{"id":47276,"date":"2019-11-03T20:56:26","date_gmt":"2019-11-03T19:56:26","guid":{"rendered":"https:\/\/www.thermal-engineering.org\/o-que-e-a-lei-de-stefan-boltzmann-constante-de-stefan-boltzmann-definicao\/"},"modified":"2020-01-23T08:37:47","modified_gmt":"2020-01-23T07:37:47","slug":"o-que-e-a-lei-de-stefan-boltzmann-constante-de-stefan-boltzmann-definicao","status":"publish","type":"post","link":"https:\/\/www.thermal-engineering.org\/pt-br\/o-que-e-a-lei-de-stefan-boltzmann-constante-de-stefan-boltzmann-definicao\/","title":{"rendered":"O que \u00e9 a lei de Stefan-Boltzmann – constante de Stefan-Boltzmann – defini\u00e7\u00e3o"},"content":{"rendered":"
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A lei de Stefan-Boltzmann fornece a intensidade radiante de um \u00fanico objeto.\u00a0A constante Stefan-Boltzmann recebe o nome de Josef Stefan e Ludwig Boltzman.\u00a0Engenharia T\u00e9rmica<\/div>\n<\/div>\n
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Direito de Stefan \u2013 Boltzmann<\/h2>\n

A<\/strong>\u00a0taxa de\u00a0transfer\u00eancia de calor por radia\u00e7\u00e3o<\/strong>\u00a0, q [W \/ m\u00a02<\/sup>\u00a0], de um corpo (por exemplo, um corpo preto) para seus arredores \u00e9 proporcional \u00e0\u00a0quarta pot\u00eancia<\/strong>\u00a0da\u00a0temperatura absoluta<\/a>\u00a0e pode ser expressa pela seguinte equa\u00e7\u00e3o:<\/p>\n

q = \u03b5\u03c3T\u00a0<\/em>4<\/sup><\/em><\/strong><\/p>\n

onde\u00a0\u03c3<\/strong>\u00a0\u00e9 uma constante f\u00edsica fundamental chamada constante de\u00a0Stefan-Boltzmann<\/strong>\u00a0, que \u00e9 igual a\u00a05.6697 \u00d7 10\u00a0<\/strong>-8<\/sup><\/strong>\u00a0W \/ m\u00a0<\/strong>2<\/sup><\/strong>\u00a0K\u00a0<\/strong>4<\/sup><\/strong>\u00a0.\u00a0A\u00a0constante Stefan-Boltzmann recebeu o<\/strong>\u00a0nome de Josef Stefan (que descobriu a lei de Stefa-Boltzman experimentalmente em 1879) e Ludwig Boltzmann (que a derivou teoricamente logo depois).\u00a0Como pode ser visto, a transfer\u00eancia de calor por radia\u00e7\u00e3o \u00e9 importante\u00a0em temperaturas muito altas<\/strong>\u00a0e\u00a0no v\u00e1cuo<\/strong> .<\/p>\n<\/div>\n<\/div>\n

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Como foi escrito, a\u00a0<\/span>lei de Stefan-Boltzmann\u00a0\u00a0<\/span>fornece a intensidade radiante de um \u00fanico objeto<\/span><\/strong>\u00a0.\u00a0Mas, usando a\u00a0<\/span>lei de Stefan-Boltzmann<\/span><\/strong>\u00a0, tamb\u00e9m podemos determinar a transfer\u00eancia de calor por radia\u00e7\u00e3o entre dois objetos.\u00a0Dois corpos que irradiam um para o outro t\u00eam um fluxo l\u00edquido de calor entre eles.\u00a0A vaz\u00e3o l\u00edquida de calor entre eles \u00e9 dada por:<\/span>Q = \u03b5\u03c3A\u00a0<\/span><\/em><\/strong>1-2<\/span><\/sub><\/em><\/strong>\u00a0(T\u00a0<\/span><\/em><\/strong>4\u00a0<\/span><\/sup><\/em><\/strong>1<\/span><\/sub><\/em><\/strong>\u00a0-T\u00a0<\/span><\/em><\/strong>4\u00a0<\/span><\/sup><\/em><\/strong>2<\/span><\/sub><\/em><\/strong>\u00a0) [J \/ s]<\/span><\/em><\/strong><\/p>\n

