State Fourier's law of heat conduction and explain its significance.

Fourier's law states that the rate of heat conduction through a material is proportional to the negative temperature gradient and the cross-sectional area: Q = -kA(dT/dx). Here, k is thermal conductivity (W/m-K), A is area, and dT/dx is the temperature gradient. The negative sign indicates heat flows from high to low temperature. This law forms the foundation for all conduction analysis in thermal engineering.