LEI Ting, WAN Ye-ran, LAI Wen-biao

College Physics. 2023, 42(11): 28. doi:10.16854 /j.cnki.1000-0712.220588

Abstract ( 466 )

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There are many methods to measure thermal conductivity of solid, but not many of them can be directly applied to ice measurements. In this paper, considering the problems related to sub-zero low temperature environment, and based on the steady-state method and Fourier law of thermal conductivity, it is attempted to design and enhance same concerned details by independent constructing the experiment equipment. The relationships between thermal conductivity and the ambient temperature, the impurity concentration (salt content) and the ratio of diameter to thickness of ice are studied. The result shows that when the ratio of diameter to thickness of ice is 12, the thermal conductivity tends to a best value of 2.25 w/m·K and the best ambient temperature is -10 ℃. When comparing with that of pure water ice, the thermal conductivity of salt water ice decreases significantly, and its value decreases gradually and becomes stable with the increase of salt content.

An equivalent solution to the internal magnetic field of a uniformly magnetized sphere from the point of view of current element

CAO Xiu-qi, SHAO Han-yong

College Physics. 2023, 42(11): 32. doi:10.16854/j.cnki.1000-0712.230083

Abstract ( 542 )

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A typical problem related to magnetic medium is to calculate the magnetic field distribution in a uniformly magnetized sphere, that is, to solve the distribution of magnetic induction B in a sphere with radius R and uniform magnetization M in the space. If the surface or volume of the sphere is taken as a microelement, it will be difficult to analytically solve the expression of the magnetic field and the upper and lower limit of the integral at a point in the sphere, but the traditional method for solving the magnetic charge often lacks a strict demonstration of the equivalence of the magnetic charge. Therefore, in this paper, under the viewpoint of current element in accordance with the essence of physics, an equivalent solution to this problem is proposed by using the idea of cut-complement and equivalent substitution, which has a good enlightening effect and reference value for the relevant teaching and research of electromagnetics.

The visualization of the quantum linear harmonic oscillator in three-dimensional space

SHI Jun-xi, FAN Xiao-xin, E Yi-ting, KONG Ling-yang, QI Xin, SHU Song

College Physics. 2023, 42(11): 35. doi:10.16854 /j.cnki.1000-0712.220612

Abstract ( 393 )

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Based on the wave functions of three-dimensional quantum linear harmonic oscillator in the Descartes and spherical coordinates, we make the sampling by Monte Carlo method through the spatial probability density function. The spatial probability cloud of the particle is visualized by Mathematica software in different coordinate systems. Further comparison and analyzation of the visualization pictures of the system from different decompositions of the quantum numbers in two coordinate systems are carried out. And the physical meaning is illustrated from the theoretical point of view of quantum mechanics.

Improvement of interference experiment of polarized light

SHANGJin-guang, CHEN Mao-xiang, LING Yun-feng, LV Shou-kun, PING Li, LIU Mei-jie

College Physics. 2023, 42(11): 42. doi:10.16854 /j.cnki.1000-0712.220543

Abstract ( 540 )

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Interference experiment of polarized light is important in college physics experiment teaching. Based on Mach-Zehnder optical path, a self-built optical path is improved to observe the interference phenomenon of polarized light, to measure the change of light intensity, to judge the polarization form of polarized light, to calculate the refractive index of air in the laboratory environment. Data processing and error analysis are completed to having an error within 5%. Compared with the traditional experimental teaching method, the improved experimental device not only has clear and intuitive principle, but also can be expanded and applied by adding some optical components, so as to improve the efficiency of college physics experiment teaching and mobilize the enthusiasm of students to participate in experiments.

An approximate calculation of candle powered turbine’s torque

GONG Yi-hong

College Physics. 2023, 42(11): 48. doi:10.16854/j.cnki.1000-0712.230022

Abstract ( 372 )

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This article comes from 2022 IYPT’s problem. Main researches are properties of flow field around a candle and the paper spiral’s torque. This article consults literature and uses data to verify the properties of flow field around a candle. The pictures of flow field are simulated by COMSOL Multiphysics and flow visualization is realized striation technique. Then two different methods are used to derive influencing factors of paper spiral’s torque. An expression for paper spiral’s rotational inertia comes from integration method. Experimental data are used to fit the curve of paper spiral’s rotation. Angular acceleration can be obtained from the curve. Finally calculate paper spiral’s torque and analyze relationship between paper density, suspension height, layers of spiral and the torque. Data also prove the theory and calculation method are rational and feasible.

Research of oscillating screw based on theoretical mechanics and computer simulation

WANG Zi-hao, BIAN Yi-jie

College Physics. 2023, 42(11): 55. doi:10.16854/j. cnki.1000-0712.230005

Abstract ( 352 )

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First, based on kinematics and dynamics equations of theoretical mechanics, a relatively complete physical model of swing screw is established, and the numerical solution of screw motion is obtained by computer simulation. Second, on this basis, the influence and law of the model parameters on the oscillation of the swing screw are further studied. It is found that for the same screw, the screw swing angle curve is mainly related to the initial swing angle, the root friction coefficient, the nut friction coefficient and the slope angle. The root friction coefficient and the nut friction coefficient have a greater impact. Thirdly, explains are given for the phenomenon of screw oscillation increasing and screw oscillation under different model parameters. Empirical formula is put forward which is consistent with the simulated experimental results. Finally, the advantages and disadvantages of the model are explained.

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