Physics IX Notes Chapter 9 Transfer of Heat

Conduction of heat:-the process by which heat energy is transferred from particle to particle by collision is called conduction of heat.

i) Mechanism of heat conduction:- there are two ways by which heat energy is transferred.

The vibration of atoms in metals:-
A metal consists of a large number of atoms which vibrate about their mean position. When heat is supplied to its one end. Then because of this heat the vibration of atoms increases. These atoms collide with their neighbour atoms and transfer their he3at to neighbouring atoms and in this way, heat is transferred to the other end.
ii) Motion of free electron in metal:-

iii) We know that a metal consists of free electrons which can carry heat from one point to another end. When heat is supplied to the metal rod. It’s K>E o atoms increase. They transfer their heat to the free electrons by collision and these free electrons transfer their heat to other free electrons and thus heat is transferred to the other end.

2) Practical application heat conduction:-
i. We use met5al pots for cooking which transfer the heat easily to the food placed inside them.
ii. Plastic foams and fibreglass are bad thermal conductors. They are used in the wales and ceiling of homes. And keep them warm in winter.
iii. Woolen clothes have fine pores which contain air. The air and wall are bad thermal conductors. Thus they avoid the flow of heat from our body to the surrounding and keep us warm in winter.

Conduction in solids:-usually metallic solids is good conductors of heat. Which contains free electrons and packed atoms and they play a very important role in the conduction of heat which can carry heat energy inside metal from one point to another point. And they are considered good conductors while plastic wood and rubber care, poor solid conductors because they have no free electrons.

Conduction in liquid:-liquids is mostly poor conductors of heat as compared to metallic solids because the intermolecular distance is larger in liquids than solids. Therefore the rate of conductivity is also smaller than solid.

Conduction in gases:– there is a large distance among gas molecules. Thus the rate of conductive collision in gas is very small as compared to solids and liquids thus gases are the poorest conductors of heat. The conductivity of air is of waters 20 times smaller than that of water.

Thermal conductivity:- The measure of the ability of a substance to conduct heat energy is called thermal conductivity (OR) the amount of heat flowing through a unit area of a substance in one second having a temperature difference of 1k across at a length of one meter is called thermal conductivity.
Explanation:-consider a rod of length “L” having an area of the cross-section “A” and difference of temperature then the amount of heat “Q” supplied depends upon the following factors.