arXiv subjects

A. M. Varonov

Publications and source records attributed to A. M. Varonov.

13 recordsLinked to original sources

arXiv2021

Tunable High-Q Resonator by General Impedance Converter

For the need of measurements focused in condensed matter physics and especially Bernoulli effect in superconductors we have developed an active resonator with dual operational amplifiers. A tunable high-Q resonator is performed in the schematics of the the General Impedance Converter (GIC). In the framework of frequency dependent open-loop gain of operational amplifiers, a general formula of the frequency dependence of the impedance of GIC is derived. The explicit formulas for the resonance frequency and Q-factor include as immanent parameter the crossover frequency of the operational amplifier. Voltage measurements of GIC with a lock-in amplifier perfectly agree with the derived formulas. A table reveals that electrometer operational amplifiers are the best choice to build the described resonator.

physics.ins-det↗

arXiv2021

Problem of the 8th Experimental Physics Olympiad, Skopje, 8 May 2021 Determination of Planck constant by LED

This is the problem of the 8$^\mathrm{th}$ International Experimental Physics Olympiad (EPO). The task of the EPO8 is to determine Plank constant $\hbar=h/2π$ using the given set-up with LED. If you have an idea how to do it, do it and send us the result; skip the reading of the detailed step by step instructions with increasing difficulties. We expect the participants to follow the suggested items -- they are instructive for physics education in general.Only the reading of historical remarks given in the first section can be omitted during the Olympiad without loss of generality. Participants should try solving as much tasks as they can without paying attention to the age categories: give your best.

physics.ed-ph↗

arXiv2020

Determination of magnon mass of neodymium magnet by temperature dependence of spontaneous magnetization

The effective mass of a magnon in a neodymium magnet is evaluated using Bloch temperature dependence of spontaneous magnetization. We used the Hall effect magnetometer and thermo-couple thermometer for measuring of the temperature of the water in which the encapsulated commercial magnet is immersed. The experiment can be repeated in every school and methodological remarks for experimental data processing are given with great detail and attention. The purpose of the paper is to illustrate the simple relation between the quantum spectrum of magnons and the easily measured magnetic field of a permanent magnet.

physics.ed-ph↗

arXiv2020

Comment on "Adiabatic Expansion of Electron Gas in a Magnetic Nozzle" by Kazunori Takahashi, Christine Charles, Rod Roswell, and Akira Ando, Phys. Rev. Lett. Vol. 120, 045001 (2018)

Heat capacities at fixed volume and pressure as a function of the degree of ionization are graphically depicted as functions of reciprocal temperature and ionization degree. The polytropic index is calculated as a function of the same variables; as it is not constant a partially ionized plasma can be only approximately polytropic fluid. In parallel, it is launched the idea that Alfvén waves can be used to heat the plasma in a propulsion jet with magnetic nozzle.

physics.plasm-ph↗

arXiv2020

Predicted electric field induced surface magnetization of vortex phase of superconductors

We predict a new effect in condensed matter surface magnetization of the vortex phase of a superconductor induced by electric field. The magnetized superconductor should be one of the plates of a plane capacitor on which a voltage has to be applied. Applying alternating voltage to the capacitor, electrostatic induction leads to oscillations of the magnetic moment which has to be measured by electromotive force inducted in a detector coil. The derived explicit formula for the magnetization contains the effective mass of Cooper pairs and a systematic investigation of the predicted magnetization will lead to a creation of an effective Cooper pair mass spectroscopy. For cleaved superconductors this effective mass is a property of the bulk material.

cond-mat.supr-con↗

arXiv2020

Robust formula for $N$-point Padé approximant calculation based on Wynn identity

The performed numerical analysis reveals that Wynn's identity for the compass $1/(N-C)+1/(S-C)=1/(W-C)+1/(E-C)=1/η$ (here C stands for center, the other letters correspond to the four directions of the compass) gives the long sought criterion, the minimal $|η|$, for the choice of the optimal Padé approximant. The work of this method is illustrated by calculation of multipoint Padé approximation by a new formula for calculation of this best rational approximation. The work of the criterion for the calculation of optimal Padé approximant is illustrated by the frequently seen in the theoretical physics problems of calculation of series summation and multipoint Padé approximation used as a predictor for solution of differential equations motivated by the magneto-hydrodynamic problem of heating of solar corona by Alvén waves. In such a way, an efficient and valuable control mechanism for $N$-point Padé approximant calculation is proposed. We believe that the suggested method and criterion can be useful for many applied problems in numerous areas not only in physics but in any scientific application where differential equations are solved. The solution of the Cauchy-Jacobi problem is illustrated by a Fortran program. The algorithm is generalized for the case of the first $K$ derivatives at $N$ nodal points.

math.NA↗

arXiv2019

Determination of Boltzmann constant by equipartition theorem for capacitors

A new experimental set-up for Boltzmann constant measurement is described. Statistically averaged square of voltage $\left $ is measured for different capacitances $C$. Boltzmann constant is determined by the equipartition theorem $C\left =k_{_\mathrm{B}}T$. For fixed capacitance, voltages could be measured for different temperatures. The set-up consists of low-noise high frequency operational amplifiers ADA4898-2. An instrumental amplifier is followed by an inverting amplifier, square of the voltage is created by an analog multiplier AD633 and finally the averaged signal is measured by a multimeter.

physics.ed-ph↗

arXiv2019

Measurement of the electron charge $q_e$ using Schottky noise. Problem of the 6-th Experimental Physics Olympiad. Sofia 8 December 2018

