College Physics

The differences between the mathematical and the physical definition of quantum bit

LIU Yi-wen, LEI Yi-an

College Physics. 2019, 38(6): 1-3. doi:10.16854 /j.cnki.1000-0712.180629

Abstract ( 1908 )

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We discuss the mathematical definition of quantum bit ( qubit) and its physical properties and find

discrepancies between them. In the context of quantum information，the definition of quantum bit is purely mathematical，

which greatly differs from its physical properties. Major differences include: the readability and writeability

of the qubit; the requirement of inifinit procision in typical algrorithms such as Shor’s algorithm; voilation of

classic entropy definition，etc

An equivalent operator of radial operator and its quantum measurement

XIAO Shi-fa, LIU Quan-hui

College Physics. 2019, 38(6): 4-7. doi:10.16854 /j.cnki.1000-0712.180490

Abstract ( 2152 )

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A new decomposition of the momentum operator in curvilinear coordinates is proposed，from which

an equivalent operator of radial operator is resulted. Though the radial operator itself is non-self-adjoint and is not

measurable in physics，the equivalent operator is in fact the difference of two self-adjoint operators，so is measurable.

Thus，a superficial contradiction between the non-self-adjointness and existence of the uncertainty for the radial

momentum operator is resolved. What is more，the Hydrogen atom ground state is used to show how to measure

the equivalent operator.

Upper limit of planetary rotation speed

YU Feng-jun, TANG Zhen-jie, TIAN Jun-long

College Physics. 2019, 38(6): 8. doi:10.16854 /j.cnki.1000-0712.180544

Abstract ( 2218 )

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Many planets have rotational motions. In a reference frame rotating with the planet，the mass point

of the planet is subjected to an inertial centrifugal force，which makes it appear as a flat rotating ellipsoid. The faster

the planet rotates，the greater the centrifugal force of inertia is. When the speed is greater than a critical value，

the planet will break. Based on the principle of minimum potential energy，the upper limit of planetary rotation

　　speed is strictly calculated.

Another description of RLC coupling loop

HUANG Wan-xia, YE Huan, YIN Jie, WANG Mao-sheng

College Physics. 2019, 38(6): 13. doi:10.16854 /j.cnki.1000-0712.180520

Abstract ( 1631 )

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We use the two-oscillator model to describe the coupling between the RLC loops. However，the two -oscillator model is described by two second-order differential equations，the solving process is complicated. Therefore， for RLC coupling loop，we study theoretically and experimentally the correlation between the coupled-mode theory ( CMT) of the first-order differential equations and the two-oscillator model of second-order equations. The result shows the CMT can generalize the electric circuit system.

The force of the charged thin ring and the conductor sphere obtained by using the potential energy method

LIAO Qi-li, DENG Ya, YU Yan, XIE Guo-ya, ZHANG Zhu-feng

College Physics. 2019, 38(6): 16. doi:10.16854 /j.cnki.1000-0712.180517

Abstract ( 1751 )

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By using the mirror image method in electromagnetism and the Mathematica software，we calculate

the electric potential outside space of the a conductor ball on the axis of the charged thin ring. The electric potential

distribution of the conductor ball outside space is drawn. Then，electric field force can be obtained by using the relationship

between electric potential energy and electric field force，and the force of electric field with distance

change is drawn.

Electrical characteristics of n ladder resistor network with an external load

TAN Zhi-zhong, ZHAO Jia-cheng, DING Lu-yu, ZHANG Qing-hua

College Physics. 2019, 38(6): 20. doi:10.16854 /j.cnki.1000-0712.180507

Abstract ( 1364 )

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The electrical characteristics of complex resistor networks are always sophisticated theses to deal

with.Using the voltage parameter to construct a recursion model， this paper studies electrical characteristics，

including the node voltage，branch current and equivalent resistor etc.，a type of the complex n-order ladder shaped

resistor network，which has an external load on an arbitrary node and a random resistor on the right boundary.By　

means of the network analysis， this paper establishes the difference equation model of the voltage，and the equation

model of the boundary compatible with the constraint conditions.After then，explicit expressions of the common solution

and the particular solution are derived， from which the formulae of voltages of arbitrary nodes，branch currents，

and the equivalent resistors are also obtained.All the formulae obtained here have multi-functions and universality，

and many special cases.

Analysis of two-dimensional hexagonal lattice structure and vibration mode using graphene as an example

WANG Qing-qing, CHENG Rong-long, GONG Hao

College Physics. 2019, 38(6): 24. doi:10.16854 /j.cnki.10000712.180487

Abstract ( 7304 )

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Graphene has become one of the most widely used new materials because of its unique hexagonal lattice

structure，which has excellent mechanical， thermal，optical and electrical properties.From the point of view of

solid state physics teaching， taking the lattice structure of single-layer graphene as an example， this paper describes

the normal lattice primitive cell and base vector， inverted lattice primitive cell and base vector，wave vector space

and first Brillouin region of two-dimensional hexagonal lattice in the form of images，and derives the vibration mode

of two-dimensional hexagonal lattice.

