Principle of Heat Conduction

It states that the instantaneous rate of heat flow is equal to the product of three factors the area A taken at right angles to the direction of heat flow the temperature gradient which represents the change in tanperature with respect to the length of path dx and the factor of proportionality A, called thermal conductivity, which is a physical property of the material. 

This equation is of value and great importance under stationary conditions, when the 

On the contrary, in transient conditions, when the tanperature and its gradient vary with space and time, Fourier s law should be written with partial derivatives  

The negative sign arises because the heat flux occurs in the direction opposite to that of increasing temperature. 

---

From a theoretical standpoint, one can have a situation where the AH < 0, but there is enough work done on the system that makes AH > 0. 149. A gas stove works by combustion of a flammable fuel. Once shut off, the heat source instantly disappears. An electric stove works by the principle of heat conduction. Even after the electricity is shut off to the heating coil, it fakes time for the coil to cool because of its heat capacity, and therefore it continues to supply heat to the pot. 150. The answer is (a). IM. The answer is (b). 

---