Conduction, as we know, is a method of heat transfer in solids caused due to free electrons the crystal structure. It is mainly caused by the vibration of the molecules about there meaning position.
Conduction in gases and fluid flow occurs by the collisions of and diffusion of the molecules during their random motion. Mathematically heat transfer is given by the equation.
Q= -k A dt/dx
Where Q is the heat transfer rate between the one end of the solid of solid wall. The expression dt/dx is known as the temperature gradient is the difference of the temperature divided by the thickness of the wall. A is the area of the wall and K is the thermal conductivity of the material on which the wall is made up of. K depends on the material to material.
The above equation is known as the Fourier equation of heat conduction.
Another useful quantity is known as the heat flux, denoted by q,it is defined as the heat transfer per unit area. The sign should always be positive of the Q so to make it positive, a minus sign also introduces.
Applying the concept of electrical heat analogy to the above heat equation we get the thermal resistance as
Thus it is obvious that the electrical heat resistance increases as the length increases. The concept of electrical resistance can be applied if there are composites of wall thickness.
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