(1) What is the final temperature if a 25-gram ice cube at T = - 15 Degree C is
ID: 1395094 • Letter: #
Question
(1) What is the final temperature if a 25-gram ice cube at T = - 15 Degree C is dropped into a Styrofoam coffee cup containing 125 g of coffee at 80 Degree C? (Assume no heat is exchanged with the environment, the heat capacity of the Styrofoam cup is negligible, and that the coffee is essentially pure water.) (2) A metal surface is 0.50 m2 in area, and has an emissivity of 0.25. What is the net rate of heat lost by this panel if its surface temperature is 50 Degree C, and it is surrounded by the walls of the room, which are at a temperature of 20 Degree C?Explanation / Answer
1)
Let T is the final temperature.
heat lost by coffee = heat gained by ice
m_water*C_water*(80 - T) = m_ice*C_ice*(15-0) + m_ice*Lf + m_ice*C_water*(T - 0)
0.125*4186*(80 - T) = 0.025*2010*15 + 0.025*3.33*10^5 + 0.025*4186*(T - 0)
0.125*4186*80 - 0.025*2010*15 - 0.025*3.33*10^5 = T*(0.025*4186+0.125*4186)
T = (0.125*4186*80 - 0.025*2010*15 - 0.025*3.33*10^5)/(0.025*4186+0.125*4186)
= 52.2 degrees celius <<<<<<<--------------Answer
2) Rate of enrgy loss = e*A*sigma*(T2^4 - T1^4)
= 0.25*0.5*5.67*10^-8*((50+273)^4 - (20+273)^4)
= 25 W or 25 J/s <<<<<<<--------------Answer
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