急!F3 physics latent heat

Rachel

2008-12-31 10:25 pm

1) 10g of steam at 100℃ is mixed with 20g of water at 25℃.Find the final temperature of the mixture.

2) An immersion heater is used to heat a cup of water.It takes 12 min to bring the water from 15℃ to its boiling pt 100℃.Find the time taken for the heater to vapouize the boiling water completely.

my ans:
(0.01)(2.26x10^6)+(0.01)(4200)(100-T)=(0.02)(4200)(T-25)
T=229℃
book ans:100℃.邊個位錯?

第二題就仲衰...可以教我做嗎?要加解釋喎: )

更新1:

但係唔可以係水轉蒸氣咩? 如果真係咁寫答案係咁寫? "since the final temperature cannot be higher than 100℃ as both substances initially are at most 100℃." 其實我條式岩唔岩? by the way thank you: )

回答 (1)

匿名

2009-01-01 4:23 am

✔ 最佳答案

1) As the mass of water is too small, it does not require a large amount of energy to change the temperature from 25℃ to 100℃. Therefore, the steam does not have to change its form. [Your answer T=229℃ > 100℃ also implies (0.01)(2.26x10^6) >>> (0.02)(4200)(T-25) ]

Remember that the final temperature cannot be higher than 100℃ as both substances initially are at most 100℃.

2)
E1 = mcΔT = m(4200)(100-15) = 357000m J
P = E1/t1 = 357000m J / (12*60)sec = 495.833J/sec = 495.833m W

E2 = mV = m(2260000) = 2260000m J

assume there is no energy lost, time taken for the heater to vaporize the boiling water: (P=E1/t1=E2/t2)
t2 = E2/P = 2260000m J / 495.833m J/sec= 4557.983 sec (or = 75.966mins)

p.s.
m = mass
c = 4200J kg^-1 K^-1
V = 2260000J kg^-1
ΔT = change in temp. (in K)

2009-01-01 22:45:56 補充：
但係唔可以係水轉蒸氣咩?
可以但在沒輔助物品(如加熱器)的情況下兩杯水加起來是沒可能變為蒸氣的

其實我條式岩唔岩?
呢條題目其實有D tricky 你計算方法同式都啱但要諗諗個答案係唔係真係有可能
你計到229℃即係表示蒸氣的熱能還沒耗盡溫度(100℃)是不會變

參考: myself

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