67 research outputs found
Time evolution of the microwave second-order response of YBaCuO powder
Transient effects in the microwave second-order response of YBaCuO powder are
investigated. The time evolution of the second harmonic signal has been
measured for about 300 s after the sample had been exposed to variations of the
DC magnetic field. We show that in different time scales the transient response
has different origin. In the time scale of milliseconds the transient response
of samples in the critical state is ascribable to processes of flux
redistribution induced by the switching on/off of the microwave field. At
longer times, the time evolution of the second harmonic signal can be ascribed
to motion of fluxons induced by the variation of the DC magnetic field. In
particular, diffusive motion of fluxons determines the response in the first 10
seconds after the stop of the magnetic field variation; magnetic relaxation
over the surface barrier determines the response in the time scale of minutes.Comment: 16 pages, 7 figures, submited to Physica
Introduzione al Laboratorio di Fisica: gli errori nelle misure sperimentali
L'attivit\ue0 di laboratorio deve fornire agli studenti la possibilita di effettuare osservazioni quantitative di grandezze fisiche facilmente individuabili e analizzabili. Per questo motivo,
bisogna pensare esperimenti operativi, cioe esperimenti che possano essere eseguiti in laboratorio allo scopo di studiare la relazione tra le varie grandezze fisiche coinvolte e
quindi determinare la relativa legge fisica. Questi esperimenti hanno valenze didattiche prevalenti per la misura, l'analisi dei dati, la formalizzazione della legge fisica e per gli
aspetti formativi in generale. Gli esperimenti illustrativi, cioe esperimenti in cui viene visualizzato un determinato fenomeno fisico, non sono indicati per attivita di laboratorio
finalizzata all'analisi dati; essi sono utili da mostrare in classe durante la lezione allo scopo di attirare l'attenzione degli studenti e stimolare la riflessione su una particolare
fenomenologia, prima di trattare formalmente lo specifico argomento. Il materiale raccolto in questo volume proviene sostanzialmente dalle lezioni di Laboratorio di Fisica tenuti dall'autore per i Corsi di Studio della ex Facolta di Scienze Matematiche, Fisiche e Naturali confluita oggi nella Scuola delle Scienze di Base ed Applicate dell'Universita degli Studi di Palermo. Nella prima parte del libro, viene affrontato il problema della stima degli errori nelle misure dirette e viene trattata la propagazione
degli errori nelle misure indirette. Nella seconda parte, vengono introdotti i metodi elementari di analisi statistica dei dati sperimentali e viene quindi introdotta la funzione
di distribuzione di Gauss per gli errori casuali. La trattazione degli argomenti e focalizzata principalmente sul significato fisico delle nozioni introdotte piu che sul formalismo
matematico-statistico, con un'esposizione non sempre completamente rigoros
Multidisciplinary learning at the University scientific museums: the Bunsen burner
Nell'articolo riportiamo un'attività di laboratorio svolta insieme a studenti di scuole superiori, al fine di aumentare l’interesse dei giovani studenti verso gli strumenti scientifici di interesse storico e stimolare il loro approccio agli studi scientifici. Viene proposta un'attività "hands on" che può essere facilmente condotta presso i musei scientifici universitari. Un gruppo di studenti selezionati ha frequentato, sotto la guida di ricercatori universitari, uno Stage estivo di una settimana organizzato presso la Collezione Storica degli Strumenti di Fisica e il Museo di Chimica dell'Università di Palermo. È stato proposto lo studio del becco Bunsen, di come funziona e come viene usato nei laboratori di chimica. In particolare, sono stati presi in considerazione alcuni esemplari appartenenti alle collezioni, che saranno illustrati nell'articolo. Alla fine dello Stage, gli studenti hanno descritto ai visitatori gli strumenti e illustrato le attività svolte.We report on a laboratory activity carried out together with secondary school (high-school) students, with the aim of increasing their interest toward historical scientific instruments and stimulate their approach to scientific knowledge. To this purpose, we propose a hands-on activity that can be fruitfully performed at the University scientific museums. We organized a one-week summer stage at the Historical Collection of Physics Instruments and at the Museum of Chemistry of the University of Palermo. A group of selected students attended it, under the tutoring of university researchers. They were showed some Bunsen burners belonging to the collections, how they are restored, how they work and how they were used in the chemical laboratories. At the end of the stage, students introduced museum visitors to these instruments, describing them and referring about the activities they had carried out
Critical-state effects on microwave losses in type-II superconductors
We discuss the microwave energy losses in superconductors in the critical
state. The field-induced variations of the surface resistance are determined,
in the framework of the Coffey and Clem model, by taking into account the
distribution of the vortex magnetic field inside the sample. It is shown that
the effects of the critical state cannot generally be disregarded to account
for the experimental data. Results obtained in bulk niobium at low temperatures
are quantitatively justified.Comment: 4 pages, 4 embedded figures, to be published on Eur. Phys. J.