The electric field of a line charge and a grounding parallel conductor plate with a vertical ridge and numerical simulation

WANG Fu-qian, YIN Yong, LIU Wei-qi, LIU Yu

College Physics. 2019, 38(6): 29. doi:10.16854 /j.cnki.1000-0712.180135

Abstract ( 1338 )

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By combining conformal mapping and plane mirror method，the electric field of a line charge and a

ground parallel conductor plate with a ridge are solved，and several special features are discussed. Further，the distribution

of the electric field is simulated by using mathematical software MATLAB.

A simple experiment to verify the momentum theorem

SANG Rui-rui, ZHANG Xiong, ZHANG Hao-jing, LI Xiao, WANG Bi-hong

College Physics. 2019, 38(6): 33. doi:10.16854 /j.cnki.1000-0712.180341

Abstract ( 2039 )

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The paper provides a simple experimental device to enable students to better understand the relationship between momentum and impulse， and to verify momentum theorem through simple mathematical processing. Teachers can use the experiment to carry out IBSE ( Inquiry Based Science Education) activities for deepen students' understanding of the basic concepts of physics and can effectively cultivate students' hands-on practical ability.

Research on the phenomenon of water bottle flipping to the ground smoothly

XUN Zhi-peng, ZHANG Xiao-li, YU Zhe-wei

College Physics. 2019, 38(6): 37. doi:10.16854 /j.cnki.1000-0712.180672

Abstract ( 4009 )

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In this paper，the flipping of partially filled plastic bottle is investigated theoretically and experimentally.Through simplifying the distribution of the water in the flipping bottles，the movement of flipping bottle is analyzed theoretically by using the conservation of angular momentum. Then the effects of the initial angular velocity，the proportion of water in bottle and the ground roughness on the success rate of the bottle landing stably are investigated experimentally. Finally，a game advice is given.

Measuring the forced vibration of a tuning fork by interference

LIU Shuo, ZHONG Hao-yuan, GAO You-hui

College Physics. 2019, 38(6): 42. doi:10.16854 /j.cnki.1000-0712.180337

Abstract ( 2099 )

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In this experiment，the character of forced vibration of a tuning fork is studied，including the displacement amplitude，velocity amplitude and phase difference between driving force and vibration. All the charater can be studied at the same time using Michelson interference method. The result shows that the resonance frequency of displacement amplitude and velocity amplitude are approximately equal; there is a π/2 phase difference between driving force and vibration when the resonance is reached. The change of the phase difference when the driving frequency is increased to the resonance can also be observed.

Solution of string vibrating problem under the resonant frequency incentive

HE Jia-qi, HAN She-jiao

College Physics. 2019, 38(6): 45. doi:10.16854 /j.cnki.1000-0712.180577

Abstract ( 1737 )

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For the string vibrating problem under the condition that one end is fixed and the other end with sinusoidal incentive，it is solved by the separation of variables under incentive of resonant frequency and non-resonant frequency. Through the study of two different forms of solutions，it is found that the solution under the resonant frequency incentive can be obtained by getting the limit of the solution under non-resonant frequency incentive. Finally，the analysis shows that the solution under resonant frequency incentive is the solution of standing wave state without damping.

Brownian motion of Millikan's oil-drop experiment

GUAN Shu-yue, ZHANG Ming, ZHANG Shi-ping, WU Ping

College Physics. 2019, 38(6): 48. doi:10.16854 /j.cnki.1000-0712.180581

Abstract ( 3438 )

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In Millikan's oil-drop experiment，if the oil drop is chosen too small，it shows obvious Brownian motion，which causes the measured dropping time to deviate from the ideal dropping time. In this article，through practical experiment and theoretical study，the stochastic differential equation of the movement of the oil drop is established by solving the Langevin equation. By introducing single and multiple time deviation，both the Brownian motion and the time deviation are quantitatively analyzed. As being indicated by the result，when the oil drop is relatively small，the influence of Brownian motion should be taken into account because of the limit of experiment times.

Simple experimental approach to suppress the backscattering peak in NaI( Tl) spectrometer

GUAN Shu-yue, ZHANG Ming, ZHANG Shi-ping, WU Ping

College Physics. 2019, 38(6): 55. doi:10.16854 /j.cnki.1000-0712.180444

Abstract ( 1794 )

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In Millikan's oil-drop experiment，if the oil drop is chosen too small，it shows obvious Brownian motion，which causes the measured dropping time to deviate from the ideal dropping time. In this article，through practical experiment and theoretical study，the stochastic differential equation of the movement of the oil drop is established by solving the Langevin equation. By introducing single and multiple time deviation，both the Brownian motion and the time deviation are quantitatively analyzed. As being indicated by the result，when the oil drop is relatively small，the influence of Brownian motion should be taken into account because of the limit of experiment times.

A magnetic force microscopy study of FePt nanowires

LIU Xin-meng, WANG Hao, WANG Yi, YE Shi-li, XU Rui, CHENG Zhi-hai

College Physics. 2019, 38(6): 60. doi:10.16854 /j.cnki.1000-0712.180549

Abstract ( 1420 )

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In this study，MFM is used to scan and observe the surface and magnetization of FePt nanowires scattered

on a SiO2 substrate and their magnetic images are also obtained. The magnetizaion distribution of FePt nanowires

is analyzed based on bright and dark regions shown on magnetic images. The aggregation of nanowires with opposite

magnetization indicates magnetic materials always tend to be in the lowest energy state. The orientation of magnetization

of FePt nanowires can be controlled and magnetic reversal is observed by applying external magnetic field.

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