{"id":5521,"date":"2019-05-13T19:28:36","date_gmt":"2019-05-13T17:28:36","guid":{"rendered":"http:\/\/liceocuneo.it\/oddenino\/?page_id=5521"},"modified":"2022-03-06T11:29:04","modified_gmt":"2022-03-06T10:29:04","slug":"si-e-costanti","status":"publish","type":"page","link":"https:\/\/liceocuneo.it\/oddenino\/fisica\/si-e-costanti\/","title":{"rendered":"SI e costanti"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">La Conferenza Generale sui Pesi e le Misure (CGPM), in occasione della sua 26\u00aa riunione, decide che, a partire dal 20 maggio 2019, il Sistema Internazionale delle Unit\u00e0 (SI) \u00e8 il sistema di unit\u00e0 in cui:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>la frequenza di transizione iperfine dello stato fondamentale imperturbato dell&#8217;atomo di cesio 133 \u0394\u03bd<sub>Cs<\/sub> \u00e8 9 192 631 770 Hz,<\/li><li>la velocit\u00e0 della luce nel vuoto <i>c<\/i> \u00e8 299 792 458 m\/s,<\/li><li>la costante di Planck <i>h<\/i> \u00e8 6,626 070 15 \u00d7 10<sup>\u201334<\/sup> J s,<\/li><li>la carica fondamentale <i>e<\/i> \u00e8 1,602 176 634 \u00d7 10<sup>\u201319<\/sup> C,<\/li><li>la costante di Boltzmann <i>k<\/i> \u00e8 1,380 649 \u00d7 10<sup>\u201323<\/sup> J\/K,<\/li><li>la costante di Avogadro <i>N<\/i><sub>A<\/sub> \u00e8 6,022 140 76 \u00d7 10<sup>23<\/sup> mol<sup>\u20131<\/sup>,<\/li><li>l&#8217;efficienza luminosa della radiazione monocromatica di frequenza 540 \u00d7 10<sup>12<\/sup> Hz, <i>K<\/i><sub>cd<\/sub>, \u00e8 683 lm\/W,<\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">dove hertz (Hz), joule (J), coulomb (C), lumen (lm) e watt (W) sono legate alle unit\u00e0 secondo (s), metro (m), kilogrammo (kg), ampere (A), kelvin (K), mole (mol) e candela (cd) dalle seguenti relazioni:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Hz = s<sup>\u20131<\/sup><\/li><li>J = kg m<sup>2<\/sup> s<sup>\u20132<\/sup><\/li><li>C = A s<\/li><li>lm = cd m<sup>2<\/sup> m<sup>\u20132<\/sup> = cd sr<\/li><li>W = kg m<sup>2<\/sup> s<sup>\u20133<\/sup> <\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Partendo dalla nuova definizione del SI descritta sopra in termini di valori numerici fissi delle costanti fondamentali, le definizioni di ciascuna delle sette unit\u00e0 di base sono dedotte prendendo, a seconda dei casi, una o pi\u00f9 di queste costanti per fornire la seguente serie di definizioni, ufficialmente in vigore dal 20 maggio 2019:<\/p>\n\n\n\n<p class=\"has-text-align-center wp-block-paragraph\"><em>in fase di traduzione<\/em><\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency \u0394\u03bd<sub>Cs<\/sub>, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s<sup>\u20131<\/sup>.<\/li><li>The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum <i>c<\/i> to be 299 792 458 when expressed in the unit m\/s, where the second is defined in terms of \u0394\u03bd<sub>Cs<\/sub>.<\/li><li>The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant <i>h<\/i> to be 6.626 070 15 \u00d7 10<sup>\u201334<\/sup> when expressed in the unit J s, which is equal to kg m<sup>2<\/sup> s<sup>\u20131<\/sup>, where the metre and the second are defined in terms of <i>c<\/i> and \u0394\u03bd<sub>Cs<\/sub>. <\/li><li>The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge <i>e<\/i> to be 1.602 176 634 \u00d7 10<sup>\u201319<\/sup> when expressed in the unit C, which is equal to A s, where the second is defined in terms of \u0394\u03bd<sub>Cs<\/sub>. <\/li><li>The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant <i>k<\/i> to be 1.380 649 \u00d7 10<sup>\u201323<\/sup> when expressed in the unit J K<sup>\u20131<\/sup>, which is equal to kg m<sup>2<\/sup> s<sup>\u20132<\/sup> K<sup>\u20131<\/sup>, where the kilogram, metre and second are defined in terms of <i>h<\/i>, <i>c<\/i> and \u0394\u03bd<sub>Cs<\/sub>.<\/li><li>The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 \u00d7 10<sup>23<\/sup> elementary entities. This number is the fixed numerical value of the Avogadro constant, <i>N<\/i><sub>A<\/sub>, when expressed in the unit mol<sup>\u20131<\/sup> and is called the Avogadro number.  <br>The amount of substance, symbol<i> n<\/i>, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. <\/li><li>The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 \u00d7 10<sup>12<\/sup> Hz, K<sub>cd<\/sub>, to be 683 when expressed in the unit lm W<sup>\u20131<\/sup>, which is equal to cd sr W<sup>\u20131<\/sup>, or cd sr kg<sup>\u20131<\/sup> m<sup>\u20132<\/sup> s<sup>3<\/sup>, where the kilogram, metre and second are defined in terms of <i>h<\/i>, <i>c<\/i> and \u0394\u03bd<sub>Cs<\/sub>.<\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Link al sito ufficiale: <a href=\"https:\/\/www.bipm.org\/en\/CGPM\/db\/26\/1\/\">https:\/\/www.bipm.org\/en\/CGPM\/db\/26\/1\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>La Conferenza Generale sui Pesi e le Misure (CGPM), in occasione della sua 26\u00aa riunione, decide che, a partire dal 20 maggio 2019, il Sistema Internazionale delle Unit\u00e0 (SI) \u00e8 il sistema di unit\u00e0 in cui: la frequenza di transizione iperfine dello stato fondamentale imperturbato dell&#8217;atomo di cesio 133 \u0394\u03bdCs \u00e8 9 192 631 770 &hellip; <a href=\"https:\/\/liceocuneo.it\/oddenino\/fisica\/si-e-costanti\/\" class=\"more-link\">Continua a leggere <span class=\"screen-reader-text\">SI e costanti<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"parent":244,"menu_order":10,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-5521","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/pages\/5521","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/comments?post=5521"}],"version-history":[{"count":0,"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/pages\/5521\/revisions"}],"up":[{"embeddable":true,"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/pages\/244"}],"wp:attachment":[{"href":"https:\/\/liceocuneo.it\/oddenino\/wp-json\/wp\/v2\/media?parent=5521"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}