想打 asked in 科學化學 · 9 years ago

熱力學定律

熱力學第一定律為 ΔE = q - w

但是下面又說

ΔE = q + w

小弟完全搞不懂

可否幫忙解釋一下

ΔE = ΔH and ΔH = qp

With systems at constant pressure (P) then:

ΔE = q + w

ie. the change in internal energy of a system is equal to the

energy that passes into the system as heat or as work

Where H = enthalpy and is effectively equivalent to the heat supplied to

the system (at constant pressure) ie qp

Δ indicates a change is determined from the difference between the

final and initial state

Conservation of energy:

the change in system’s energy is balanced by an equal and opposite

change in the energy of its surroundings.

Note: if heat or work is transferred out of the system into the surroundings

this is indicated by giving the quantity a negative sign. The sign only

indicates the direction of energy flow

Update:

以上來源出自於ppt檔案

檔案內文來源可能為biochemistry 出版社:brooks/cole

1 Answer

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  • 9 years ago
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    ΔE是內能

    Q是熱能

    W是作功

    單單從公式上來看

    內能就是熱能和功的總和

    先來講熱

    放熱的時候內能會減少

    所以熱能會小於零,吸熱的時候則反之

    同樣的道理

    作功也是如此

    ΔE = q + w

    ΔE = q - w

    這兩條哪一個對?

    每一本原文書都不一樣

    但是道理是一樣

    先看第一條公式

    ΔE = q + w

    當系統對外作功的時候W<0,當環境對系統作功的時候W>0

    記好上面

    再來看看第二條

    ΔE = q - w

    當系統對外作功的時候W>0,當環境對系統作功的時候W<0

    解釋物理意義會隨著公式而變

    但是存在的真理卻是不變

    還是不懂的話

    我就非常簡單的說

    "當系統對外作功時,內能會減少,當環境對系統作功時則反之"

    然後你再仔細比較那兩條公式

    再來你可能會問

    那考試要寫哪一條?

    就像我紅色的字寫出來讓閱卷老師知道就可以

    哪一條公式都是對的

    端看你怎麼去詮釋它

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