The Experts below are selected from a list of 1575 Experts worldwide ranked by ideXlab platform

Namik Yener - One of the best experts on this subject based on the ideXlab platform.

  • Permittivity of Vacuum and speed of light in Vacuum which vary with relative speeds of media in uniform rectilinear motion with respect to each other
    2011
    Co-Authors: Namik Yener
    Abstract:

    Having determined in a series of articles that the principle of constancy of speed of light in Vacuum of the Special Relativity Theory is false and has to be put aside because it cannot account for the loss in one of the two media which are in uniform rectilinear motion with respect to each other, it has become necessary to use difierent speeds of light in Vacuum for difierent Galilean reference systems. In this paper this is demonstrated again by obtaining of a relation that relates the speed of light in Vacuum c, constitutive parameters of the medium, frequency and the speeds of two media in uniform rectilinear motion with respect to each other. This in turn necessitates the revision of the concept of the Permittivity of Vacuum for difierent Galilean reference systems. Therefore flrst we determine the speed of light in Vacuum for one of the two moving media, and deflne the Permittivity of Vacuum for that medium using the speed of light in Vacuum found for that medium. We assume the permeability of Vacuum is an invariant quantity given as "0 = 4… £ 10 i7 (H/m). The same procedure is repeated also for the second medium. The two media which are in relative motion with respect to each other and which are taken up in this paper, are a simple medium with loss and a perfectly conducting medium fllling a half space. Their interface is an inflnite plane perpendicular to the direction of the uniform rectilinear motion of the second medium which is constituted of the perfectly conducting half space. For simplicity we have assumed the incident wave that impinges on the interface has a wave vector that makes an angle of µ = …=2 with the direction of the velocity of the moving medium and hence we can make use of the time dilation formula to transform the frequencies between the Galilean reference systems. The Permittivity of Vacuum and speed of light in Vacuum results obtained are particular to this electromagnetic system.

Bonić Ivan - One of the best experts on this subject based on the ideXlab platform.

  • Superluminalna propagacija u umjetnim elektromagnetskim strukturama.
    University of Zagreb. Faculty of Electrical Engineering and Computing. Department of Wireless Communications., 2020
    Co-Authors: Bonić Ivan
    Abstract:

