First slide

Radiation

Question

An object kept in a large room having air temperature of 25°C takes 12 minutes to cool from 80°C to 70°C.
The time taken to cool for the same object from 70°C to 60°C would be nearly,

Moderate

A

15 min
B

10 min
C

12 min
D

20 min

Solution

Using average method in Newton's law of cooling
$\frac{(T_{1} - T_{2})}{t} = K (\frac{T_{1} + T_{2}}{2} - T_{0}) \frac{(80 - 70)}{12} = K (75 - 25) \dots \frac{(70 - 60)}{t^{'}} = K (65 - 25) \dots (i)$
Divide eq. (i) by (ii)
$t^{'} = 12 \times \frac{5}{4} = 15 \min$

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