q = \u03b5\u03c3 (T\u00a0<\/span><\/em><\/strong>4\u00a0<\/span><\/sup><\/em><\/strong>1 \u2212<\/span><\/sub><\/em><\/strong>\u00a0T\u00a0<\/span><\/em><\/strong>4\u00a0<\/span><\/sup><\/em><\/strong>2<\/span><\/sub><\/em><\/strong>\u00a0) [J \/ m\u00a0<\/span><\/em><\/strong>2<\/span><\/sup><\/em><\/strong>\u00a0s]<\/span><\/em><\/strong><\/p>\n

O fator de \u00e1rea A\u00a0<\/span>1-2<\/span><\/sub>\u00a0\u00e9 a \u00e1rea visualizada pelo corpo 2 do corpo 1 e pode se tornar bastante dif\u00edcil de calcular.<\/span><\/p>\n<\/div>\n<\/div>\n

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Radia\u00e7\u00e3o de corpo negro<\/span><\/h2>\n

Sabe-se que a quantidade de energia de radia\u00e7\u00e3o emitida a partir de uma superf\u00edcie em um determinado comprimento de onda depende do\u00a0<\/span>material<\/span><\/strong>\u00a0do corpo e das condi\u00e7\u00f5es de sua\u00a0<\/span>superf\u00edcie<\/span><\/strong>\u00a0, bem como da\u00a0<\/span>temperatura<\/span><\/strong><\/a>\u00a0da superf\u00edcie\u00a0.\u00a0Portanto, v\u00e1rios materiais emitem quantidades diferentes de energia radiante, mesmo quando est\u00e3o na mesma temperatura.\u00a0Um\u00a0<\/span>corpo<\/span><\/strong>\u00a0que emite a\u00a0<\/span>quantidade m\u00e1xima<\/span><\/strong>\u00a0de calor para sua temperatura absoluta \u00e9 chamado de\u00a0<\/span>corpo negro<\/span><\/strong>\u00a0.<\/span><\/p>\n

\"Radia\u00e7\u00e3o<\/a>Um\u00a0<\/span>corpo negro<\/span><\/strong>\u00a0\u00e9 um corpo f\u00edsico idealizado, com propriedades espec\u00edficas.\u00a0Por defini\u00e7\u00e3o, um corpo negro em equil\u00edbrio t\u00e9rmico tem uma\u00a0<\/span>emissividade<\/span><\/strong>\u00a0de\u00a0<\/span>\u03b5<\/span><\/em>\u00a0= 1,0<\/span><\/strong>\u00a0.\u00a0Objetos reais n\u00e3o irradiam tanto calor quanto um corpo preto perfeito.\u00a0Eles irradiam menos calor que um corpo preto e, portanto, s\u00e3o chamados corpos cinzentos.<\/span><\/p>\n

A superf\u00edcie de um corpo negro emite radia\u00e7\u00e3o t\u00e9rmica a uma taxa de aproximadamente 448 watts por metro quadrado em temperatura ambiente (25 \u00b0 C, 298,15 K).\u00a0Objetos reais com emissividades inferiores a 1,0 (por exemplo, fio de cobre) emitem radia\u00e7\u00e3o a taxas correspondentemente mais baixas (por exemplo, 448 x 0,03 = 13,4 W \/ m\u00a0<\/span>2<\/span><\/sup>\u00a0).\u00a0<\/span>A emissividade<\/span><\/strong>\u00a0desempenha papel importante nos problemas de transfer\u00eancia de calor.\u00a0Por exemplo, coletores solares de calor incorporam superf\u00edcies seletivas com emissividades muito baixas.\u00a0Esses coletores desperdi\u00e7am muito pouco da energia solar atrav\u00e9s da emiss\u00e3o de radia\u00e7\u00e3o t\u00e9rmica.<\/span><\/p>\n

