HEAT~~~~~~~~~

Question

我見書話:
有塊合金(打橫的睇)上半部係鐵,下半係銅,
當遇熱時,銅會比鐵澎脹的多,所以會彎向上。
我想問如果冷卻又會怎樣?
(書就說是彎向下,但不是在任何溫度時.銅都比鐵澎脹的多嗎?)

Answer 1

書沒有錯啊!

在接近常温下, 當遇熱時.銅都比鐵澎脹得多也是沒有錯的,

你可以這樣想, 起初時是一塊彎向下的這種合金在冷卻的環境, 後來把那塊合金放在常温下, 因為當遇熱時.銅都比鐵澎脹的多, 因此合金會變得平坦, 把這塊平坦的合金放回冷卻的環境, 它就會還原, 即變得彎向下! 所以若在常温下, 把平坦的這種合金冷卻, 它就會彎向下!

所以應該是,
當遇熱時.銅都比鐵澎脹得多
當遇冷時.銅都比鐵收縮得多

若用數式顯示,

dl = α L dt
where dl = expansion; L = length ; dt = temperature difference; α = linear expansion coefficient

因銅的linear expansion coefficient比鐵大, 因此若下降相同温度 (dt), 銅比鐵收縮得多 (dl)

Reference:
http://en.wikipedia.org/wiki/Volume_Expansion_Coefficient#Linear_thermal_expansion_coefficient

Answer 2

Waht is said in your book is right.

Metal with high expansivity (coeficient of expansion) expands more or contracts more than metal with lower expansivity. Copper has a higher expansivity than iron.

Thus on contraction, copper contracts more than iron. The alloy strip will then become a downward curved arc. When temperature rises, because copper expands more than iron, the curved alloy strip will become flat again. Further rise of temperature will make the strip curved upward.