IEMDaily - Video Lecture Notes

Definition

Conductors and insulators

Substances through which heat can pass easily are called good conductors of heat. Examples: metals, mercury, etc.
Substances through which heat cannot pass easily are called insulators of heat. Examples: glass, wool, etc.

Definition

Conduction

Conduction is heat transfer between two bodies in contact and at different temperatures. It usually occurs between solids. Heat transfer by conduction is a slow process.

Definition

Thermal conductivity

Thermal conductivity is defined as the rate at which heat passes through a specified material, expressed as the amount of heat that flows per unit time through a unit area with a temperature gradient of one degree per unit distance.(

W / (m K

)) and

B T U

h r - f t - F

) are some units of thermal conductivity.

Formula

Coefficient of thermal conductivity

Formula :
Coefficient of thermal conductivity:

= \frac{Q d}{A (θ_{2} - θ_{1}) t}

Dimension :

[M^{1} L^{1} T^{- 3} K^{- 1}]

Unit :

W m^{- 1} K^{- 1}

Definition

Coefficient of thermal conductivity

Coefficient of thermal conductivity:

= \frac{Q d}{A (θ_{2} - θ_{1}) t}

Cefficient of thermal conductivity of the material of solid is equal to the rate of flow heat per unit area per unit temperature gradient across the solid.

Definition

Proportionality of rate of heat transfer and temperature gradient at steady state

In steady state, the rate of flow of heat

H

from one point to another is proportional to the temperature difference (

T_{1} - T_{2}

) and the area of cross section

A

and is inversely proportional to length L:

H = K A \frac{T_{1} - T_{2}}{L}

The constant of proportionality

K

is the thermal conductivity of the material.

Definition

Heat flow in series conductors

The attached figures show the heat flow in parallel conductors. Series configuration in conductors is defined by same heat flow in the two conductors across all cross sections.
In the given figure,

q = \frac{k_{1} A (T_{1} - T_{2})}{L_{1}} = \frac{k_{2} A (T_{2} - T_{3})}{L_{2}}

Formula

Series thermal resistance

R_{s e r i e s} = \sum R_{i}

where

R_{i} = \frac{L}{k_{i} A}

is the resistance of the

i^{t h}

layer

Definition

Heat flow in parallel conductors

The attached figures show the heat flow in parallel conductors. Parallel configuration in conductors is defined by same temperature at the two junctions of the conductors. Total heat flowing through a cross section is the sum of heat flow in each conductor.
In the given figure,