Microwave Second-Harmonic Response of Ceramic MgB2 Samples
Nonlinear microwave response of different ceramic MgB2 samples has been
investigated by the technique of second-harmonic emission. The second-harmonic
signal has been investigated as a function of temperature, DC magnetic field
and input microwave power. The attention has mainly been devoted to the
response at low magnetic fields, where nonlinear processes arising from motion
of Abrikosov fluxons are ineffective. The results show that different
mechanisms are responsible for the nonlinear response in the different ranges
of temperature. At low temperatures, the nonlinear response is due to processes
involving weak links. At temperatures close to Tc, a further contribution to
the harmonic emission is present; it can be ascribed to modulation of the order
parameter by the microwave field and gives rise to a peak in the temperature
dependence of the harmonic signal.Comment: 16 pages, 8 figures, submitted to Physica
Fluxon dynamics by microwave surface resistance measurements in MgB2
Field-induced variations of the microwave surface resistance, Rs(H), have
been investigated in high-density ceramic MgB2. At low temperatures, several
peculiarities of the Rs(H) curves cannot be justified in the framework of
models reported in the literature. We suggest that they are ascribable to the
unconventional vortex structure in MgB2, related to the presence of two gaps.
On the contrary, the results near Tc can be accounted for by the Coffey and
Clem model, with fluxons moving in the flux-flow regime, provided that the
anisotropy of the upper critical field is taken into due account.Comment: 6 pages, 4 figure
Depinning frequency in a heavily neutron-irradiated MgB2 sample
The magnetic-field-induced variations of the microwave surface resistance
have been investigated in a heavily neutron-irradiated MgB2 sample, in which
the irradiation has caused the merging of the two gaps into a single value. The
experimental results have been analyzed in the framework of the Coffey and Clem
model. By fitting the experimental data, we have determined the field
dependence of the depinning frequency, omega_0, at different values of the
temperature. Although the pinning is not particularly effective, the value of
omega_0 obtained at low temperatures is considerably higher than that observed
in conventional low-temperature superconductors.Comment: 15 pages, 6 figures; to be published in Physica
The Silver Voltameter: An Essential Instrument for the Definition of the Unit of Electric Current
We discuss the restoration of a silver voltameter belonging lo the Historical Collection of Physics Instruments of the University of Palermo. We stress the essential role this instrument had in the definition of the unit of
electric current
Comparative study of historical woods from XIX century by thermogravimetry coupled with FTIR spectroscopy
Thermal and structural properties of historical woods from apparatuses of the Historical Collection of the Physics Instruments of the University of Palermo have been investigated by FTIR spectroscopy coupled with thermogravimetric analysis. Specifically, the wooden portions of apparatuses from XIX century have been studied. The investigated woods belong to different taxa (Swietenia mahagoni, Picea abies and Juglans regia). The thermal behavior of the wooden materials has been successfully interpreted on the basis of specific indexes determined by the quantitative analysis of the FTIR spectra. The kinetics of the wood pyrolysis have been investigated by using a non-isothermal approach based on model-free isoconversional procedures, such as Kissinger–Akahira–Sunose (KAS) and Friedman methods. Interestingly, the activation energy of the pyrolysis process reflects both the peculiar composition (related to the specific wooden taxon) and the conservation state of the historical woods. In this regards, we estimated that the average activation energies obtained from KAS analysis are 203, 156 and 43 kJ mol−1 for Swietenia mahagoni, Picea abies and Juglans regia woods, respectively. The thermogravimetric parameters have been correlated to the lignin index of the woods by proper experimental equations, which can be considered as a novel protocol to estimate the preservation conditions of historical woods from different taxon
Time relaxation of microwave second order response of superconductors in the critical state
Relaxation of the microwave second order response of YBa2Cu3O7 and
Ba0.6K0.4BiO3 crystals in the critical state is studied. The samples are
exposed to static and pulsed microwave magnetic fields. The second harmonic
signals decay during the time in which the microwave pulse endures. The decay
times depend on the supercon-ductor investigated and on the way the value of
the static field has been reached.Comment: 2 pages, 1 figur
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