Como a\u00a0<\/span>absor\u00e7\u00e3o<\/span><\/strong>\u00a0e a\u00a0<\/span>emissividade<\/span><\/strong>\u00a0est\u00e3o interconectadas pela\u00a0<\/span>Lei de Kirchhoff da radia\u00e7\u00e3o t\u00e9rmica<\/span><\/strong>\u00a0, um\u00a0<\/span>corpo negro<\/span><\/strong>\u00a0tamb\u00e9m \u00e9 um absorvedor perfeito da radia\u00e7\u00e3o eletromagn\u00e9tica.<\/span><\/p>\n

Lei de Kirchhoff da radia\u00e7\u00e3o t\u00e9rmica<\/span><\/strong>\u00a0:<\/span><\/em><\/p>\n

Para um corpo arbitr\u00e1rio que emite e absorve radia\u00e7\u00e3o t\u00e9rmica em equil\u00edbrio termodin\u00e2mico, a emissividade \u00e9 igual \u00e0 absor\u00e7\u00e3o.<\/span><\/em><\/p>\n

emissividade \u03b5 = absortividade \u03b1<\/span><\/strong><\/p>\n

Um\u00a0<\/span>corpo negro<\/span><\/strong>\u00a0absorve toda a radia\u00e7\u00e3o eletromagn\u00e9tica incidente, independentemente da frequ\u00eancia ou \u00e2ngulo de incid\u00eancia.\u00a0Sua\u00a0<\/span>absor\u00e7\u00e3o<\/span><\/strong>\u00a0\u00e9, portanto, igual \u00e0 unidade, que tamb\u00e9m \u00e9 o valor mais alto poss\u00edvel.\u00a0Ou seja, um\u00a0<\/span>corpo negro<\/span><\/strong>\u00a0\u00e9 um\u00a0<\/span>absorvedor perfeito<\/span><\/strong>\u00a0(e um\u00a0<\/span>emissor perfeito<\/span><\/strong>\u00a0).<\/span><\/p>\n

Note que a radia\u00e7\u00e3o vis\u00edvel ocupa uma faixa muito estreita do espectro de 0,4 a 0,76 nm, n\u00e3o podemos fazer julgamentos sobre a escurid\u00e3o de uma superf\u00edcie com base em observa\u00e7\u00f5es visuais.\u00a0Por exemplo, considere o papel branco que reflete a luz vis\u00edvel e, portanto, parece branco.\u00a0Por outro lado, \u00e9 essencialmente preto para radia\u00e7\u00e3o infravermelha (capacidade de\u00a0<\/span>absor\u00e7\u00e3o \u03b1 = 0,94<\/span><\/strong>\u00a0), pois eles absorvem fortemente a radia\u00e7\u00e3o de comprimento de onda longo.<\/span><\/p>\n<\/div>\n<\/div>\n

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Este artigo \u00e9 baseado na tradu\u00e7\u00e3o autom\u00e1tica do artigo original em ingl\u00eas. Para mais informa\u00e7\u00f5es, consulte o artigo em ingl\u00eas. Voc\u00ea pode nos ajudar. Se voc\u00ea deseja corrigir a tradu\u00e7\u00e3o, envie-a para: translations@nuclear-power.com ou preencha o formul\u00e1rio de tradu\u00e7\u00e3o on-line. Agradecemos sua ajuda, atualizaremos a tradu\u00e7\u00e3o o mais r\u00e1pido poss\u00edvel. Obrigado.<\/p>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

A lei de Stefan-Boltzmann fornece a intensidade radiante de um \u00fanico objeto.\u00a0A constante Stefan-Boltzmann recebe o nome de Josef Stefan e Ludwig Boltzman.\u00a0Engenharia T\u00e9rmica Direito de Stefan \u2013 Boltzmann A\u00a0taxa de\u00a0transfer\u00eancia de calor por radia\u00e7\u00e3o\u00a0, q [W \/ m\u00a02\u00a0], de um corpo (por exemplo, um corpo preto) para seus arredores \u00e9 proporcional \u00e0\u00a0quarta pot\u00eancia\u00a0da\u00a0temperatura absoluta\u00a0e … Ler mais<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[14],"tags":[],"yoast_head":"\nO que \u00e9 a lei de Stefan \u2013 Boltzmann - constante de Stefan-Boltzmann - defini\u00e7\u00e3o<\/title>\n<meta name=\"description\" content=\"A lei de Stefan-Boltzmann fornece a intensidade radiante de um \u00fanico objeto. 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