Thermal conductivity is a material property that describes how readily a substance conducts heat. It is the rate at which heat energy passes through a unit area of a material for a unit temperature gradient, expressed in watts per metre-kelvin (W/m·K). A high thermal conductivity means the material transfers heat quickly (like copper), while a low value means it resists heat flow and acts as an insulator (like glass wool or air).
For engineering, polytechnic, and science students, thermal conductivity is one of the foundational properties studied in heat transfer and thermodynamics. It governs how cooking utensils, heat exchangers, building insulation, electronic heat sinks, and internal-combustion engines behave. This article defines the term precisely, gives the governing equation, lists units, compares common materials, and explains how the property is measured and demonstrated in a teaching laboratory.
What does thermal conductivity actually mean?
Thermal conductivity, given the symbol k (sometimes written as λ), is an intrinsic property of a material — it does not depend on the shape or size of the object, only on the substance itself and its temperature. It is the constant of proportionality in Fourier’s law of heat conduction, which relates the heat flow through a material to the temperature difference driving that flow.
Conduction is heat transfer through direct molecular and electronic interaction, without bulk movement of the material. In metals, free electrons carry most of the thermal energy, which is why good electrical conductors are usually good thermal conductors. In non-metals and gases, heat is carried mainly by lattice vibrations (phonons) and molecular collisions, which is far less efficient.
Fourier’s law and the formula
The one-dimensional form of Fourier’s law of conduction for steady-state heat flow through a flat slab is:
Q = k × A × (T1 − T2) / L
Where:
- Q = rate of heat transfer (watts, W)
- k = thermal conductivity of the material (W/m·K)
- A = cross-sectional area normal to heat flow (m²)
- T1 − T2 = temperature difference across the slab (K or °C)
- L = thickness of the slab in the direction of heat flow (m)
Rearranging the equation lets you solve directly for the property when the other quantities are measured in the laboratory:
k = (Q × L) / (A × (T1 − T2))
The negative sign that appears in the strict differential form of Fourier’s law (q = −k dT/dx) simply indicates that heat flows from hot to cold, that is, down the temperature gradient. For magnitude calculations in a typical lab experiment, the positive arrangement above is used.
What are the units of thermal conductivity?
In the SI system, thermal conductivity is measured in watts per metre-kelvin (W/m·K), which is dimensionally equivalent to W/m·°C because a temperature difference of one kelvin equals one degree Celsius. You may also encounter other unit systems in older textbooks and imported datasheets.
| Unit | System | Conversion to W/m·K |
|---|---|---|
| W/m·K | SI (standard) | 1 |
| W/m·°C | SI (equivalent) | 1 |
| cal/(s·cm·°C) | CGS | ≈ 418.4 |
| kcal/(h·m·°C) | Metric (older) | ≈ 1.163 |
| BTU/(h·ft·°F) | Imperial | ≈ 1.731 |
For most teaching and design work in India, results are reported in W/m·K. Two related properties are often discussed alongside it: thermal resistance (R-value), which is how much a layer resists heat flow, and thermal diffusivity, which combines conductivity with density and specific heat to describe how fast a temperature change propagates.
How do different materials compare?
Thermal conductivity spans several orders of magnitude, from highly conductive metals to insulating gases. The approximate values below are typical room-temperature figures used for teaching; exact values vary with temperature, purity, and structure.
| Material | Approx. k (W/m·K) | Class |
|---|---|---|
| Copper | ~385–400 | Excellent conductor |
| Aluminium | ~205–235 | Good conductor |
| Brass | ~110–125 | Good conductor |
| Mild steel | ~45–50 | Moderate conductor |
| Stainless steel | ~15–20 | Poor conductor (metal) |
| Glass | ~0.8–1.0 | Insulator |
| Water | ~0.6 | Insulator (liquid) |
| Brick / concrete | ~0.5–1.5 | Building material |
| Wood | ~0.1–0.2 | Insulator |
| Glass wool / cork | ~0.04–0.05 | Strong insulator |
| Air (still) | ~0.025 | Insulator (gas) |
This ordering explains many everyday observations: a copper-bottomed pan heats food evenly, an insulated wall keeps a building cool in the Indian summer, and trapped still air inside glass wool or a double-glazed window makes an effective thermal barrier.
How is thermal conductivity measured in a teaching lab?
In engineering and science laboratories, thermal conductivity is determined experimentally by setting up a controlled heat flow and measuring the resulting temperatures. Two classic approaches are common, one for good conductors and one for poor conductors.
Methods used for solids
- Searle’s bar apparatus — used for good conductors such as metal rods. Heat is supplied at one end (commonly by steam or an electric heater) and removed by cooling water at the other. By measuring the steady-state temperatures at two points, the water flow rate, and its temperature rise, students calculate k from the heat balance.
