[問題] 擴散
嗨 各位
最近重新看到關於擴散的定義(Fick's law)
發現之前好像都學錯了
擴散是從分子通量(flux)從高濃度到低濃度移動
但是如果單從各個分子來看時,各分子是隨機的運動(熱運動)
也就是在某瞬間,分子受到碰撞後往四面八方的機率是相同
例如 溶質分子受到周遭許多水分子撞擊(合力方向無法預知)
那為什麼整體通量會往低濃度移動呢?
因為在濃度高的粒子往濃度低的地方移動的總數比濃度低到高的粒子總數多
舉例來說,左邊有100顆分子 右邊10顆 每個分子往左右的機率相同(50%)
所以左邊有 50顆往右,右邊則有5顆往左
因此會慢慢擴散....
我的疑問是,如果把Fick's law 應用在氣體
為什麼不是因為粒子感受左邊的壓力大(濃度高),右邊壓力小
所以每顆會向右運動,壓力差的觀念
感覺粒子受到左邊的碰撞機會遠大於右邊(合力向右的機率比較高吧?)
但答案卻是每個粒子在任何瞬間感受到的合力是隨機的
這樣想到底哪裡不對?
還是因為壓力的觀念只適用於巨觀的表現,無法解釋微觀呢?
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※ 編輯: denisck123 (98.253.77.140), 11/06/2016 09:06:43
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※ 編輯: denisck123 (98.253.77.140), 11/06/2016 11:55:54
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