First slide

Radiation

Question

A body cools from 50⁰C to 40⁰C in 5 min. The surrounding temperature is 20⁰C. In what further time (in minute) will it cool to 30⁰C?

Moderate

A

5
B

$\frac{15}{2}$
C

$\frac{25}{3}$
D

10

Solution

Applying, $\frac{T_{1} - T_{2}}{t} = α (\frac{T_{1} + T_{2}}{2} - T_{0}), we get$
$\frac{50 - 40}{5} = α [\frac{50 + 40}{2} - 20]$ …(i)
and $\frac{40 - 30}{t} = α [\frac{40 + 30}{2} - 20]$ …(ii)
Solving these two equations, we get
$t = \frac{25}{3} min$

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