{"id":48255,"date":"2019-11-08T20:25:29","date_gmt":"2019-11-08T19:25:29","guid":{"rendered":"https:\/\/www.thermal-engineering.org\/o-que-e-a-formula-da-primeira-lei-da-termodinamica-definicao\/"},"modified":"2020-01-28T14:11:19","modified_gmt":"2020-01-28T13:11:19","slug":"o-que-e-a-formula-da-primeira-lei-da-termodinamica-definicao","status":"publish","type":"post","link":"https:\/\/www.thermal-engineering.org\/pt-br\/o-que-e-a-formula-da-primeira-lei-da-termodinamica-definicao\/","title":{"rendered":"O que \u00e9 a F\u00f3rmula da Primeira Lei da Termodin\u00e2mica – Defini\u00e7\u00e3o"},"content":{"rendered":"
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Equa\u00e7\u00f5es – F\u00f3rmulas da Primeira Lei da Termodin\u00e2mica.\u00a0Existem muitas formas e equa\u00e7\u00f5es da primeira lei da termodin\u00e2mica.\u00a0V\u00e1rios formul\u00e1rios est\u00e3o listados aqui.\u00a0Engenharia T\u00e9rmica<\/div>\n<\/div>\n
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F\u00f3rmulas da Primeira Lei da Termodin\u00e2mica<\/h2>\n

O aumento da energia interna de um sistema fechado \u00e9 igual ao calor fornecido ao sistema menos o trabalho realizado por ele.<\/strong><\/em><\/p>\n

F\u00f3rmula:<\/strong><\/p>\n

IntE\u00a0int<\/sub>\u00a0= Q – W<\/strong><\/p>\n

Esta \u00e9 a\u00a0Primeira Lei da Termodin\u00e2mica<\/strong>\u00a0e \u00e9 o\u00a0princ\u00edpio da conserva\u00e7\u00e3o de energia<\/a>\u00a0, o que significa que a\u00a0energia\u00a0<\/a>n\u00e3o<\/strong>\u00a0pode\u00a0ser criada nem destru\u00edda<\/strong>\u00a0, mas transformada em v\u00e1rias formas \u00e0 medida que o fluido dentro do volume de controle est\u00e1 sendo estudado.<\/p>\n

\u00c9 a lei mais importante para an\u00e1lise da maioria dos sistemas e a que quantifica como\u00a0a energia t\u00e9rmica<\/strong><\/a>\u00a0\u00e9 transformada em outras\u00a0formas de energia<\/a>\u00a0.\u00a0Segue-se que m\u00e1quinas perp\u00e9tuas de movimento do primeiro tipo s\u00e3o imposs\u00edveis.<\/p>\n<\/div>\n<\/div>\n

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Forma diferencial:<\/h2>\n

Forma diferencial:<\/strong><\/p>\n

dE\u00a0int<\/sub>\u00a0= dQ – dW<\/strong><\/p>\n

A energia interna E\u00a0int<\/sub>\u00a0de um sistema tende a aumentar se a energia \u00e9 adicionada como calor Q e tende a diminuir se a energia \u00e9 perdida como o trabalho W realizado pelo sistema.<\/p>\n<\/div>\n<\/div>\n

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Primeira lei em termos de entalpia dH = dQ + Vdp<\/h2>\n
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A\u00a0<\/span>entalpia<\/span><\/strong><\/a>\u00a0\u00e9 definida como sendo a soma da\u00a0<\/span>energia interna<\/span><\/a>\u00a0E mais o produto da\u00a0<\/span>press\u00e3o P<\/span><\/a>\u00a0e\u00a0<\/span>volume V<\/span><\/a>\u00a0.\u00a0Em muitas an\u00e1lises termodin\u00e2micas, a soma da energia interna U e o produto da press\u00e3o pe volume V aparece; portanto, \u00e9 conveniente atribuir \u00e0 combina\u00e7\u00e3o um nome,\u00a0<\/span>entalpia<\/span><\/strong>\u00a0e um s\u00edmbolo distinto, H.<\/span><\/p>\n

H = U + pV<\/span><\/strong><\/em><\/p>\n

Veja tamb\u00e9m:\u00a0<\/span>Entalpia<\/span><\/a><\/p>\n

A\u00a0<\/span>primeira lei da termodin\u00e2mica<\/span><\/strong>\u00a0em termos de\u00a0<\/span>entalpia<\/span><\/strong>\u00a0mostra-nos por que os engenheiros usam a entalpia em ciclos termodin\u00e2micos (por exemplo,\u00a0<\/span>ciclo de Brayton<\/span><\/strong>\u00a0ou\u00a0<\/span>ciclo de Rankine<\/span><\/strong>\u00a0).<\/span><\/p>\n

