735 research outputs found
Methanol Masers as Tracers of Circumstellar Disks
We show that in many methanol maser sources the masers are located in lines,
with a velocity gradient along them which suggests that the masers are situated
in edge-on circumstellar, or protoplanetary, disks. We present VLBI
observations of the methanol maser source G309.92+0.48, in the 12.2 GHz
transition, which confirm previous observations that the masers in this source
lie along a line. We show that such sources are not only linear in space but,
in many cases, also have a linear velocity gradient. We then model these and
other data in both the 6.7 GHz and the 12.2 GHz transition from a number of
star formation regions, and show that the observed spatial and velocity
distribution of methanol masers, and the derived Keplerian masses, are
consistent with a circumstellar disk rotating around an OB star. We consider
this and other hypotheses, and conclude that about half of these methanol
masers are probably located in edge-on circumstellar disks around young stars.
This is of particular significance for studies of circumstellar disks because
of the detailed velocity information available from the masers.Comment: 38 pages, 13 figures accepted by Ap
Casimir attraction in multilayered plane parallel magnetodielectric systems
A powerful procedure is presented for calculating the Casimir attraction
between plane parallel multilayers made up of homogeneous regions with
arbitrary magnetic and dielectric properties by use of the Minkowski
energy-momentum tensor. The theory is applied to numerous geometries and shown
to reproduce a number of results obtained by other authors. Although the
various pieces of theory drawn upon are well known, the relative ease with
which the Casimir force density in even complex planar structures may be
calculated, appears not to be widely appreciated, and no single paper to the
author's knowledge renders explicitly the procedure demonstrated herein.
Results may be seen as an important building block in the settling of issues of
fundamental interest, such as the long-standing dispute over the thermal
behaviour of the Casimir force or the question of what is the correct stress
tensor to apply, a discussion re-quickened by the newly suggested alternative
theory due to Raabe and Welsch.Comment: 13 pages, 6 figures. Version 2: Updated contact details. Minor
changes and correction
ANALISIS PEMAHAMAN KONSEP MATEMATIKA SISWA DENGAN MODEL PEMBELAJARAN PROBLEM BASED LEARNING PADA POKOK BAHASAN SEGITIGA (Study deskriptif di Kelas VII SMP Negeri 2 Plered Kab. Cirebon)
Anggun Maya Sari. NIM 59451058. “Analisis Pemahaman Konsep
Matematika Siswa dengan Model Pembelajaran Problem Based Learning
pada Pokok Bahasan Segitiga”. (Studi Deskriptif di Kelas VII SMP Negeri 2
Plered).
Pembelajaran yang efektif adalah pembelajaran yang menyediakan
kesempatan kepada siswa untuk belajar mandiri, sehingga dalam prosesnya siswa
dapat memperoleh pemahaman dan pengetahuan dengan lebih mendalam. Akan
tetapi proses kegiatan pembelajaran matematika yang berlangsung disekolah, pada
umumnya guru hanya sekedar penyampai informasi tanpa mempertimbangkan
seberapa jauh pemahaman siswa terhadap konsep matematika dari pokok bahasan
segitiga yang disampaikan.
Tujuan penelitian ini adalah 1) mengetahui seberapa besar pemahaman
konsep yang dimiliki peserta didik; 2) mengetahui factor pendukung dan
penghambat peserta didik dalam memahami konsep yang diberikan; 3)
mengetahui deskripsi aktivitas siswa dalam memahami konsep matematika
melalui model pembelajaran Problem Based Learning; 4) mengetahui respon
siswa setelah melaksanakan model pembelajaran Problem Based Learning; 5)
mengetahui sejauhmana pemahaman konsep matematika siswa setelah mengikuti
proses pembelajaran Problem Based Learning.
Penerapan model pembelajaran Problem Based Learning (PBL)
merupakan salah satu model pembelajaran yang dapat memberikan kondisi belajar
aktif kepada siswa. Dengan diterapkannya pembelajaran matematika dengan
model pembelajaran PBL, diharapkan dapat membantu siswa dalam memahami
konsep matematika.
Penelitian ini menggunakan pendekatan kualitatif dan kuantitatif dengan
metode deskriptif. Pengumpulan data menggunakan lembar observasi, pedoman
wawancara, angket dan tes. Informan dalam penelitian ini adalah kelas VII yang
sudah dibentuk kelompok kelas, yaitu VII A, VII D dan VII F. dengan
menggunakan proportionate stratified random sampling, maka dari masingmasing
kelompok kelas didapat 18 siswa sebagai informan penelitian.
