Consider a metal slab of area of cross-section A and thickness d. Let the end faces (ABCD & EFGH) be maintained at temperatures θ1 and θ2 (θ1 > θ2).
The amount of heat conducted (Q) from the hotter to colder face is,
1. directly proportional to the area of cross-section (A)
2. directly proportional to the time for which heat flows (t)
3. directly proportional to the temperature difference between the faces (θ1 – θ2)
4. inversely proportional to the distance between the faces (d) This is called the law of thermal conductivity.
According to the law Q ∝ 
K is a constant of proportionality called coefficient of thermal conductivity.
