0708_conduction_convection_radiation.ppt

Heat Transfer
The heat transfer is defined as the transmission of heat energy from one region to another region as a result of temperature gradient. In actual practice, the heat transfer always takes place from the region of high temperature to the region of low temperature.

Modes of Heat Transfer
The transfer of heat energy from one region to another region takes place by the following three modes −

Conduction

Convection

Radiation

In a majority of real situations, heat transfer occurs as a result of a combination of these modes of heat transfer.

Conduction
When the heat transfers from one part of the substance to another part without the movement in the molecules of the substance, it is called the conduction mode of heat transfer.

In this mode, the rate of heat transfer, i.e., the rate of conduction of heat along the substance depends upon the temperature gradient.

Mathematically, for a cubic body with parallel faces with

Thickness=tmeters
Crosssectionalarea=Am2
Temperatureoftwofaces=T1∘CandT2∘C
Timeforwhichheattransfertakespalce=Thours
Then, the amount of heat passed through this cubic body is given by,

H=kAt(T1−T2)T
Where, H is measured in Mega Joules (MJ) and k is the coefficient of the thermal conductivity for the substance and it is measured in MJ/m3/∘C/hr
.

Examples of heat transfer through the conduction mode are refractory heating, the heating of insulating material, etc.

Convection
When the transfer of heat takes place from one part to another part of the substance or fluid due to the actual motion of the molecules, it is called the convection mode of heat transfer. In this method, the rate of heat transfer depends mainly upon the difference in the fluid density at different temperature.

The transfer of heat energy from immersion water heater to water is an example of convection mode of heat transfer. The amount of heat absorbed by water from the heater through convection partly depends upon the temperature of the heating element and partly upon the position of the heater.

Mathematically, the heat dissipation in convection mode of heat transfer is given by the following expression −

H=a×(T1−T2)bW/m2
Where,

'a' and 'b' are the constants whose values depend upon the heating surface.

T1
and T2
are the temperatures of heating element and the fluid / substance in °C.

Radiation
When the heat transfer takes place from the source of heat to the substance to be heated without direct contact between them, it is called the radiation mode of heat transfer. The heat transfer through the radiation depends upon the surface. The transfer of heat in case of solar heater is an example of radiation mode of heat transfer.

Mathematically, the rate of heat dissipation through radiation is given by Stefan's law, that is

H=(5.72×104)×ke[(T11000)4−(T21000)4]W/m2
Where,

σ = (5.72× 104) is the Stefan's constant

'e' is the emissivity (it is 1 for ideal radiator)

T1
is the temperature of source in Kelvin

T2
is the temperature of the subs

1. Understanding Heat Transfer,
Conduction, Convection and
Radiation

2. Heat Transfer
•Heat always moves from a warmer place to
a cooler place.
•Hot objects in a cooler room will cool to
room temperature.
•Cold objects in a warmer room will heat up
to room temperature.

3. Question
•If a cup of coffee and a red popsickle were
left on the table in this room what would
happen to them? Why?
•The cup of coffee will cool until it reaches
room temperature. The popsickle will melt
and then the liquid will warm to room
temperature.

4. Heat Transfer Methods
•Heat transfers in three ways:
–Conduction
–Convection
–Radiation

5. Conduction
When you heat a metal strip at one end, the heat
travels to the other end.
As you heat the metal, the particles vibrate, these
vibrations make the adjacent particles vibrate, and so on
and so on, the vibrations are passed along the metal and
so is the heat. We call this?Conduction

6. Metals are different
The outer e______ of metal atoms
drift, and are free to move.
When the metal is
heated, this ‘sea of
electrons’ gain k_____
energy and transfer it
throughout the metal.
Insulators, such as w___ and p____, do not
have this ‘sea of electrons’ which is why they
do not conduct heat as well as metals.
lectrons
inetic
ood lastic

7. Why does metal feel colder than wood, if they
are both at the same temperature?
Metal is a conductor, wood is an insulator. Metal
conducts the heat away from your hands. Wood
does not conduct the heat away from your hands as
well as the metal, so the wood feels warmer than
the metal.

8. Convection
What happens to the particles in a liquid or a
gas when you heat them?
The particles spread out and
become less dense.
This effects fluid movement.What is a fluid?A liquid or gas.

9. Fluid movement
Cooler, more d____, fluids
sink through w_____, less
dense fluids.
In effect, warmer liquids and
gases r___ up.
Cooler liquids and gases s___.
ense
armer
ise
ink

10. Water movement
Hot water
rises
Cooler
water sinks
Convection
current
Cools at the
surface

11. Why is it windy at the seaside?

12. Cold air sinks
Where is the
freezer
compartment
put in a fridge?
Freezer
compartment
It is put at the
top, because
cool air sinks,
so it cools the
food on the
way down.
It is warmer
at the
bottom, so
this warmer
air rises and
a convection
current is
set up.

13. The third method of heat transfer
How does heat energy get
from the Sun to the Earth?
There are no particles
between the Sun and the
Earth so it CANNOT
travel by conduction or
by convection.
?
RADIATION

14. Radiation
Radiation travels in straight lines
True/False
Radiation can travel through a vacuum
True/False
Radiation requires particles to travel
True/False
Radiation travels at the speed of light
True/False

15. Emission experiment
Four containers were filled with warm water. Which
container would have the warmest water after ten minutes?
Shiny metal
Dull metal
Dull black
Shiny black
The __________ container would be the warmest after ten
minutes because its shiny surface reflects heat _______ back
into the container so less is lost. The ________ container
would be the coolest because it is the best at _______ heat
radiation.
shiny metal
radiation
dull black
emitting

16. Absorption experiment
Four containers were placed equidistant from a heater. Which
container would have the warmest water after ten minutes?
The __________ container would be the warmest after ten
minutes because its surface absorbs heat _______ the best.
The _________ container would be the coolest because it is
the poorest at __________ heat radiation.
dull black
radiation
shiny metal
absorbing
Shiny metal
Dull metal
Dull black
Shiny black

17. Convection questions
Why are boilers placed beneath hot water
tanks in people’s homes?
Hot water rises.
So when the boiler heats the water, and the hot water
rises, the water tank is filled with hot water.
Why does hot air rise and cold air sink?
Cool air is more dense than warm air, so the
cool air ‘falls through’ the warm air.

18. Radiation questions
Why are houses painted white in hot countries?
White reflects heat radiation and keeps the house cooler.
Why are shiny foil blankets wrapped around marathon
runners at the end of a race?
The shiny metal reflects the heat radiation from the runner
back in, this stops the runner getting cold.

19. 1.Which of the following is not a
method of heat transfer?
A.Radiation
B.Insulation
C.Conduction
D.Convection

20. 2.In which of the following are
the particles closest together?
A.Solid
B.Liquid
C.Gas
D.Fluid

21. 3.How does heat energy reach
the Earth from the Sun?
A.Radiation
B.Conduction
C.Convection
D.Insulation

22. 4.Which is the best surface for
reflecting heat radiation?
A.Shiny white
B.Dull white
C.Shiny black
D.Dull black

23. 5.Which is the best surface for
absorbing heat radiation?
A.Shiny white
B.Dull white
C.Shiny black
D.Dull black