f.3/4 physics(10 points)

2008-03-25 7:06 am

A fish keeper performed a 75% water change (75% of the tank water is discarded and replaced by tap water).

If the temperatures of the tank water and tap water were 25'C and 12'C respectively, what was the water temperature right after the water change?

I need steps please.
The suggested answer is 12.8'C
I am not sure if it is correct or not.

更新1:

15.75'C is what I found too...... i am just wondering if the ans is correct or not. PS.The mass of the whole water tank of water is 100kg water specific heat capcity is 4200

回答 (2)

[匿名]

2008-03-25 8:34 am

✔ 最佳答案

Assume there is no energy is lost to the surrounding

Let m be the mass of the whole tank of water
T be the final temperature

By conservational of energy,
Energy lost = Energy gain
25% m (4200) ( 25-T) = 75% m (4200) (T-12)
25 (25-T) = 75 (T-12)
625 - 25T = 75T - 900
1575 = 100T
T = 15.8 oC

er................我計吾到12.8 oC喎................
可能兩種水gd capacity都吾同

另外...你個anwser可能真係有d問題，依常理諗都吾會tap Water升得o個0.8 oC掛

2008-04-06 15:49:53 補充：
SOR.....MY ANWSER IS WRONG..
Energy lost = Energy gain
25% m (4200) ( 25-T) = 75% m (4200) (T-12)
25 (25-T) = 75 (T-12)
625 - 25T = 75T - 900
1525 = 100T
T = 15.25 oC

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用戶179770

2008-03-27 6:18 pm

I'll like to ask if it has mentioned for how many times did he change the water?

1. the answer is 15.25 degree celsius, not 15.75, according to your formula
2. if you use 15.25 degree celsius as the new temp. of tank water and then perform another 75% water change using the formula stated in the above answer, you should find the answer of 12.8125 degree Celsius.

參考: myself, and the answer above

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