AIPMT | 2014Heat TransferHard
Question
Certain quantity of water cools from 70oC to 60oC inthe first 5 minutes and to 54oC in the next 5 minutes. The temperature of the surroundings is
Options
A.45o C
B.20o C
C.42o C
D.10o C
Solution
Newtons law of cooling 
First ⇒
= K [65 - θ0]
⇒ 2 = K [65 - θ0] ....(i)
Next ⇒
= K[57 - θ0] ....(ii)
Diving (i) and (ii)
⇒ 285 - 5θ0 = 195 - 3θ0
⇒ 2θ0 = 90
θ0 = 45o
First ⇒
= K [65 - θ0]⇒ 2 = K [65 - θ0] ....(i)
Next ⇒
= K[57 - θ0] ....(ii)Diving (i) and (ii)
⇒ 285 - 5θ0 = 195 - 3θ0
⇒ 2θ0 = 90
θ0 = 45o
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