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  4. A sphere, a cube and a thin circular plate, all made of same material and having the same mass are initially heated to the same temperature of 200° C, which of them cools fastest when left in air at room temperature:

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Q. A sphere, a cube and a thin circular plate, all made of same material and having the same mass are initially heated to the same temperature of $200^{\circ} C$, which of them cools fastest when left in air at room temperature:

J & K CETJ & K CET 2000

Solution:

Rate of flow of heat is given by $H=\frac{K A \Delta \theta}{l}$
where $A$ is area, $K$ the coefficient of thermal conduction,
$A 6$ the change in temperature.
$\therefore H \propto $ Area $(A)$
Since, surface area is maximum for circular plate, hence it cools fastest.