Physics urgent !!!

2013-12-25 4:21 am

A metal block is first immersed in boiling water for some time and it's then transferred to a cup of cold water.After a while, the temperature of the water is measured and the results are as follows :

Mass of metal block: 0.8 kg
Mass of water in the cup: 0.3 kg
Initial temp of the water in the cup: 23 degree C
Final temp of water in the cup : 38 degree C
SHC of water: 4200

The SHC of the metal is .381
1. Which of the following can explain that the actual value of SHC value to be larger than the value found in question 1 ?Why ?State the reason.
(1) Heat loss to surroundings
(2) Metal doesnt reach the temp of boiling water when it's transferred
(3) Water is not mixed well after the metal is dropped into it.

2. Martin bought an electric cooker and he measured the power of the cooker by the following method.He first poured 1.5 L of water into the cooker and measured its temperature.He found it to be 25 degree C.He switched on the cooker for 10 minutes and measured the temperature again and he found 95 degree C.(The SHC of water is 4200)

-Predict the extra time taken to boil the water .

回答 (1)

天同

2013-12-25 7:39 am

✔ 最佳答案

1. Statement (1) is correct.
By the Heat balance equation: m(Ca)T = Hw + Q
where m is the mass of metal block
Ca is the actual SHC of the metal block
T is the drop in temperature of the block
Hw is the heat gained by water
Q is the heat lost to the surroundings

Hence, Ca = (Hw + Q)/mT
But the experimental value of SHC, Ce = Hw/mT
thus, Ca > Ce

Statement (2) is correct.
Ce = Hw/mT
If the metal doesn't reach boiling water temperature, the value of T is smaller. Hence, Ca = Hw/mT' with T' < T
i.e. Ca > Ce

Statement (3) is wrong.
If water was mixed well, the final temperature would be lower than it should be in the experiment. Hence, the it is obvious that Ca < Ce

2. Heat gained by water = 1.5 x 4200 x (95-25) J = 441000 J
Hence, power of cooker = 441000/(10x60) w = 735 w

Thus, extra time needed to boil the water
= 1.5 x 4200 x (100-95)/735 s
= 42.86 s

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