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急!F3 physics latent heat

發問:

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... 顯示更多 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: 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: )

最佳解答:

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

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