Phy Question

2007-06-07 6:17 am

I don't know how to calculate, please help me.

John adds ice cubes, each of mass 0.01kg and at 0 C, to 0.1kg of hot coffee in a polystyrene cup at 70 C.

Find the least number of ice cubes needed to cool the cup of coffee to below 10 C when all the ice cubes added are completelyll melted. ( Assumem the specific hest capacity of coffee is the samem as that of water.)

回答 (1)

用戶40379

2007-06-07 7:06 am

✔ 最佳答案

John adds ice cubes, each of mass 0.01kg and at 0 C, to 0.1kg of hot coffee in a polystyrene cup at 70 C.
Find the least number of ice cubes needed to cool the cup of coffee to below 10 C when all the ice cubes added are completelyll melted. ( Assumem the specific hest capacity of coffee is the samem as that of water.)
冰的熔解熱是L = 334000Jkg-1
水及架啡的比熱容 c = 4200 Jkg-1℃-1
設冰的質量為 m
冰吸熱
mL + mc(10 - 0)
水放熱
m’c(70 – 10)
若過程中能量不會產生及散失則，吸熱 = 放熱
mL + mc(10 - 0) = m’c(70 – 10)
334000m + 10x4200m = 0.1 x 4200 x (70 – 10)
376000m = 25200
m = 0.067kg
每塊冰質量是 0.01kg
所以最少要0.067 / 0.01 = 6.7 (即要 7 粒冰)

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