10-2.Transmission of Heat
medium

In $5\, minutes,$ a body cools from $75^{\circ} {C}$ to $65^{\circ} {C}$ at room temperature of $25^{\circ} {C}$. The temperature of body at the end of next $5\, minutes$ is $......\,{ }^{\circ} {C} .$

A

$57$

B

$60$

C

$61$

D

$570$

(JEE MAIN-2021)

Solution

By newton's law of cooling (with approximation)

$\frac{\Delta {T}}{\Delta {t}}=-{C}\left({T}_{{avg}}-{T}_{{s}}\right)$

$1^{\text {st }}\, \frac{-10^{\circ} {C}}{5\, {min}}=-{C}\left(70^{\circ} {C}-25^{\circ} {C}\right)$

${C}=\frac{2}{45}\, {min}^{-1}$

$2^{\text {nd }} \,\frac{{T}-65}{5 {min}}=-{C}\left(\frac{{T}+65}{2}-25\right)=-\left(\frac{2}{45}\right)\left(\frac{{T}+15}{2}\right)$

$9({T}-65)=-({T}+15)$

$10 {T}=570$

${T}=57^{\circ} {C}$

Standard 11
Physics

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