分子間的力和勢能的關係

1)分子間的力和勢能有咩關係?
Intermolecular force, similar to gravitational force, is an attractive force. The strength of the forces decreases with increase of separation between molecules following a rather complex manner. In solids where the separation is small, intermolecular force is strong and hence a small increase in separation would bring a substantial increase in potential energy of molecules. The strength of force becomes much smaller in liquids because of wider separation between liquid molecules. This is the reason why liquids generally expand more in volume when heated than that in solids.

The intermolecular force decreases rapidly when the temperature of the liquid is close to its boiling point. As such, any slight increase of potential energy leads to a much much wider separation between molecules. In the gas state, molecules are so far apart such that the action of intermolcular force is almost negligible. The gas molecules are thus free to move. The potential energy of molecules reaches a maximum value.

2)點解氣體既p.e最大,但又會話佢係0,有冇矛盾???
This is a convention, similar to that used in gravitational potential.

When molecules are so far apart so that they are not acted upon by any intermolecular force, they should have no potential energy whatsoever, because potential energy only have a meaning under the action of a force. It is therefore appropriate to say that the molecules in gas state possess zero potential energy merely because they are no longer acted by any force.

In the liquid and solid states, molecules are subject to the action of intermolecular force, hence they possess potential energy. Potential energy of molecules arising from intermolecular force decreases with the decrease of molecular separation.Thus, molecules in liquid and solids have potential energy with values less than zero. In mathematics, any value less than zero is -ve. This means that molecules in solids and liquids possess -ve potential energy.

我比個簡單既答案你啦:p
1)分子間的力和勢能有咩關係?
假設有兩個核子在平衡位置,兩者關係可由下式表示:
dU = -Fdx , 負號表示一個正移離核子受固定位置核子吸引
其中F = -kx
這式表示每一點勢能微元的改變都是由外力(等值F)抵抗分子間的引力F移動一小微元距離而作功引起
睇下面個網 ,
http://zh.wikipedia.org/zh-tw/File:PotentialEnergy_Intermolecular.png

左面張圖係分子間的力對距離既圖,接近平衡位置ro附近slope係k同時面積等於分子間勢能 右面張圖slope就係F

總括而言,分子距離愈大,分子間勢能負得愈小,但係分子間吸引力會先增加 其後先會減少.


2)點解氣體既p.e最大,但又會話佢係0,有冇矛盾???
去番相同個網睇:
http://zh.wikipedia.org/zh-tw/File:PotentialEnergy_Intermolecular.png
睇右邊 兩分子間勢能對距離圖,簡述+解釋如下:
設無限遠為0 , 當分子間距離d大於ro (張圖上面個一點,表示分子間平衡距離)和不斷增加,分子間勢能會負得更小,即係增加啦,如果機械能守恆(外界無能量輸入),平均分子動能會因此不斷減少,所以粒子之間既距離愈大,其p.e愈大 講法正確,在以上推論下集體而言,固體p.e<液體p.e<氣體p.e都是正確
不過.....
對於理想氣體而言, 分子間引力是略去不計,由於以上分子間勢能是由兩分子間作用力定義出來,即係無力--->無勢能,因此分子內能只由分子自身的動能這個因素決定‧

PS.我答左第二題先打第一題 次序亂左小小 sor