Before starting the lesson just think over the questions given below.
Q.1 why the bottom of a pan used in the kitchen usually made by
copper?
Q.2 why the handle of steel a pan used in the kitchen is covered
by plastic or wood?
Q.3 why we use blankets or woolen clothes in
cold?
Q.4 why the blanket initially cold but, it
become warm after some time?
Have you got the answer?
The answer of the entire questions is one because of the conduction.
Conduction is the phenomenon of heat transfer from a heated
particle (molecule) to the colder particle by the collision of particle and
flow of electrons.
Conduction may occur in all the three phase solid, liquid and
gas.When we examine at microscopic level, in solid phase all the molecules are in contact with each
other. They can not flow or change their positions. When a pan or a
solid body is heated from one side, then the molecules of that side tends to move
but they resist to move. So it starts to vibrate at their own position and some
electron of these molecules start to flow. Because of these vibration and flow
of electron the adjacent layer of molecules gets heated and this process carries on until all the layer of solid surface is heated.
In liquid and gases the molecules can flow, so when some molecules gets heated up, they start to flow and collide with the colder molecules. Because of this collision heat is transferred to the cooler molecules.
The property of a material to conduct heat is known as
thermal conductivity.
By Fourier law, thermal conductivity is given by the heat
transfer by unit surface area when the unit temperature gradient exists.
k = -Q / A*(dX/dt)
Thermal conductivity is higher in metals but lower in
non-metals expect diamond. Copper and Silver are the highest thermal conducting metal. Some materials like cotton, wood, asbestos, woolen etc are thermal
insulators.
The thermal conductivity of some materials are given below:
Problem:
Consider a pan placed on a burner the lower surface
temperature of pan is 3000C and the upper surface temperature is 2000C.
The surface area is 20 cm2 and the heat transfer rate is 2000 W. Thickness
of the pan is 10 mm. Find the thermal conductivity of metal if the flow is steady and one dimension?
Solution:
Given:
The temperature at heated side (T1) = 300 0C
The temperature at cooler side (T2) = 200 0C
Thickness of surface (L) = 10 mm =10-2 m
Surface area (A) = 20 cm2 = 2*10-3 m
Heat transfer rate = 2000 W
Thermal conductivity =?
We know by furrier law of conduction that-
k = -Q *L / (A*(T2-T1))
k = (2000*10-2) / (2*10-3 *100)
