In the previous section we saw heat transfer by conduction. In this section, we will see convection and radiation
• First we will see convection
• Convection can occur only in fluids
• In convection, there is actual movement of particles
♦ The particles which are in direct contact with the heat source, become hot
♦ They move away towards colder parts of the fluid mass
• This process can be explained in detail, by considering the formation of sea breeze and land breeze. It can be explained in 17 steps:
1. The heat from the sun travels through the atmosphere and reaches the surface of the earth
2. The heat traveling from sun to earth is in the form of short waves
• The molecules in the atmosphere cannot absorb these short waves. They can absorb only long waves
• So, while traveling from sun to earth, the sun’s rays cannot heat up the atmosphere
• Then how does the atmosphere get it’s heat?
3. The answer can be written in 4 steps:
(i) The earth gets heated up first
(ii) The earth then emits heat back into the atmosphere
♦ This heat is in the form of long waves
(iii) The molecules in the atmosphere can absorb these long waves
(iv) Thus the atmosphere gets heated
4. However, our present discussion is about sea breeze and land breeze
• For that discussion, we do not need to consider the heating of atmosphere by the long waves emitted by the earth
• What we need to remember at this stage is that:
♦ the earth’s surface gets heated first by the suns rays
5. Now, the earth’s surface has both land and water
• The soil particles in the land has much smaller specific heat capacity when compared to water
• So land gets heated more quickly than the seas
(Recall that, water is used as a coolant in automobiles because, it can absorb large quantities of heat with out becoming hot)
6. So a difference in heating occurs:
♦ The atmospheric air in contact with the land
♦ gets heated more
♦ Than the atmospheric air in contact with the sea
7. Note that, this heating is achieved by conduction
♦ The vibrating soil particles
♦ set up vibrations in the air particles
♦ which are in contact with the soil
8. When the air is heated in this way, it expands
• Expansion means: ‘increase in volume’ and consequent ‘decrease in density'
• The 'hot air' thus has a lesser density than cold air
• The lesser dense air rises up due to buoyancy
• This upward motion is marked as ‘A’ in the fig.11.18 below:
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| Fig.11.18 |
9. So a vacant space is created above the land
• The air above the sea will move towards that vacant space
♦ This movement of air from sea to land is known as sea breeze
♦ This is marked as ‘B’ in the fig.11.18 above
10. This ‘B’ creates a vacant space above sea
• The air high up above the sea settles downwards to this vacant space
♦ This downward movement is marked as ‘C’
11. This ‘C’ creates a vacant space high up above the sea
• The air already present high up above the land moves horizontally towards this vacant space
♦ This horizontal movement is marked as ‘D’
12. Thus the process continues in a cyclic manner
• This cyclic process continues as long as the sun heats up the land
13. During night, there is no heat from the sun
• The land cools down more quickly than the sea
(Recall that, water is used in hot water bags because, it cools down slowly)
14. So a difference in heating occurs:
♦ The air above sea
♦ will be hotter
♦ Than the air above the land
15. The hot air above the sea rises up
This is marked as 'A' in fig.11.19 below:
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| Fig.11.19 |
• The air above land moves towards the sea
♦ This movement of air from land to sea is called land breeze
♦ This is marked as 'B' in the fig.11.19 above
16. We see that, the movements in fig.11.19 are just the opposites of the movements in the previous fig.11.18
• So we can write:
In the night, the cycle is reversed
17. Thus we see that, in ‘heat transfer by convection’, there indeed is actual movement of particles from one part of the fluid body to other parts
Next we will see heat transfer by radiation. It can be written in steps:
1. Consider a piece of iron
• Let it be heated gradually. It will pass through several stages:
• Stage 1:
♦ The iron becomes a little hot
♦ This hotness can be detected if we move our hands closer to the iron
• Stage 2:
♦ As the temperature increases, the iron becomes red hot
• Stage 3:
♦ As the temperature increases further, the iron becomes white hot
2. For our present discussion, we are interested in stage 1
• In the stage 1, the heat reached us even though we did not touch the iron
• How is that possible?
• The following steps from (3) to (7) will give the answer:
3. When a body is heated, it's molecules, atoms, electrons, protons etc., begin to vibrate
• Due to the vibration of the charged particles, electromagnetic waves are produced
(Some basic details can be seen here)
• These waves begin to radiate out from the body
4. Such electromagnetic waves do not need a medium to travel
• They can travel through vacuum
• They travel at the speed of light
• In our present case, the electromagnetic waves radiated out from the iron and reached our hands
• Thus we felt the heat
5. The ‘frequency of the waves’ depend on the energy content of the iron piece
♦ If the energy content is high, the emitted waves will have higher frequency
♦ If the energy content is low, the emitted waves will have lower frequency
6. In stage 1, the energy content is low. So the emitted radiation have a low frequency
• Radiations of lower frequencies are detected in the form of heat
7. In stages 2 and 3, the energy contents are high. So the emitted radiations have high frequencies
• Radiations of higher frequencies are detected in the form of light
♦ So they appear red and white in color
8. So now we know how the heat from the iron piece reached us
■ All bodies (solid, liquid and gas) which have a temperature above absolute zero, will emit heat radiations
■ The electromagnetic radiation emitted by a body by virtue of it’s temperature is called thermal radiation
■ The heat that we receive from the sun is thermal radiation
9. Consider the 'earth together with the atmosphere' as one unit
• Then, the space between this unit and the sun is a vacuum
• All radiations (including thermal radiation) emitted from the sun, can travel through this vacuum and reach the earth
10. Let us see the relation between black color and thermal radiation:
♦ A body with black color can absorb most of the thermal radiation falling on it
♦ A body with white color can reflect most of the thermal radiation falling on it
• So we prefer to wear:
♦ white or light colored cloths in summer
♦ black or dark colored cloths in winter
• The bottom side of cooking vessels is given a black color
♦ This will enable the vessels to absorb maximum heat from the flame
11. Let us see the working of a Dewar flask. It can be written in 7 steps:
(i) A Dewar flask consists of two flasks, kept one inside the other
• Some images can be seen here
(ii) The inner flask is double walled
♦ The space between the two walls is a vacuum
(iii) The inner surface of the inner wall is coated with silver
♦ So heat from the contents will be reflected back
(iv) The outer surface of the outer wall is also coated with silver
♦ So heat from the surroundings will be reflected back
(v) The vacuum space between the walls prevents any heat movement by conduction or convection
(vi) The inner flask is fixed to the outer flask using insulating materials like cork
(vii) All the above steps will help to keep hot contents hot and cold contents cold for a longer duration
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