heat

2. WHAT IS HEAT?
 A form of energy associated with the motion
of atoms or molecules.
 Transferred from objects at a higher
temperature to objects at a lower
temperature.
 Hot objects in a cooler room will cool to
room temperature.
 Cold objects in a warmer room will heat up
to room temperature.

3. There are three ways in which
heat can be transferred:

4. Types of Heat Transfer:

5. TYPES OF HEAT TRANSFER:

6.  Conduction is heat transfer by means of molecular
agitation within a material without any motion of the
material as a whole. It occurs through direct contact.
 If one end of a metal rod is at a higher temperature, then
energy will be transferred down the rod toward the colder
end because the higher speed particles will collide with
the slower ones with a net transfer of energy to the
slower ones.
 As long as the objects are in contact, transfer of heat will
continue until the temperature of the objects is the same.
Heat Conduction

7. CONDUCTION = CONTACT
Conduction

8. 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
Heating a metal strip:
When you heat a metal strip at one end,
the heat travels all the way to the other
end as well!

9.  Think of a metal spoon in a pot of water being heated.
 The fast-moving particles of the fire collide with the
slow-moving particles of the cool pot.
 Because of these collisions, the slower particles move
faster and heat is transferred.
 Then the particles of the pot collide with the particles
in the water, which collide with the particles at one
end of the spoon.
 As the particles move faster, the metal spoon gets
hotter. This process of conduction is repeated all
along the metal until the entire spoon is hot.
Example of Conduction

10.  A piece of cheese melts as heat is transferred from
the meat to the cheese
Another Example

11.  The outer electrons of metal atoms drift, and are free to
move.
 When the metal is heated, this ‘sea of electrons’ gain
kinetic energy and transfer it throughout the metal.
 Insulators, such as wood and plastic do not have this ‘sea
of electrons’ which is why they do not conduct heat as
well as metals.
Conduction in metals

12. 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.
Why does metal feel colder?

13. Heat Convection
Convection is heat transfer by mass motion of a fluid such
as air or water when the heated fluid is caused to move
away from the source of heat, carrying energy with it.
Convection that occurs above a hot surface occurs
because hot air expands, becomes less dense and rises
and is called natural convection. Convection assisted by
breeze, pump or fan is called forced convection.
Hot water is likewise less dense than cold water and rises,
causing convection currents which transport energy.

14. Convection currents
 When particles in the air spread out, they become less
dense and generally rise above the unheated, more dense
particles around them.
 The denser masses of the gas or liquid move in to fill the
space left by the heated particles.
 The particles that move away from the source of heat
become cooler and more dense.

15. Example of Convection
When the water at the bottom of a pot is heated, its particles move
faster, and they also move farther apart. As a result, the heated
water becomes less dense. A less dense fluid will float on top of a
more dense one. Therefore, the heated water rises in the pot. The
surrounding cooler water flows into its place.
This flow creates the circular motion, known as convection currents.

16. Sea Breeze and Land breeze
Convection currents cause sea breeze and
land breeze

17. 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.
Application

18. Convection currents in the earth
• There is a lot of heat within the earth.
• The surface of the earth is cooler than the lower mantle.
• Heat is transferred to the upper layers.
• Spots where the material conducts more heat become
hotter and less dense, so they rise towards the surface.
• Denser material falls.

19. Heat Radiation
• Energy transferred in the form of rays or waves
or particles.
• Radiation is the transfer of energy by
electromagnetic waves
• Radiation does NOT require a medium to
transfer thermal energy

20. Radiation from the sun
You can feel the sun warm your skin on a sunny day.
This is because the energy causes the particles in
your skin to move faster = more heat energy

21. Radiation from other sources
We all have experience that lovely warm feeling
sitting by the bonfire. The heat that we receive
from the fire comes to us by RADIATION.

22. Radiators
Radiators are heat exchangers designed to transfer thermal
energy from one medium to another for the purpose of space
heating. The heating radiator was invented by Franz San
Galli in 1855, a Prussian-born Russian businessman living in
St. Petersburg.

23. Applications:
Buildings which are white-washed or painted in light colors keep
cooler in summer, since the light surfaces reflect radiant heat from
the sun.
Many factory roofs are aluminium-painted. The bright surface
reduces the heat lost in winter, and keeps the interior cool in
summer.
We ourselves choose light-colored clothing in summer for the
same reason.
Glass in greenhouses is often painted white in summer.
Artificial satellites are highly polished.
You can control your TV with a remote controller.
Firemen search for people in smoke filled buildings with infra red
detectors.