Thermal conductivity is a material property that indicates its ability to conduct heat. It is represented by k and measured in watts per kelvin per meter. There are two main methods to measure thermal conductivity - steady state methods which measure materials at thermal equilibrium, and transient state methods which measure during heating. Steady state is more accurate but slower, while transient state does not require equilibrium but has more difficult analysis. Thermal conductivity values vary widely between materials from over 400 W/K-m for metals like silver to below 0.1 W/K-m for insulators like styrofoam.

2. THERMAL:
MEANS
“HEAT.”
Conductivity:
Means
“ABILITY TO CONVEY”.

3. DEFINITION:
It is the property of a material that
indicates its ability to conduct heat.
 It is represented by k and is
measured in watts per kelvin per metre
(W·K−1·m−1).
The reciprocal of thermal conductivity
is thermal resistivity.

6. x
Q / t Q / t
T1
T2
= T1-T2
Thermodynamics
Assume that no heat is lost
through the edges of the disc.

7. Thermodynami
c
s
x
Q/ t Q/ t
T1 T2
= T1-T2
For a uniform rod the rate of flow of heat through a
conductor ( Q/ t) is proportional to
•the cross sectional area (A) of the conductor
•the temperature gradient ( q/ x)
The constant of proportionality, k, is the thermal conductivity BUT note that heat
flows down a temperature gradient so we also introduce a negative sign to account
for this and obtain:
Q
T
kA
x

8. Thermodynami
c
s
k depends on the material and is called the thermal conductivity.
Rearranging in terms of k we can evaluate the units of k:
k
Q
t A
x
W m -2
K -1
m W m -1
K - 1
So k is defined as the rate of flow of
heat through unit area of cross
section of 1m of material when the
temperature difference between the
surfaces is 1K.

9. Methods to measure thermal conductivity.
• Main methods:
1.STEADY STATE METHODS.
2.TRANSIENT STATE
METHODS

10. STEADY STATE METHODS.
Definition:
• These methods are used when the materials are in
equilibrium state( means when temperature of the
materials is constant).
• Advantage :
• Accurate readings can be taken.
• It steady state implies constant
signals.
• Disadvantage:
As material take to long time to reach
equilibrium state so it is slow method.

11. TRANSIENT STATE METHOD:
Definition:
• These methods are used during
the heating of material.
Advantage:
• Non-steady-state methods to measure the
thermal conductivity do not require the
signal to obtain a constant value.
• Readings can be taken during heating of
material.
Disadvantage:
• Readings are not accurate
• Mathematical analysis of the data is in
general more difficult.

12. THERMAL CONDUCTIVITY OF MATERIALS
Material  (W K-1 m-1)
Silver 422
Copper 391
Gold 295
Aluminum 205
Stainless Steel 10–25
Glass, Concrete,Wood 0.5–3
Many Plastics ~0.4
G-10 fiberglass 0.29
Stagnant Air 0.024
Styrofoam 0.01–0.03