    This thesis is devoted to electromagnetic metamaterials (MTM), which are artificial structures with special electromagnetic properties that are not normally found in naturally occurring materials. Besides their unusual physical properties, some MTM-based electromagnetic structures can support superluminal propagation (propagation with velocities faster than the speed of light in Vacuum c). Main goal is engineering of active superluminal structures and characterization of their electromagnetic properties. Superluminal propagation phenomena found in some passive MTMs are compared to the novel concept of active broadband non-Foster-based superluminal MTMs that support broadband phase or group velocity. A causal model of non-Foster-based epsilon-near-zero (ENZ) active MTM is developed and tested analytically and numerically. These active ENZ MTMs are ultra-wideband and support superluminal group velocity and superluminal phase velocity. In the end, the proposed theoretical approach is simulated and non-Foster negative capacitors in RF regime are designed, built and optimized. Furthermore, the proposed transmission-line-based non-Foster ENZ 1D MTM was built and tested in RF 2 MHz to over 40 MHz band. The experiments were conducted in both frequency and time domain, confirming the stable operation and the Permittivity that is lower than the Permittivity of Vacuum (the ENZ behavior). The extractions further show that this medium has ultra-wideband EM characteristics that cannot be achieved with any existing passive MTMs. The post processing of extracted parameters revealed simultaneous superluminal phase velocity and superluminal group velocity within the bandwidth of more than four octaves. The superluminal behavior of the prototype was also demonstrated by measurements in time domain with the sinusoidal and Gaussian burst excitation signals. These measurements also revealed the superluminal behavior of pulses that are qualitatively very similar to previous “fast light” time-domain superluminal experiments in physics. The achieved theoretical and experimental results enable causal modeling of future stable 1D and 2D volumetric active non-Foster based MTMs that support ultra-wideband superluminal pulse propagation.Ovaj doktorski rad je posvećen elektromagnetskim metamaterijalima (eng. MTM) koji su umjetne strukture sa specijalnim elektromagnetskim svojstvima koja ne nalazimo u prirodi. Uz njihova specijalna fizikalna svojstva, neke elektromagnetske strukture bazirane na metamaterijalima mogu podržavati superluminalnu propagaciju s brzinama većim od brzine svjetlosti u vakuumu c. Glavni cilj je dizajn i razvoj aktivnih superluminalnih struktura te karakterizacija njihovih elektromagnetskih svojstava. Fenomen superluminalne propagacije koji nalazimo u nekim pasivnim metamaterijalima je uspoređen s novim konceptom aktivnih širokopojasnih nefosterovskih superluminalnih metamaterijala koji podržavaju širokopojasnu faznu ili grupnu brzinu. Kauzalni model nefosterovskog aktivnog metamaterijala s permitivnosti blizu nule (eng. ENZ) je razvijen i testiran analitički i numerički. Ovi aktivni metamaterijali s permitivnosti blizu nule su ultraširokopojasni i podržavaju superluminalnu grupnu brzinu i superluminalnu faznu brzinu. Na kraju, predloženi teorijski pristup je simuliran te su razvijeni, napravljeni i optimizirani nefosterovski negativni kondenzatori u RF frekvencijskom području. Nadalje, predloženi nefosterovski 1D ENZ metamaterijal baziran na prijenosnim linijama je napravljen i testiran u RF području od 2 MHz do preko 40 MHz. Eksperimenti su napravljeni u frekvencijskoj i vremenskoj domeni te potvrđuju stabilan rad bez oscilacija te permitivnost koja je manja od permitivnosti u vakuumu (ENZ ponašanje). Ekstrakcije nadalje pokazuju da ovaj medij ima ultraširokopojasna elektromagnetska svojstva koja se ne mogu postići s pasivnim metamaterijalima. Postprocesiranjem ekstraktiranih parametara pokazana je istovremeno superluminalna fazna i superluminalna grupna brzina u frekvencijskom opsegu od preko četiri oktave. Superluminalno ponašanje ovog prototipa je također potvrđeno mjerenjima u vremenskoj domeni uz pomoć sinusnih i Gaussovih impulsnih pobudnih signala. Ova mjerenja također otkrivaju superluminalano ponašanje impulsa što je kvalitativno vrlo slično postojećim eksperimentima superluminalnog „brzog svjetla“ u fizici. Postignuti teorijski i eksperimentalni rezultati omogućuju kauzalno modeliranje budućih stabilnih 1D i 2D volumetrijskih aktivnih nefosterovskih metamaterijala koji podržavaju ultraširokopojasnu superluminalnu propagaciju impulsa

Johner Albert - One of the best experts on this subject based on the ideXlab platform.

  • Interaction between like-charged surfaces in water: does mean-field (Poisson Boltzmann) always apply for monovalent counterions?
    'Journal Club for Condensed Matter Physics', 2020
    Co-Authors: Johner Albert
    Abstract:

    International audienceCharged interfaces in water are widespread in industry and biology. The interactions of two like-charged interfaces, colloid/water/colloid[1, 2] or cell(vesicle)/water/cell(vesicle)[3] for example, received continuous attention since the start of the 20th. century[4]. It was early realized that several type of "interactions" are at work besides electrostatics, like van der Waals interactions, in the celebrated DLVO theory[4], or short range hydration forces (defined from neutral interfaces). In practice, for want of better options, those interactions are usually added up. The standard treatment of the electrostatic part uses the continuous Poisson-Boltzmann (P-B) theory, which is a mean-field theory. There are many, potentially important, corrections to the standard P-B treatment: non-electrostatic counterion-surface interactions, discreteness of the surface charge (both noted early[5]), non-uniform dielec-tric constant, correlations between counterions. * The paper by Schlaich et al. combines chemically realistic Molecular Dynamics (MD) simulations and analytical theory (with the additional input of Monte Carlo (MC) for pure electrostatics) to address all these points and essentially solve the problem (for polar surfaces). It offers much more than suggested by the, unavoidably reductive, title. This comment presents some of Schlaich et al.'s results in relation to a posterior experimental paper on deposited bilayers[6] and a much older one on biliquid foams[7]. In continuous theories, the strength of electrostatics for a given solvent characterized by the Bjerrum length l B = e 2 /(4π 0 k B T) decreases with increasing dielectric constant , 0 being the Permittivity of Vacuum. Electrostatics of an isolated pair of elementary charges separated by more than l B is overcome by thermal noise. The P-B theory adds a second length * Reading, for exemple, the book by Verwey and Overbeek[4], it is clear that the community knew about the weaknesses of the Poisson-Boltzmann treatment for electrostatics, already by that time. But the success of DLVO steadily reported since, at least for monovalent counterions, is striking. At this point it is fair to say that in most experimental setups the conditions of the experiments are not easy to control and the parameters not always known with precision, or even fitted

Grard R. - One of the best experts on this subject based on the ideXlab platform.

