Heat carried away from a fire sideways mainly by radiation. Above the fire, heat is carried by both radiation and convection of air. But convection carries much more heat than radiation. So, it is much hotter above a fire than by its sides

The air between the two walls of the thermo flask is evacuated. This prevents heat loss due to conduction and convention. The loss of heat due to radiation is minimized by silvering the inside surface of the double wall. As the loss of heat due to the three processes is minimized the tea remains hot for a long time.

As the heat capacities are equal, so m₁c₁ = m₂c_{2 .}
Let C be the specific heat of the composite body. Then,
(m₁ + m₂) C = m₁c₁ + m₂c₂
= m₁c₁ + m₁c₁ = 2m₁c₁
 \( C=\frac { 2{ m }_{ 1 }{ C }_{ 1 } }{ { m }_{ 1 }+{ m }_{ 2 } } =\frac { 2{ m }_{ 1 }{ C }_{ 1 } }{ { m }_{ 1 }+{ m }_{ 1 }\frac { { C }_{ 1 } }{ { C }_{ 2 } } \ } \)
\(\\=\frac { 2{ C }_{ 1 }{ C }_{ 2 } }{ { C }_{ 1 }+{ C }_{ 2 } } \)

A cooking utensil should be of suitable material, which possesses small valve of specific heat (C), large value of thermal conductivity (K) and low coefficient of expansion \((\alpha )\)

The coefficient of linear expansion of copper is more than steel. On heating, the expansion in copper strip is more than the steel strip. The bimetallic strip will bend with steel strip on inner (concave) side.