A forma cl\u00e1ssica da lei \u00e9 a seguinte equa\u00e7\u00e3o:<\/span><\/p>\n

dU = dQ – dW<\/span><\/strong><\/em><\/p>\n

Nesta equa\u00e7\u00e3o,\u00a0<\/span>dW<\/span><\/strong>\u00a0\u00e9 igual a\u00a0<\/span>dW = pdV<\/span><\/strong>\u00a0e \u00e9 conhecido como\u00a0<\/span>trabalho de fronteira<\/span><\/strong>\u00a0.<\/span><\/p>\n<\/div>\n<\/div>\n

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Como\u00a0<\/span>H = U + pV<\/span><\/em><\/strong>\u00a0, portanto\u00a0<\/span>dH = dU + pdV + Vdp<\/span><\/strong><\/em>\u00a0e substitu\u00edmos\u00a0<\/span>dU = dH – pdV – Vdp<\/span><\/strong><\/em>\u00a0na forma cl\u00e1ssica da lei:<\/span>dH – pdV – Vdp = dQ – pdV<\/span><\/strong><\/em><\/p>\n

Obtemos a lei em termos de entalpia:<\/span><\/p>\n

dH = dQ + Vdp<\/span><\/em><\/strong><\/p>\n

ou<\/span><\/p>\n

dH = TdS + Vdp<\/span><\/em><\/strong><\/p>\n

Nesta equa\u00e7\u00e3o, o termo\u00a0<\/span>Vdp<\/span><\/strong><\/em>\u00a0\u00e9 um\u00a0<\/span>trabalho de processo de fluxo.\u00a0<\/span><\/strong>Este trabalho, \u00a0\u00a0<\/span>Vdp<\/span><\/strong><\/em>\u00a0, \u00e9 usado para\u00a0<\/span>sistemas de fluxo aberto,<\/span><\/strong>\u00a0como uma\u00a0<\/span>turbina<\/span><\/strong>\u00a0ou uma\u00a0<\/span>bomba<\/span><\/strong>\u00a0na qual existe um\u00a0<\/span>“dp”<\/span><\/strong>\u00a0, ou seja, mudan\u00e7a de press\u00e3o.\u00a0N\u00e3o h\u00e1 altera\u00e7\u00f5es no\u00a0<\/span>volume de controle<\/span><\/a>\u00a0.\u00a0Como pode ser visto, essa forma de lei\u00a0<\/span>simplifica a descri\u00e7\u00e3o da transfer\u00eancia de energia<\/span><\/strong>\u00a0.\u00a0<\/span>A press\u00e3o constante<\/span><\/strong>\u00a0, a\u00a0<\/span>mudan\u00e7a de entalpia<\/span><\/strong>\u00a0\u00e9 igual \u00e0\u00a0<\/span>energia<\/span><\/strong>\u00a0transferida do ambiente atrav\u00e9s do aquecimento:<\/span><\/p>\n

Processo isob\u00e1rico (Vdp = 0):<\/span><\/strong><\/p>\n

dH = dQ \u00a0 \u00a0\u00a0\u00a0<\/span>\u2192<\/span>\u00a0\u00a0 \u00a0 \u00a0Q = H\u00a0<\/span><\/strong>2<\/span><\/sub><\/strong>\u00a0– H\u00a0<\/span><\/strong>1<\/span><\/sub><\/strong><\/p>\n

Na entropia constante<\/span><\/strong>\u00a0, ou seja, no processo isentr\u00f3pico, a\u00a0<\/span>mudan\u00e7a de entalpia<\/span><\/strong>\u00a0\u00e9 igual ao\u00a0<\/span>trabalho do processo de fluxo<\/span><\/strong>\u00a0realizado no ou pelo sistema:<\/span><\/p>\n

Processo isentr\u00f3pico (dQ = 0):<\/span><\/strong><\/p>\n

dH = Vdp\u00a0 \u00a0 \u00a0\u00a0<\/span>\u2192<\/span>\u00a0\u00a0 \u00a0 \u00a0W = H\u00a0<\/span><\/strong>2<\/span><\/sub><\/strong>\u00a0– H\u00a0<\/span><\/strong>1<\/span><\/sub><\/strong><\/p>\n

\u00c9 \u00f3bvio que ser\u00e1 muito \u00fatil na an\u00e1lise de ambos os ciclos termodin\u00e2micos usados \u200b\u200bna engenharia de energia, ou seja, no ciclo de Brayton e no ciclo de Rankine.<\/span><\/p>\n<\/div>\n<\/div>\n

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Trabalho p\u0394V<\/span><\/h2>\n
\"Trabalho<\/a>
p\u0394V O trabalho \u00e9 igual \u00e0 \u00e1rea sob a curva do processo plotada no diagrama press\u00e3o-volume.<\/span><\/figcaption><\/figure>\n

