THERMAL ENERGY TRANSFER.
Conduction and convection
By Munkonge c. jr
THERMAL ENERGY TRANSFER
Are ways in which heat can travel. These are
conduction, convection and radiation.
Conduction: is the transfer of heat through collision of
particles;
It is the transfer of heat from one part of matter to
another without movement of particles.
• meaning there must be contact between atoms(bodies).
Therefore,
conduction is also defined as the transfer of heat
through solids,
though it also occurs in liquids, but hardly in gases
because atoms are too far apart, while no conduction
occurs in a vacuum.
EXPERIMENT TO SHOW THE BEST
CONDUCTOR OF HEAT.
STICK A PIN AT THE END OF EACH ROD USING WAX OR VASELINE
AND HEAT THE RODS WITH UNIFORM HEAT AT THE OTHER END
FROM THE PINS.
THE ONE THAT DROPS THE PIN FIRST IS THE BEST CONDUCTOR
WHILE THE LAST ONE TO DO SO IS THE LEAST CONDUCTOR.
EXPERIMENT TO SHOW THAT WATER IS
A POOR CONDUCTOR OF HEAT
SET THE APPARATUS AS SHOWN IN THE DIAGRAM.
HEAT THE WATER AT THE TOP OF THE TEST TUBE UNTIL THE WATER
BOILS.
OBSERVATION: WATER AT THE TOP BOILS WHILE
AT THE BOTTOM IT REMAINS COLD AND THE ICE
DOES NOT MELT.
NOTE: ICE IS WRAPPED IN A WIRE GAUZE TO MAKE IT
SINK TO THE BOTTOM SINCE ICE FLOATS ON WATER.
 Conduction is fastest in metals because they have many
electrons that are free to move.
 These electrons quickly move when heated and pass on the heat
to other electrons and atoms in cooler parts.
 Materials that allow heat to easily pass through are called
conductors of heat, e.g. metals.
 Materials that do not allow heat to easily pass through are
known as insulators e.g. plastic, wood, rubber and air.
 Metals and concrete feel cooler when you touch them because
it is transferred from the human body to the metal or concrete.
USES OF CONDUCTORS
 MAKING SAUCEPANS, BOILERS, KETTLES, POTS, RADIATORS
USES OF INSULATORS
 WOOD IS USED IN MAKING HANDLES OF SAUCEPANS AND POTS.
 CORK IS USED FOR TABLE MATS.
Uses of Air as an insulator
 Air is one of the best insulators.
 Certain houses have double walls separated by an air
space. Some windows are double glazed with an airspace in
between too.
 The air traps heat, hence such buildings keep warmer in
winter and cooler in summer.
 Wool, fur, feathers, fiberglass and polystyrene foam are
very good heat insulators because they trap air and are
used for lagging(insulating hot water pipes, cylinders,
ovens, refrigerators and walls and roofs of houses.
 A pullover/jersey made of wool fibres keeps one warm
because it traps air which is an excellent insulator itself.
CONVECTION
 Is the transfer of heat by the movement of hot particles to cooler
regions. It occurs in fluids (Liquids and gases).
 Convection in liquids: when water particles at the bottom of a pot are
heated, they move faster and further apart making them less dense,
so they rise to the surface of the liquid. The cooler particles at the
surface are denser hence sink to the bottom, become heated and less
denser hence also rise and push the now cooler particles down.
 The transfer of heat by movement of all particles in a fluid
forming a cycle is called a Convection current.
CONVECTION IN AIR
 when air near the ground is heated, it becomes less dense and
rises. The cooler air goes down.
❖ CONVECTION CANNOT OCCUR IN
SOLIDS BECAUSE PARTICLES DO
NOT MOVE FREELY but ONLY
VIBRATE IN A FIXED POSITION.
sea breeze
❖ During the day, the land
heats up quickly, while water
being a poor conductor remains
cold. Air on land becomes hot,
less dense, rises and is
replaced by cool air from the
sea.
This is why the beach is usually
cool during the day.
Land breeze: At night, the land
cools fast, the water remains
warm. So warm air above the
sea rises and
is replaced by cool air from the
land.