---
title: "Define thermal conductivity"
subject: "Civil Engineering Materials"
chapter: 1
key: "semester_3/civil_engineering_materials:property-thermal-conductivity"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/property-thermal-conductivity
site: BCT Engineering
---

# Define thermal conductivity

Civil Engineering Materials · Chapter 1: Introduction to Civil Engineering Material

- Asked in: 2066 Shrawan (old course), 2 marks
- Video: [Presentation - Thermal Properties of Building Materials](https://www.youtube.com/watch?v=ch0d5Hb-_qA) — CertainTeed
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/property-thermal-conductivity
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-1

## Question

Define thermal conductivity as a property of materials, with sketch where necessary.

## Short answer

Thermal conductivity is the rate of heat flow through a unit thickness per unit area per unit temperature difference, given by Fourier's law. Low values (timber, cork) indicate good insulators; high values (steel, copper) indicate good conductors.

## Answer

**Thermal conductivity ($k$)** is the rate of heat flow through a unit thickness of a material, per unit area, per unit temperature difference between its faces. By Fourier's law:

$$
Q = \frac{k\,A\,(T_1-T_2)\,t}{L}
$$

where $Q$ is the heat conducted in time $t$ through a slab of area $A$ and thickness $L$ whose faces are at temperatures $T_1$ and $T_2$. The unit of $k$ is W/(m·K). A low value means a good insulator (timber, cork, hollow bricks); a high value means a good conductor (steel, copper).

```text
  T1 (hot)  +---------------+  T2 (cold)
   ======>  |  area A       |  ======>
   heat Q   +---------------+
            <------ L ------>
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Thermal conductivity k is heat flow per unit
   A. mass and time only
   B. volume only
   C. load
   D. thickness, area and temperature difference
   Answer: D. Defined by Fourier's law. (Key point: Definition)

2. Fourier's law is
   A. Q = k A (T1 - T2) t / L
   B. Q = k L / (A dT)
   C. Q = A L / k
   D. Q = k (T1 - T2) / A
   Answer: A. Heat grows with k, A, dT, t and falls with L. (Key point: Formula)

3. A good insulator has
   A. high thermal conductivity
   B. low thermal conductivity
   C. high density always
   D. high hardness
   Answer: B. Timber and cork are good insulators. (Key point: Insulators)

## Related in Civil Engineering Materials

- [Important properties of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aimportant-properties-civil-materials) — Civil Engineering Materials, chapter 1, asked 8×
- [Constituents of brick earth and functions](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aconstituents-brick-earth-functions) — Civil Engineering Materials, chapter 3, asked 9×
- [Define creep in brief](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aproperty-creep) — Civil Engineering Materials, chapter 1, asked 8×

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- [Fourier law and thermal conductivity](https://www.bctengineer.com/question.md?key=semester_5%2Fheat_transfer%3Afourier-law-thermal-conductivity) — Heat Transfer, chapter 2
- [Plane wall with temperature dependent conductivity](https://www.bctengineer.com/question.md?key=semester_5%2Fheat_transfer%3Avariable-thermal-conductivity-plane-wall) — Heat Transfer, chapter 2
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- [Thermal stress in a restrained bar](https://www.bctengineer.com/question.md?key=semester_4%2Fstrength_of_materials_mechanical%3Abar-between-rigid-walls-temperature) — Strength of Materials, chapter 2