First slide

Radiation

Question

A hot liquid takes 5 minutes to cool from 70°C to 60°C. How long will it take to cool from 60°C to 50°C ?

Moderate

A

5 minutes
B

more than 5 minutes
C

less than 5 minutes
D

less or more than 5 minutes depending on the nature of liquid

Solution

We know that rate of loss of heat c. temperature difference
$ms \frac{(70 - 60)}{5} \propto (\frac{70 + 60}{2} - T_{0}) \propto (65 - T_{0})$
and $ms \frac{(60 - 50)}{t} \propto (\frac{60 + 50}{2} - T_{0}) \propto (55 - T_{0})$
This show that t should be greater than 5 minutes.

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