Exemplo:<\/span><\/strong><\/p>\n

Considere-se um pist\u00e3o de atrito que \u00e9 usado para proporcionar uma press\u00e3o constante de\u00a0<\/span>500 kPa<\/span><\/strong>\u00a0num cilindro contendo vapor (\u00a0<\/span>vapor sobreaquecido<\/span><\/a>\u00a0) de um volume de\u00a0<\/span>2 m\u00a0<\/span><\/strong>3<\/span><\/sup><\/strong>\u00a0\u00a0a\u00a0<\/span>500 K<\/span><\/strong>\u00a0.<\/span><\/p>\n

Calcule a temperatura final, se\u00a0<\/span>3000 kJ<\/span><\/strong>\u00a0de\u00a0<\/span>calor<\/span><\/strong>\u00a0forem adicionados.<\/span><\/p>\n

Solu\u00e7\u00e3o:<\/span><\/strong><\/p>\n

Usando\u00a0<\/span>tabelas de vapor<\/span><\/a>\u00a0, sabemos que a\u00a0<\/span>entalpia espec\u00edfica<\/span><\/strong>\u00a0desse vapor (500 kPa; 500 K) \u00e9 de cerca de\u00a0<\/span>2912 kJ \/ kg<\/span><\/strong>\u00a0.\u00a0Como nessa condi\u00e7\u00e3o o vapor possui densidade de 2,2 kg \/ m\u00a0<\/span>3<\/span><\/sup>\u00a0, sabemos que h\u00e1 cerca de\u00a0<\/span>4,4 kg de vapor<\/span><\/strong>\u00a0no pist\u00e3o na entalpia de 2912 kJ \/ kg x 4,4 kg =\u00a0<\/span>12812 kJ<\/span><\/strong>\u00a0.<\/span><\/p>\n

Quando usamos simplesmente\u00a0<\/span>Q = H\u00a0<\/span><\/strong>2<\/span><\/sub><\/strong>\u00a0– H\u00a0<\/span><\/strong>1<\/span><\/sub><\/strong>\u00a0, a entalpia resultante do vapor ser\u00e1:<\/span><\/p>\n

H\u00a0<\/span>2<\/span><\/sub>\u00a0= H\u00a0<\/span>1<\/span><\/sub>\u00a0+ Q =\u00a0<\/span>15812 kJ<\/span><\/strong><\/p>\n

Nas\u00a0<\/span>tabelas de vapor<\/span><\/strong>\u00a0, esse vapor superaquecido (15812 \/ 4,4 = 3593 kJ \/ kg) ter\u00e1 uma temperatura de\u00a0<\/span>828 K (555 \u00b0 C)<\/span><\/strong>\u00a0.\u00a0Uma vez que nesta entalpia do vapor tem a densidade de 1,31 kg \/ m\u00a0<\/span>3<\/span><\/sup>\u00a0, \u00e9 \u00f3bvio que se expandiu em cerca de 2,2 \/ 1,31 = 1,67 (+ 67%).\u00a0Portanto, o volume resultante \u00e9 de 2 m\u00a0<\/span>3<\/span><\/sup>\u00a0x 1,67 = 3,34 m\u00a0<\/span>3<\/span><\/sup>\u00a0e \u2206V = 3,34 m\u00a0<\/span>3<\/span><\/sup>\u00a0– 2 m\u00a0<\/span>3<\/span><\/sup>\u00a0= 1,34 m\u00a0<\/span>3<\/span><\/sup>\u00a0.<\/span><\/p>\n

A\u00a0parte\u00a0<\/span>p\u2206V<\/span><\/strong>\u00a0da entalpia, ou seja, o trabalho realizado \u00e9:<\/span><\/p>\n

W = p\u2206V = 500 000 Pa x 1,34 m\u00a0<\/span><\/strong>3<\/span><\/sup><\/strong>\u00a0= 670 kJ<\/span><\/strong><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n

……………………………………………………………………………………………………………………………….<\/p>\n

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","protected":false},"excerpt":{"rendered":"

Equa\u00e7\u00f5es – F\u00f3rmulas da Primeira Lei da Termodin\u00e2mica.\u00a0Existem muitas formas e equa\u00e7\u00f5es da primeira lei da termodin\u00e2mica.\u00a0V\u00e1rios formul\u00e1rios est\u00e3o listados aqui.\u00a0Engenharia T\u00e9rmica F\u00f3rmulas da Primeira Lei da Termodin\u00e2mica O aumento da energia interna de um sistema fechado \u00e9 igual ao calor fornecido ao sistema menos o trabalho realizado por ele. F\u00f3rmula: IntE\u00a0int\u00a0= Q – W … 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 F\u00f3rmula da Primeira Lei da Termodin\u00e2mica - Defini\u00e7\u00e3o<\/title>\n<meta name=\"description\" content=\"Equa\u00e7\u00f5es - F\u00f3rmulas da Primeira Lei da Termodin\u00e2mica. Existem muitas formas e equa\u00e7\u00f5es da primeira lei da termodin\u00e2mica. 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