Define coefficient of thermal conductivity or thermal conductivity of a substance.

Answer

### Step-by-Step Text Solution:

1. **Understanding Thermal Conductivity**:
- Thermal conductivity refers to the ability of a material to conduct heat. It is a measure of how well heat is transferred through a substance.

2. **Defining Coefficient of Thermal Conductivity**:
- The coefficient of thermal conductivity (often denoted as \( k \)) is defined as the rate of heat transfer through a unit area of a material per unit temperature difference across that area.

3. **Mathematical Expression**:
- The coefficient of thermal conductivity can be expressed mathematically as:
\[ k = \frac{Q}{A \cdot \Delta T} \]
where:
- \( k \) = coefficient of thermal conductivity
- \( Q \) = amount of heat transferred
- \( A \) = area through which heat is transferred
- \( \Delta T \) = temperature difference across the material

4. **Practical Example**:
- Consider a cubical material with two faces (Phase 1 and Phase 2). When heat is supplied to Phase 1, it flows to Phase 2. The coefficient of thermal conductivity can be understood as the amount of heat required to raise the temperature of the material by 1 degree Celsius when the heat is transferred from Phase 1 to Phase 2.

5. **Conclusion**:
- In summary, the coefficient of thermal conductivity is a crucial property of materials that indicates how effectively they can conduct heat, which is essential in various applications in physics and engineering.