  • First results of on-comet observations by SESAME/PP
    2014
    Co-Authors: Schmidt Walter, Seidensticker, Klaus J., Le Gall Alice, Hamelin Michel, Caujolle-bert Sylvain, Lethuillier Anthony, Grard R.
    Abstract:

    The Permittivity Probe (PP) as part of the SESAME instrument group performed measurements directly before Philae's separation from Rosetta, during its descent and on the first day on the comet surface at Philae's final landing position. The purpose of PP is the monitoring of the electrical properties of the comet's subsurface material down to a depth of about 2 m and their variation with temperature and distance from the Sun. The used implementation is especially sensitive to the presence and properties of water ice inside the observed volume. As a secondary objective the instrument tries to observe low-frequency plasma waves below 20 kHz as generated by interaction between the comet and the solar wind. The descent measurements were mainly intended to calibrate the instrument in its final configuration with deployed landing gear and without being influenced by the vicinity of the Rosetta structure, while the environmental properties like Permittivity of Vacuum were known. Possible deviations from this assumption were monitored by RPC and will be taken into account during the detailed data analysis. Unfortunately these measurements were heavily distorted by saturation of the sensitive pre-amplifiers in the landing gear feet, which were for the first time confronted with CONSERT radio pulses operating with deployed antenna. While PPís own transmitter for injecting currents into the medium worked nominally and the measured data can be used for calibrating the injected currents on the comet surface, probably only a small sub-set of the receiver data can be used for the intended stray capacitance calibration after post processing to extract the few time intervals where the receivers were not in saturation. The on-surface measurements were all performed during the initial safe-mode interval while the illuminated period changed into comet night, allowing the observation of the temperature dependence of the measurement parameters. Though the second transmitter electrode attached to the MUPUS PEN and the APXS sensor, not-yet deployed at that time, could not be utilized, the first interpretation of data collected in this reduced configuration indicate the presence of cold water ice at least underneath the -Y foot of Philae. The distance between the +Y foot and the surface is either too large to see a similar effect or the subsurface material in the partly sun-lit area of the current location is depleted of ice. Once the additional transmitter electrodes at the deployed MUPUS PEN and APXS can be utilized during a later LTS mission, the initial measurements can be re-evaluated in more detail

Dai-ning Fang - One of the best experts on this subject based on the ideXlab platform.

  • FRACTURE CRITERION FOR PIEZOELECTRIC MATERIALS WITH DEFECTS BASED ON ENERGY DENSITY THEORY
    2016
    Co-Authors: Ai Kah Soh, Kwok Lun Lee, Dai-ning Fang
    Abstract:

    A failure criterion, which was established based on the energy density factor, is extended for an elliptical cavity or a line crack embedded in an infinite piezoelectric solids, subjected to a combined in-plane electrical and mechanical loading. In the present analysis, the exact electric boundary conditions are applied at the rim of the cavity/crack. The direction of crack initiation or subsequent post- failure, and the critical loads for fracture, can be predicted using the total energy density factor, S. This factor is a function of the aspect ratio of the elliptical cavity, the electromechanical loading, core region outside the crack tip, Permittivity of Vacuum and material constants. The results obtained agree with the experimental observation, i.e. a positive electric field enhances crack growth while a negative electric field impedes crack growth. Moreover, the results indicate that the critical fracture loads are under-estimated by the assumption of impermeable crack and over-estimated when the crack is assumed to be permeable for Ez app>0, where Ez app is the applied electric field. However, the fracture loads are over-estimated by the assumption of impermeable crack and under-estimated when the crack is assumed to be permeable for Ez app<0. The energy density criterion has the advantage of possessing the capability to implement the exact electric boundary conditions. This is due to the fact that the criterion can link the behavior of a crack to that of an elliptical cavity by consistent application of this criterion to a thin layer near the cavity/crack boundary