i dont know how th do these
1. the resistance of a metal wire is 25 unots when it is placed in melting ice and 80 units when placed over boiling water. what is its resistance when it is placed in water in 40℃? (assume the resistance increase liearly with temperature)
2. an iron sphere of mass 1.2kg is cooled by immersin it in a tank of water. the mass of water is 3kg and its initial temperature is 12℃. after a certain time, both the iron sphere and the water are at 15℃.
(specific heat capacity of iron is= 480 J kg-1 °C-1)
please~~~~~~
Physics heat and gases15points
2012-11-09 4:08 am
回答 (3)
2012-11-09 7:14 am
✔ 最佳答案
The resistance of the wire R varies with temperature T can be represented by the linear equation:R = aT + b
where a and b are constants.
At 0'C (melting ice), R = 25 units
hence, 25 = a(0) + b
i.e. b = 25 units
At 100'C (boiling wtaer), R = 80 units
hence, 80 = a(100) + 25
a = 0.55 units/C
The equation then becomes: R = 0.55T + 25
At 40'C, R = (0.55 x 40 + 25) units = 47 units
2. I suppose you want to find the initial temperature of the iron sphere.
Heat gained by water = 3 x 4200 x (15-12) J = 37800 J
where 4200 J/kg.C is the specific heat capacity of water
Heat lost by iron sphere = 1.2 x 480 x (T - 15)
where T is the initial temperature of the iron sphere
Hence, 1.2 x 480 x (T - 15) = 37800
T = 80.6'C
2012-11-20 6:23 am
thank you so much
2012-11-16 1:25 am
1.
let R= a*T + b
Using these two data
(0, 25)
(100, 80),
You can find out a and b.
Put T=40, you can find R.
(This is only a math question-_-)
2.
Do you mean that you want to find the original temp of iron??
By energy conservation,
The energy loss of iron goes to the water until thermal equilibrium.
Energy loss of iron=Energy gain of water
(1.2)(480)(15-To)=(3)(C)(15-12)
C is the specific heat cap of water.
You can find To, the original temperature of iron.
let R= a*T + b
Using these two data
(0, 25)
(100, 80),
You can find out a and b.
Put T=40, you can find R.
(This is only a math question-_-)
2.
Do you mean that you want to find the original temp of iron??
By energy conservation,
The energy loss of iron goes to the water until thermal equilibrium.
Energy loss of iron=Energy gain of water
(1.2)(480)(15-To)=(3)(C)(15-12)
C is the specific heat cap of water.
You can find To, the original temperature of iron.
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