Physics Q - Ans.fyi 參考答案

Physics Q

sl_ye

2007-06-07 11:17 pm

1. If ocean water has a density of 1.01 (10)^3 kg/m^3, how large must a block of ice be in order to just support a 45kg child? (The density of ice is 0.92 kg/m^3)
In liquid water the molecular separations (distances btw centres) are approximately equal to the molecular diameter, d. When one mL of water becomes 1000mL of steam, what is the average distance btw molecules now?

更新1:

A block of copper (mass 0.5kg) at 500C is placed on a large block of ice at 0C. The specific heat of the copper is c=385J/kg per degree, and the latent heat of fusion of ice is 333 (10)^3 J/kg. How much water is formed?

回答 (1)

魏王將張遼

2007-06-07 11:30 pm

✔ 最佳答案

(1) According to the Archimede's principle, if 1 m3 of ice is completely immersed into the ocean, the upthrust on it will be equivalent to a force of supporting mas 1010 kg,
Therefore, reducing its own mass (or weight), the net force (upward) on the ice will be equivalent to a force of supporting mass 1010 - 920 = 90 kg.
So, to support a child of 45 kg, 0.5 m3 of ice will be enough.
(2) From the given, we can see that the average distance between molecules in water state is d.
Then, in the state of steam, its volume is 1000 times before and suppose that the average distance now is D, we have:
D3 : d3 = 1000 : 1
D : d = 10 : 1
Therefore the average separation between the molecules is 10d.
(3) Suppose that there's no heat exchange with the surroundings, then:
Heat gain of the ice = Heat loss of the copper
Suppose that the amount of ice is enough that its temperature can remain at 0 °C:
Heat loss of copper = 0.5 × 385 × 500 = 96250 J (falling from 500 °C to 0 °C)
Then mass of water formed will be 96250/333000 = 0.289 kg or 289 mL.

參考: My physics knowledge

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