phy一問(中三)

0.2kg of water at 30℃ is put into a freezer to make ice. How much energy has to be removed from the water to freeze it?
所放出的能量包括
由30℃下降至0℃所放出的能量(mcΔT) + 結冰時所放出的能量(mL)
已知水的
比熱容 c = 4200J Jkg-1 ℃-1
冰凝結比潛熱 L = 334,000Jkg-1
所以放出的能量是
E = mcΔT + mL
E = 0.2 x 4200 x (30 - 0) + 0.2 x 334000
E = 92,000J

E= (0.2)(4200)(30-0)
E= 25200J

The energy removed from water means cooling, so energy has to be taken from the water to make it cool down. Hence energy is removed from water.