QUESTION 18 A piece of copper at 350 oC is dropped into 1.9 kg of water at 17 oC

Question

QUESTION 18 A piece of copper at 350 oC is dropped into 1.9 kg of water at 17 oC. If the equilibrium temperature is 22 OC, what is the mass of the copper? You may assume that no energy is exchanged with the surroundings. (a) 0.15 kg (b) 20.6 kg (c) 0.31 kg (d) 0.20 kg QUESTION 19 A cylindrical log 15 cm in diameter and 78 cm long is glowing red hot in a fireplace. If it's emitting radiation at the rate of 34 kW, what is its temperature? The logs emissivity is essentially 1. (a) 200 K (b) 930 K (c) 110K (d) 1100 K

Explanation / Answer

Note :

Specific heat of water : 4.186 J/gm

Specific heat of copper : 0.386 J/gm

the energy amount going out of the warm water is equal to the energy amount going into the cool water. This means:

qlost = qgain

Let the mass of copper be "X"

(X) (350 - 22)(0.386) = (1900) (22 - 17) (4.184)

Solving for X we get :

X = 313.96 gm

Ans (C)

Q19)

The lateral surface area is = pi*(0.15 m)*(0.78 m) = 0.25038 m2
The surface area of the two ends is = 2*pi*(0.075 m)2 = 0.024075 m2
The total surface area is 0.274455 m2
Therefore, the radiation intensity in W/m^2 = 34 kW/(0.274455 m^2) = 123.88 kW/m2.

This should be equal to sigma*T4 ,where sigma is the Stefan-Boltzmann constant
and T is the absolute temperature.
sigma = 5.67 x 10^(-8) W/(m2 K4)
Hence,
T4 = 2.184 x 1012 K4
T = 1100 K approximately

Ans (D)

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