- Guarded hot plate / Lee’s disc method — used for poor conductors and insulators such as glass, rubber, cardboard, or building materials, supplied as a thin disc clamped between heated and cooled plates. A guard ring or symmetrical arrangement ensures heat flows one-dimensionally through the sample so the measured value is accurate.
Typical experimental procedure
- Mount the specimen and establish a steady, one-dimensional heat flow.
- Allow the system to reach steady state, where temperatures stop changing with time.
- Record temperatures with thermocouples or thermometers at fixed positions, the heat input (electrical wattage or calorimetric measurement), the sample dimensions, and the area.
- Substitute the readings into k = (Q × L) / (A × ΔT) and compute thermal conductivity.
- Repeat at different power levels or temperatures to study how k varies and to estimate experimental error.
These experiments teach not only the property itself but also steady-state analysis, calorimetry, thermocouple measurement, and error handling — core skills for any heat-transfer course. Apparatus for both conductor and insulator measurement, along with composite-wall, lagged-pipe, and natural/forced-convection setups, forms a standard part of a thermodynamics and heat-transfer laboratory.
Manufacturers of such teaching equipment supply the apparatus with the heater, instrumentation, and a manual covering theory, procedure, and sample calculations. Thermodynamics Lab Equipment for thermal-conductivity and heat-transfer experiments is among the categories produced by Scientico India, an ISO 9001:2015 and CE certified manufacturer based in Ambala, Haryana, supplying engineering and technical institutions in India and exporting to over 60 countries since 1993. Each unit can be supplied with calibration and conformity documentation on request.
Why does thermal conductivity matter in engineering?
Understanding k is essential whenever heat must be moved efficiently or blocked deliberately. Engineers rely on it to size heat exchangers and radiators, design CPU heat sinks and engine cooling, select insulation for furnaces, pipelines, and buildings, and predict how composite walls and lagged surfaces lose heat. For students, mastering the definition, the formula, and the laboratory measurement builds the foundation for the entire field of heat transfer, including conduction, convection, and radiation.
Key takeaways
- Thermal conductivity (k) measures how well a material conducts heat, in W/m·K.
- It is the constant in Fourier’s law: Q = k·A·ΔT / L.
- Metals conduct well; gases, plastics, and fibrous insulators conduct poorly.
- It is measured in the lab using steady-state methods such as Searle’s bar (conductors) and the guarded hot plate or Lee’s disc (insulators).
For institutions planning to equip or upgrade a heat-transfer laboratory, a quote-based CIF proforma invoice can typically be issued within 24 hours, and queries are welcomed on WhatsApp at +91-7015865225.
Frequently Asked Questions
What is thermal conductivity in simple words?
Thermal conductivity is a measure of how easily heat passes through a material. A material with high thermal conductivity, like copper, lets heat flow through quickly, while one with low conductivity, like wood or glass wool, resists heat flow and acts as an insulator. It is measured in watts per metre-kelvin (W/m·K).
What is the formula for thermal conductivity?
Thermal conductivity comes from Fourier’s law of heat conduction: Q = k × A × (T1 − T2) / L, where Q is the heat transfer rate, A is the area, (T1 − T2) is the temperature difference, and L is the thickness. To find the property itself, rearrange it to k = (Q × L) / (A × ΔT), measured in W/m·K.
What is the SI unit of thermal conductivity?
The SI unit of thermal conductivity is the watt per metre-kelvin (W/m·K), which is equivalent to W/m·°C because a temperature difference of one kelvin equals one degree Celsius. Older datasheets may use cal/(s·cm·°C), kcal/(h·m·°C), or BTU/(h·ft·°F).
Which material has the highest thermal conductivity?
Among common engineering materials, copper has one of the highest thermal conductivities at roughly 385–400 W/m·K, followed by aluminium and brass. Among all known materials, diamond conducts heat even better. Insulators such as air, glass wool, and cork sit at the opposite end, around 0.025–0.05 W/m·K.
How is thermal conductivity measured in a laboratory?
In a teaching lab, thermal conductivity is measured under steady-state heat flow. Searle’s bar apparatus is used for good conductors like metal rods, while the guarded hot plate or Lee’s disc method is used for poor conductors and insulators. Students record the heat input, sample dimensions, and steady-state temperatures, then calculate k = (Q × L) / (A × ΔT).
Lab Equipment Featured in This Guide
Manufactured in-house by Scientico India — ISO 9001:2015 & CE certified, exported to 60+ countries. Request a CIF quote within 24 hours.
Thermal and Light Radiation | ThermoFlux – 7031View details & get quote →
Steam Power Plant | ThermoFlux – 7090View details & get quote →
Thermal Expansion Apparatus | ThermoFlux-26View details & get quote →
Reciprocating Air Compressor Test RigView details & get quote →
Engine Test Bed | ThermoFlux – 7080View details & get quote →
Steam Power Plant with Steam Engine | ThermoFlux – 7087View details & get quote →