First slide

Radiation

Question

A sphere, a cube and a thin circular plate are heated to the same temperature. If they are made of same material and have equal masses then which one of the three objects cools faster

Easy

A

sphere
B

cube
C

circular plate
D

cannot be decided

Solution

According to Stefan's law, the rate of cooling is given by
$- \frac{dT}{dt} = \frac{eσA}{mc} [T^{4} - T_{0}^{4}]$
or $- \frac{dT}{dt} \propto A$
Since the surface area of the plate is maximum for the given mass and hence, the circular plate cools the fastest.

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