(a) Define thermal conductivity and clearly state its SI unit.
(b) Starting from the definition of thermal conductivity, obtain an expression for the steady–state rate of heat flow through a uniform slab in terms of its cross-sectional area, thickness, and the temperature difference between its two faces.
(c) A certain insulating board is required to limit the heat loss through a wall of area 6.0 m² to not more than 120 W when the temperature difference across it is 30 K. Explain, without doing any numerical calculation, whether the required thickness of the board will be larger or smaller if a material of lower thermal conductivity is chosen in place of the original one. Justify your answer qualitatively.