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Wednesday, 12 August 2015

THERMAL PROPERTIES OF MATERIALS

Thermal properties

There are basically three types of thermal properties.

Heat capacity

 C= ∆Q/∆T    
Heat capacity shows the ability of material to absorb thermal energy.

We can say that heat capacity is defined as the ratio of heat, added or removed from the system, to the change in temperature. Unit = J/0C

Specific heat c,   C= mc

Heat capacity can be calculated in two conditions, 
Heat capacity at Constant pressure, CP = (∆Q/∆T) P
Heat capacity at Constant volume, CV = (∆Q/∆T) v

Thermal expansion 

If materials are heated then there will be expansion and if materials are cooled then there will be contraction.

 ∆L/Li = a ∆T,
Where,
Li is original length of bar
∆T change in temperature (T-Ti)
a is linear coefficient of thermal expansion.  a= ∆L/ (L∆T)

Thermal stress

When materials are heated or cooled, there will be some changes in dimension of materials and a stress will be produced which is known as thermal stress because it is produced due to thermal action.
For example,
For a rod the thermal stress is   б=E a ∆T, where E is young modulus

The co-efficient of linear expansion

Let us take a rod of length Li at temperature Ti. Let us think that it is heated up to a temperature of Tf and therefore final length of bar after heating will be Lf.

∆L = Lf – Li and ∆T = T– Ti
a= ∆L/ (L∆T)   

So, we may say that the coefficient of linear expansion is defined as the change in length of unit length of bar due to change in temperature by unit degree.  
Unit = 0C-1 or K-1

The co-efficient of thermal conductivity

According to Fourier law we have, q = - λ (dT/dX)
Where q is amount of heat flow per unit area per unit time 

dT/dX is temperature gradient.  If we think dT/dX = 1 then, q = - λ

Co-efficient of thermal conductivity is defined as amount of heat flow per unit area per unit time with unit temperature gradient.

Construction materials
Co-efficient of thermal expansion
Co-efficient of thermal conductivity (k - W/(m. K))
Concrete
 10-6     0C-1
0.1 To 1.7
Timber
3 to 5 x10-6     K-1
0.048
Steel
13 x 10-6    0C-1
54

Let us see 

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