Simple Kinetic Molecular Model of Matter - PowerPoint by malj


									Transfer of Thermal Energy

    Name: ________________

    Class: _________________

    Index: ________________
Learning Objectives
• show understanding that thermal energy is transferred from a region
of higher temperature to a region of lower temperature.
• describe, in molecular terms, how energy transfer occurs in solids.
• describe, in terms of density change, convection in fluids.
• explain that energy transfer of a body by radiation does not require a
material medium and the rate of energy transfer is affected by:
        (i) colour and texture of the surface.
        (ii) surface temperature.
        (iii) surface area.
• apply the concept of thermal energy transfer to everyday
• In general, heat travels from higher
  temperature region to lower temperature

• There are three processes by which heat may
   – Conduction,
   – Convection, and
   – Radiation
Conduction of Heat
Conduction is the process by which heat is
transmitted through a medium from one
particle to another.
Heat travels through some material better than others.

                                               Pins are hold by wax

    Material           Time taken for pin to drop(s)
       wood                    did not drop
       copper                        14
       steel                        73
       aluminium                     16
A good conductor has a high conductivity.

Metals are good conductors of heat. Silver and
copper have highest conductivity.

Non-metals, such as plastic and air, are poor
Conductor (insulators).

Liquids and gases are usually poor conductors
The poorest conductor is vacuum.
Test Yourselves
• Heat a test tube of water near the top with a
 ‘weighted’ ice cube near the bottom. Even when
 the water at the top starts boiling, the ice cube
 does not melt. Why ?

                                    Ans.: Water is a poor
                                          conductor of heat.
• Insulators are used to lag pipes,
  lofts, hot water tanks and many
  other objects.

• Insulators are very poor

• Wrapped round hot objects,
  they act as barrier, reducing
  the heat loss.

• Fibre glass insulation is a
  popular choice for insulating
  the loft of homes.

Lagging in a cavity wall
provides extra insulation
Convection of Heat
  Convection is the process by which heat is
  transmitted from one place to another by
  the movement of heated particles of a gas
  or liquid due to difference in density.
Convection in Liquids

           • To demonstrate convection in water, drop a
             few tiny crystals of potassium permanganate
             into a flask filled with water.
           • Gently heat the flask, purple streaks of water
             will rise upwards and then fan outwards.
           • The water becomes uniformly purplish after
             some time.
           • The circulation of a liquid in this matter is
             called a convection current.

           Thinking :
           Why hot water rises and cold water sinks ?
Convection in Liquids
Convection in Gases

The figure below shows a simple demonstration of
convection of gas. The hot gases from the burning candle
go straight up the chimney above the candle. Cold air is
drawn down the other chimney to replace the air leaving
the room.
Concept on ventilation of a room with
fire place
  Sea Breeze

  Land Breeze

(continue on next slide)
Sea Breeze
• During the day the sun heats the land must faster than the sea.
  (Why ?)
• The air above the land is heated, expands and rises.
• Cold air from the sea moves inland to take its place.
• Hence, sea breeze is obtained.

Discussion : How land breeze is produced ?
Test Yourselves
1. Usually, it is better to install air conditioner
   in the higher portion of the wall. Please
   explain this statement with reasons.

2. How are winds forms ?

3. Why is the heating coil of an electric kettle
   placed near the bottom of the vessel ?
Radiation of Heat
Radiation is a method of heat transfer that does not
require any medium.
It can take place in a vacuum.
In radiation, heat transmits energy in the form of
Heat Radiation

  • All objects emit heat radiation.
  • Heating an object up make it radiate more
  • A dull dark surfaces is a better emitter or
    radiator than a shiny one.
What type of surface is the best absorber
of heat
  • Fig. below shows one way to test different surfaces.
  • Results from this type of test show that,
    a dull black surface is the best absorber of radiation,
    a shiny silvery surface is the worst absorber of radiation.
What type of surface is the best absorber of
A black body is defined as a body that absorbs all radiation that falls
on its surface. Actual black bodies don't exist in nature - though its
characteristics are approximated by a hole in a box filled with highly
absorptive material.
A black body is a hypothetic body that completely absorbs all
wavelengths of thermal radiation incident on it. Such bodies do not
reflect light, and therefore appear black if their temperatures are low
enough so as not to be self-luminous. All blackbodies heated to a
given temperature emit thermal radiation.
The radiation energy per unit time from a blackbody is proportional
to the fourth power of the absolute temperature and can be expressed
with Stefan-Boltzmann Law as
q = σ T4 A      (Not Tested)
q = heat transfer per unit time (W)
σ = 5.6703 10-8 (W/m2K4) - The Stefan-Boltzmann Constant
T = absolute temperature Kelvin (K)
A = area of the emitting body (m2)

Therefore, the rate of energy transfer increases with temperature and
surface area.
Worked Example
 Vacuum Flask
• The flask is a double-walled
  glass bottle.

• The space between the two
  walls is a vacuum. This can
  stop energy transfer out by
  conduction and convection.

• The shiny bright silvering
  coated glass surfaces can
  reduced the heat lost by
  radiation. (Why ?)
Test Yourselves
 1. Brightly polished kettle do not lose much energy
    by radiation. Why ?

 2. The cooling fins on the back of a refrigerator,
    in a car radiator should be dull black. Why ?

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