The last of the basics to go over is Fourier's Law.

This rectifies the missing piece you may have noticed in our first set of equations that were pointed out in Ohm's equations. We need to calculate for the area of any material, not just the thickness.

We'll use a derived version of Fourier's Law that functions with constant parameters to keep it simple.

**Rθ = x / A * k**

This gives us the relationship between the absolute thermal resistance (Rθ) and the length of our material (x) which is divided by the area (A) and multiplied by the conductivity (k) as we've seen before.

Heat energy will try to spread out across any surface it comes into contact with. This is unlike an electrical current, which takes the shortest possible path to complete a circuit.

Even though heat will dissipate across a whole surface, it still moves a distance over time. This means that a long piece of metal in contact with a flame will be cool at the opposite end but will still be quite hot across the width.

The transfer of heat from one surface to another will prevent a total heating of a material across its entire area. This is because the heat is still moving.

If heat were to have no place to go put into the resistance material, the entire material would eventually normalize at a temperature.

### Bottom Line

Understanding any concept starts with the basics. There will always be exceptions and further complexity to find.

A solid grasp on the fundamentals makes adapting to these real-world changes easier to do and makes finding mistakes intuitive.

For help converting between thermal resistance units, it helps to work with our thermal resistance calculator.