# 21-July. Basic Concepts 3. Thermal Resistance

From the Heat Flow \(\Phi\) relation

\( \Phi=UA\,\Delta T\)

we get

\( q=U\,\Delta T\)

We can interpret the \(\Delta \)**T** appearing in this relation as the *cause* that produces the *effect* of heat flow density **q** through the wall.

The constant of proportionality that connects the cause to its effect is **U**. Once the cause \(\Delta \)**T** is fixed, the bigger is **U**, the bigger is the effect (**q**). We call **R** the reciprocal of **U**:

\(R=\frac{1}{U}\).

This time, the bigger **R**, the lower the effect! That’s why **R** is called the* Thermal Resistance* of the wall.

**UNITS**

Thermal Resistance R | Km \( ^2\)/ W | With \(\Delta \)T fixed, the bigger R, the lower the flow density q |

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