Berdasarkan dari hasil wawancara tentang siswa dalam pemahaman konsep
matematika, diperoleh siswa masih kurang dan masih perlu banyak bimbingan.
Aktivitas siswa selama pembelajaran melalui model pembelajaran PBL diperoleh
dari hasil lembar observasi dengan nilai rata-rata dari semua aspek sebesar 54,39%
yang termasuk dalam kategori sedang. Berdasarkan angket respon siswa setelah
dilaksanakan model pembelajaran PBL termasuk dalam kategori baik dengan nilai
rata-rata 76,44%. Pemahaman konsep matematika siswa setelah mengikuti proses
pembelajaran dengan model pembelajaran PBL diperoleh siswa lebih rajin dalam
mencari bahan untuk menyelesaikan soal-soal yang diberikan dan menambah
pemahaman siswa mengenai konsep matematika, meski masih belum 100% benar,
dan hasil tes didapat nilai rata-rata 39% yang termasuk dalam kategori sangat
kurang.
Kata Kunci: Pemahaman konsep matematika, Problem Based Learning, Segitig
Frequency-dependent Drude damping in Casimir force calculations
The Casimir force is calculated between Au thin films that are described by a
Drude model with a frequency dependent damping function. The model parameters
are obtained from available experimental data for Au thin films. Two cases are
considered; annealed and nonannealed films that have a different damping
function. Compared with the calculations using a Drude model with a constant
damping parameter, we observe changes in the Casimir force of a few percent.
This behavior is only observed in films of no more than 300 thick.Comment: Proceedings of the meeting "60 years of Casimir effect", Brasilia,
200
A Compact Extreme Scattering Event Cloud Towards AO 0235+164
We present observations of a rare, rapid, high amplitude Extreme Scattering
Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is
compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance
of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a
minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and
timescale of the ESE observed here, we suggest that at least one of the
transients reported by Bower et al. (2007) may be attributed to ESEs.Comment: 11 pages, 2 figure
Sign of the Casimir-Polder interaction between atoms and oil-water interfaces: Subtle dependence on dielectric properties
We demonstrate that Casimir-Polder energies between noble gas atoms
(dissolved in water) and oil-water interfaces are highly surface specific. Both
repulsion (e.g. hexane) and attraction (e.g. glycerine and cyclodecane) is
found with different oils. For several intermediate oils (e.g. hexadecane,
decane, and cyclohexane) both attraction and repulsion can be found in the same
system. Near these oil-water interfaces the interaction is repulsive in the
non-retarded limit and turns attractive at larger distances as retardation
becomes important. These highly surface specific interactions may have a role
to play in biological systems where the surface may be more or less accessible
to dissolved atoms.Comment: 5 pages, 6 figure
Cliophysics: Socio-political Reliability Theory, Polity Duration and African Political (In)stabilities
Quantification of historical sociological processes have recently gained
attention among theoreticians in the effort of providing a solid theoretical
understanding of the behaviors and regularities present in sociopolitical
dynamics. Here we present a reliability theory of polity processes with
emphases on individual political dynamics of African countries. We found that
the structural properties of polity failure rates successfully capture the risk
of political vulnerability and instabilities in which 87.50%, 75%, 71.43%, and
0% of the countries with monotonically increasing, unimodal, U-shaped and
monotonically decreasing polity failure rates, respectively, have high level of
state fragility indices. The quasi-U-shape relationship between average polity
duration and regime types corroborates historical precedents and explains the
stability of the autocracies and democracies.Comment: 4 pages, 3 figures, 1 tabl
Casimir-Foucault interaction: Free energy and entropy at low temperature
It was recently found that thermodynamic anomalies which arise in the Casimir
effect between metals described by the Drude model can be attributed to the
interaction of fluctuating Foucault (or eddy) currents [Phys. Rev. Lett. 103,
130405 (2009)]. We show explicitly that the two leading terms of the
low-temperature correction to the Casimir free energy of interaction between
two plates, are identical to those pertaining to the Foucault current
interaction alone, up to a correction which is very small for good metals.
Moreover, a mode density along real frequencies is introduced, showing that the
Casimir free energy, as given by the Lifshitz theory, separates in a natural
manner in contributions from eddy currents and propagating cavity modes,
respectively. The latter have long been known to be of little importance to the
low-temperature Casimir anomalies. This convincingly demonstrates that eddy
current modes are responsible for the large temperature correction to the
Casimir effect between Drude metals, predicted by the Lifshitz theory, but not
observed in experiments.Comment: 10 pages, 1 figur
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