HEAT TRANSFER
Heat is a form enery that brings about the sensation of warmth.

Methods Of Heat Tranfer
1.CONDUCTION : Conduction of heat is the process by which heat is passed along a stationary solid material,the heated particle remaining in position.

Thermal Conductivity in liquids
In general,liquids are poor conductors heat,but metal in liquid form,for example mecury,are good conductors.However,heat is transferred through liquids mainly by the process of CONVECTION.
We can confirm that water is a bad conductor by the following experiment.
A piece of ice is wrapped with a metal gauze and dropped into a test-tube full of water.The tube is held inclined,and the top of the is heated with a Bunsen flame.
It will be observed that the water at the top of the tube may be boiling,the ice at the bottom remains unaffected.Little heat is conducted from the top to the bottom of the water,and this show that water is a POOR CONDUCTOR of heat.

Good and Bad Conductors and their Applications
1.In our home we notice that cooking pots and frying pans are made of metals,such as aluminium which is a good conductor.When place on the flame,they conduct the heat quickly and cause the food to cook quickly.

2.We also notice that the handles of the pots and pans are made of wood or plastic material.These materials are poor conductors of heat,or insulators.Were are therefore able to handle the utensils when they are hot.

3.A stone floor or a cemented floor feels cold to the feet but a rug or a carpet on the same floor feels warm.This is explained by noting that the stone is a good conductor and conduct heat away from our bare feet.The feet having lost some heat to the surroundings feel cold.
The rug is a poor conductor of heat and does not conduct heat away from the feet as rapidily as stone.
Consequently the feet feel warmer on the rug than on the bare floor.

4.It is also observed that the thatched-roof houses are cooler inside than iron-roofed houses.This is because the iron conducts most of the heat from the sun to the inside of the house.In some modern houses,insulation in the ceiling is used to keep them cool.Asbestos is such an insulator and prevents the heat that has passed through the roof from getting to the space below.

CONVECTION : Convection is the process by which heat enery is transferred in a liquid or gas by the actual movement of the heated fluid.

Convection in Gases
Convection current are more easily set up in gases because gases expand more radily when heated.
Experimental demostration of Convection in gases.
A wooden box with a plane glass front has two large glass tubes or chimneys A and B projecting from the top.A short candle is lit at the base of the box near A.When a piece of smouldering paper is placed near the top of tube B,the smoke is observed to flow downwards through B and upward through A.This indicates the direction of the convection current.
The air above A becomes warm when heated by the candle and rises upwards.Cold air moves down the box through the tube B to take the place of warm air.As a result of both movements,a convection current is set up.
Thus the chimneys in bringing about the circulation of fresh air in a room is based on the above principle.

RADIATION : Radiation is the process by which heat is conveyed from one place to another without heating the intervening medium

practical Consequences of radiation
1.It is not advisable to wear a dark-coloured jacket in the tropical sun.The dark jacket absorbs the heat from the sun and makes the wearer feel hot and uncomfortable.Roofs of factories are coated with aluminium paint to reflect heat during the day and thus keep the interior cool.
In our hot climate, we wish to keep houses cool during the day.We therefore use shining roofs and paint the outside walls with light colours.
In sunny countries,solar panels are used to collect radiant heat from the sun and thus heat water.The water flows back and forth along blackened pipes,which absorb radiant heat energy efficiently.