Several consecutive experiments are described with a printed circuit board PCB set-up, especially designed for these experiments. Doing the consecutive experimental tasks opens up possibility to determine the value of electron charge $q_e.$ The fluctuations of the voltage $U(t)$ should be measured for different illuminations of a photodiode. The voltage is amplified 1 million times $Y=10^6$. The amplified voltage $YU(t)$ is applied to the device, which gives the result of the value of the time averaged square of the voltage $U_\mathrm{S}=\left<(Y U(t))^2\right>/U_0$. This voltage $U_\mathrm{S}$ is measured with a multimeter. The series of measurements gives the possibility to determine the $q_e$ using the well known Schottky formula for the spectral density of the current noise $(I^2)_f=2q_e\left .$ For the junior high school students, the basic problem is to analyze the analog squaring. Students' work is separated and graded in four categories S, M, L, XL divided by age of students. For the last XL categories, the tasks contain problems oriented to physics university education program and include theoretical research of the PCB set-up as an engineering device. This is the problem of EPO6, December 2018 ``Day of the Charge'' considered. EPO6 is organized by Sofia branch of Union of physicists in Bulgaria in cooperation with Faculty of physics of Sofia University and Society of Physicists of Republic of Macedonia.

physics.ed-ph↗

arXiv2019

Master equation for operational amplifiers: stability of negative differential converters, crossover frequency and pass-bandwidth

The time dependent master equation from the seminal article by Ragazzini, Randall and Russell [J. R. Ragazzini, R. H. Randall and F. A. Russell, "Analysis of Problems in Dynamics by Electronic Circuits", Proc. of the I.R.E., Vol. 35, pp. 444--452, (1947)] is recovered as necessary tool for the analysis of contemporary circuits with operational amplifiers. This equation gives the relation between time dependent the output voltage $U_0(t)$ and the difference between the input voltages ($U_{+}(t)$ and $U_{-}(t)$). The crossover frequency $f_0$ is represented the time constant $τ_{_0}$ of this equation. The work of the master equation is illustrated by two typical examples: a) the stability criterion of the devices with negative impedance converters, which we consider as a new result b) the frequency dependence of the amplifiers with operational amplifiers given in the technical specifications without citations of time dependent equation. A simple circuit for determination of $f_0$ is suggested and the method is illustrated by determination of crossover frequency for the low-noise and high speed ADA4898 operational amplifier. It is concluded that for an exact calculation of the pass bandwidth of amplifiers with active filters the 70 years old master equation is a useful technique implicitly included in the contemporary software. The frequency dependent formulae for the amplification coefficient of inverting and non-inverting amplifiers are given for the case of non-zero conductivity between the inputs of the operational amplifiers.

physics.app-ph↗

arXiv2019

Generation of Teraherz Oscillations by Thin Superconducting Film in Fluctuation Regime

Explicit analytical expressions for conductivity of a superconducting film above and below critical temperature in an arbitrary electric field are derived in the frameworks of the time dependent Ginzburg-Landau theory. It is confirmed that slightly below critical temperature the differential conductivity of superconducting film can become negative for small enough values of electric field. This fact may cause generation of electromagnetic oscillations if the superconducting film is appropriately coupled of with a resonator. Their maximal frequency is proportional to the value of critical temperature of superconducting transition. The obtained results can stimulate the development of Terahertz generators on the basis of high temperature superconducting films.

cond-mat.supr-con↗

arXiv2018

Callen-Welton fluctuation dissipation theorem and Nyquist theorem as a consequence of detailed balance principle applied to an oscillator

We re-derive the Nyquist theorem and Callen-Welton fluctuation-dissipation theorem (FDT) as a consequence of detailed balance principle applied to a harmonic oscillator. The usage of electrical notions in the beginning makes the consideration understandable for every physicists. Perhaps it is the simplest derivation of these well-known theorems in statistical physics. The classical limit is understandable as a consequence of Waterston-Herapath equipartition theorem.

cond-mat.stat-mech↗

arXiv2017

Nonmodal analysis of helical and azimuthal magnetorotational instabilities

Helical and azimuthal magnetorotational instabilities operate in rotating magnetized flows with relatively steep negative or extremely steep positive shear. The corresponding lower and upper Liu limits of the shear, which determine the threshold of modal growth of these instabilities, are continuously connected when some axial electrical current is allowed to pass through the rotating fluid. We investigate the nonmodal dynamics of these instabilities arising from the non-normality of shear flow in the local approximation, generalizing the results of the modal approach. It is demonstrated that moderate transient/nonmodal amplification of both types of magnetorotational instability occurs within the Liu limits, where the system is stable according to modal analysis. We show that for the helical magnetorotational instability this magnetohydrodynamic behavior is closely connected with the nonmodal growth of the underlying purely hydrodynamic problem.

physics.plasm-ph↗

arXiv2016

An undergraduate laboratory experiment on measuring the velocity of light with a catastrophic machine

An experimental setup for electrostatic measurement of $\varepsilon_0$, separated magneto-static measurement of $μ_0$ and determination of the velocity of light $c=1/\sqrt{\varepsilon_0 μ_0}$ according to Maxwell theory with percent accuracy is described. No forces are measured with the experimental setup therefore there is no need of a scale and the experiment price less than \pounds20 is mainly due to the batteries used. Multiplied 137~times, this experimental setup was given at the fourth open international Experimental Physics Olympiad (EPO4) and a dozen high school students did very well. This article, however, focuses on the catastrophe theory, which is the basis of the methodology.

physics.ed-ph↗