Loi en carré inverse | EPFL Graph Search

En physique, une loi en carré inverse est une loi physique postulant qu'une quantité physique (énergie, force, ou autre) est inversement proportionnelle au carré de la distance de l'origine de cette quantité physique. Histoire Cette loi fut d'abord suggérée en 1645 par l'astronome français Ismaël Boulliau dans son livre Astronomica Philolaica, puis mise en forme par Isaac Newton en 1687 après que Robert Hooke lui eut proposé l'idée dans une lettre datée du . Hooke accusa plus tard Newton de plagiat lors de la publication en 1687 de ses Principia Mathematica dans lesquels il introduisait et établissait le même concept. Champs d'application En acoustique Acoustique La loi du carré inverse se trouve en propagation du son lorsqu'il s'agit de donner son intensité à une distance donnée d'une source sonore. La pression sonore d'un front d'onde sphérique irradiant d'un point source décroit d'un facteur ½ lorsque la distance r est doublée, ou,

Urban Greenhouse Gas Emissions Monitoring in Davos, Switzerland, Before, During and After the World Economic Forum Annual Meeting 2012

Michael Lehning

Efforts to reduce anthropogenic greenhouse gas emissions require validation. Atmospheric measurements capture all emissions, and provide a unique and powerful means of continuous validation and feedback. To demonstrate the utility of real time greenhouse gas measurements, in-situ GHG mixing ratio instruments were deployed in Davos, Switzerland to measure emissions from the city before, during and after theWorld Economic Forum (WEF). Three Instruments were deployed at two separate locations over 3 months (late December 2011 to February 2012). One site was located in the middle of the Davos urban area and a second site was located out of the valley in the surrounding mountains. Carbon Dioxide (CO2), Methane (CH4), Carbon Monoxide (CO) and water vapor (H2O) were measured continuously by Picarro G2401 instruments at both sites. Additionally, a Picarro flux analyzer was deployed in the city to evaluate the inverse fluxes. The mesoscale atmospheric model, WRF nudged to meteorological observations (WRF-FDDA), was used to simulate the transport of GHG over the valley of Davos at 1.3km resolution. A Mini Micro Pulse LiDAR (MiniMPL) from Sigma Space was deployed to evaluate the simulated planetary boundary layer depth from the WRF-FDDA model. The initial flux estimates for CO2 were constructed based on inventories reported for 2005. CO2 mixing ratio measurements prior to WEF suggest the difference between modeled (real-time) and inventory (annual) emissions to be on the order of +40%. The enhancement is likely due to the increased use of heating fuel in the winter.We present here the temporal variability in the inverse fluxes, which are correlated with a cold wave severely affectingWestern Europe during the past winter, as well as changes in anthropogenic activities during the week of the WEF meeting. Also presented are new analyses of composite diurnal cycles of hourly CO/CO2 ratios, which provide additional information on the contributions of traffic relative to heating fuel. The absence of traffic peaks during the WEF meeting, indicate a change in road emissions potentially responsible for the observed decrease in the city emissions during the meeting. Acknowledgments: Calibration tanks were provided by C. Sweeney, NOAA ESRL.

2013

Atomic-scale sensing of the magnetic dipolar field from single atoms

Donat Fabian Natterer

Spin resonance provides the high-energy resolution needed to determine biological and material structures by sensing weak magnetic interactions(1). In recent years, there have been notable achievements in detecting(2) and coherently controlling(3-7) individual atomic-scale spin centres for sensitive local magnetometry(8-10). However, positioning the spin sensor and characterizing spin-spin interactions with sub-nanometre precision have remained outstanding challenges(11,12). Here, we use individual Fe atoms as an electron spin resonance (ESR) sensor in a scanning tunnelling microscope to measure the magnetic field emanating from nearby spins with atomic-scale precision. On artificially built assemblies of magnetic atoms (Fe and Co) on a magnesium oxide surface, we measure that the interaction energy between the ESR sensor and an adatom shows an inverse-cube distance dependence (r(-3.01+/-0.04)). This demonstrates that the atoms are predominantly coupled by the magnetic dipole-dipole interaction, which, according to our observations, dominates for atom separations greater than 1 nm. This dipolar sensor can determine the magnetic moments of individual adatoms with high accuracy. The achieved atomic-scale spatial resolution in remote sensing of spins may ultimately allow the structural imaging of individual magnetic molecules, nanostructures and spin-labelled biomolecules.

Nature Publishing Group2017

Urban Greenhouse Gas Emissions Monitoring in Davos, Switzerland, Before, During and After the World Economic Forum Annual Meeting 2012

Michael Lehning

2013