termodynamic se opg og resultater

Mrs Smith wants to prepare soup for the family’s dinner and therefore she takes a block of soup out of the refrigerator (msoup = 2 kg, trefrigerator = -18°C). She drops the block of soup into an aluminium pot (mAl pot = 0.75 kg, tstart,,Al pot = 22°C) and heat it up to 80°C on an iron hot plate (mFe hot plate = 0.60 kg, tstart, Fe hot plate = 22 °C). You may assume that the soup has the same thermodynamic properties as H2O.

1. Calculate the time required in order to reach the final temperature (tfinal = 80 °C) for the entire system when the heat exchange rate of the hotplate is 2.5 kW. (no heat is lost to the surroundings)

m = cp * (tend-t, start)

2 kg (soup) * 2,10J/kg°C * 18°C = 75,6 kj

mAL pot0,75kg  * 9,00 j/kg°C *(80-22)= 391,5kj

4,186kj/kgC * 2 kg * 80 = 669,76

333 *2 kg

0,750 kg  0,9kgj/kg * 58 = 39,15

Hot plate 

mFe Hotplate 0,60 kg * 58° * 0,450(Iron) = 15,66 kj

Total= 1857,67 kj

1857 / 25 W = 74,28 / 60 = 1,238

772,2 / 2,5 = 308 sec.