Methods of transfer of thermal energy  
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Heat transfer may be defined as the transmission of the energy from one region to another  
region as a result of the temperature gradient.  
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Heat can be transferred from one place to another in three ways, these include  
1. Conduction  
2. Convection  
3. Radiation  
Heat transfer by conduction  
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Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time,  
the handle of the spoon will become hot. This is due to transfer of heat energy from molecule  
to molecule or from atom to atom.  
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Conduction is a very effective method of heat transfer in metals. However, air conducts heat  
poorly.  
Thus; Conduction is the transfer of heat energy from one substance to another or within a  
substance OR;  
Conduction Is the transfer of heat though matter from a region of higher temperature to a  
region of lower temperature  
How conduction takes place?  
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In solids, molecules are closely arranged so that they cannot move freely. When one end of the  
solid is heated, molecules at that end absorb heat energy and vibrate fast at their own positions.  
These molecules in turn collide with the neighboring molecules and make them vibrate faster  
and so energy is transferred. This process continues till all the molecules receive the heat  
energy.  
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The process of transfer of heat in solids from a region of higher temperature to a region of lower  
temperature without the actual movement of molecules is called Conduction  
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Good and Bad Conductors  
Good Conductors  
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Are the materials that allow heat to flow through them easily.  
For example, silver, alluminium, iron etc.  
Silver – you can find spoons made of silver. If use a spoon to eat some hot soup, heat will be  
transferred to the spoon from the soup and it will become very hot.  
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Aluminium – some saucepans are made of aluminium, allowing the food in the saucepan to get  
heated quickly.  
Iron – when we iron a shirt on an ironing board, heat from the shirt is conducted to the shirt to  
remove any creases.  
Conductors have different rate of heat conduction. Example, copper is the best conductor of  
heat while steel is the poorest conductor.  
Bad Conductors (Thermal Insulators)  
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Are materials that cannot allow heat to flow through them easily  
For example, air, wood, plastic, wool, glass etc.  
Air – air is a very bad conductor of heat. This explains why we find it in between the two  
panes of glass in double-glazed windows, as less heat will be lost.  
Wood – we often see the handles of saucepans made of wood. This means less heat will be  
transferred from the metal pan to the handle, so we don’t burn our hands when we hold the  
handle.  
Plastic – found on the handle of an electric kettle. So when you boil water to make a cup of  
tea, the plastic acts as an insulator so you won’t burn yourself when pouring the water into  
your mug.  
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Wool – insulators like wool can trap air to reduce heat loss, for example in fleece winter  
jackets  
Factors affecting the rate of Conduction  
1. Length of the material  
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The length of the material the energy must flow through can affect the rate at which it flows.  
The shorter the length, the faster it will flow. Therefore, the conducted heat,  
Q
is inversely  
ퟏ
proportional to the length of the bar,  
l
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푸 ∝  
풍
2. Cross-sectional area that is perpendicular to the heat flow  
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The larger the cross-sectional area, the faster the rate of heat conduction. A bar twice as wide  
conducts twice the amount of heat.  
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In general, the amount of heat conducted, Q, is proportional to the cross-sectional area, A i.e,  
푸 ∝ 푨  
3. Difference in temperature between the two ends of the material  
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The greater the difference in temperature between the two ends of the bar, the greater the  
rate of thermal energy transfer, so more heat is transferred. The heat, Q, is proportional to the  
difference in temperature, i.e,, 푸 ∝ ∆푻  
4. Thermal conductivity of the material  
This is a measure of the rate at which a material conducts heat. The higher the thermal  
conductivity of the material, the faster the rate of conduction  
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Minimizing heat losses by Conduction  
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In a system where heat needs to be conserved, heat losses by conduction can be minimized  
by thermal insulation  
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This involves the use of poor conductors of heat used in boilers, hot – water pipes and in the  
textile industry  
In the house is achieved by using double – glazed windows , carpets curtains and draught  
excluders  
Application of Good and Bad Conductors in everyday  
Application of Good conductors  
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Metals are good conductors of heat, therefore cooking utensils, kettles, irons, boilers, are  
always made up of iron, copper, Aluminum, or their alloys.  
Aluminium is used in making motor engines, pistons and cylinders due to its low density and  
high thermal conductivity  
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Soldering iron is made of iron with a tip made of copper because copper is a much better  
conductor of heat than iron.  
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When ironing a piece of clothing, the iron is hot and the heat is transferred to the clothing  
A heat exchanger uses a hot fluid to conduct heat to a cooler fluid without the two actually  
coming into direct contact  
Application of Bad conductors  
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The handles of saucepans, teapots, kettles, etc, are always made from substances like wood,  
plastic, etc, to prevent heat from entering the hand so that it can he held comfortably  
Air is a poor conductor of heat, this is why heat from our bodies does not escape easily  
to the atmosphere  
Fur, wool, cotton wool, cork, feathers, sawdust, trapped air in birds’ feathers etc. act as good  
insulators.  
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Fur and feathers keep most animals and birds warm during winter.  
Birds flutter their feathers in cold weather to trap air between their feathers  
Woolen clothes or blankets are used to keep people warm on cold days (winter).  
In cold countries, glass windows in houses are double paned  
Refrigerators, ice boxes, ovens, solar heater panels and connecting pipes are made of double  
walls with filling of cork, air, cotton wool or polyurethane in between their walls.  
Sawdust is used to cover up ice blocks because it has good insulating properties.  
Building materials like brick, asbestos, mud, grass, etc., are bad conductors of heat. They  
do not permit heat and cold to pass through the walls of bricks. They keep the houses warm in  
winter and cool in summer. Roof sheds are made of asbestos for the same reason  
Vehicles carrying inflammable materials such as petrol are covered with materials of bad  
conductors of heat. Otherwise, the petrol can get heated up and catch fire.  
The bottoms of cooking pots need periodic cleaning to remove layers of soot, which impede  
the flow of heat  
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Convection of Heat  
Convection Is the movement of heat through fluids (liquids or gases) caused by movement  
of liquid from the hotter to the colder parts  
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How does convection Occurs?  
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The Figure below shows how convection occurs, using hot water in a pot as an example. When  
particles in one area of a fluid (in this case, the water at the bottom of the pot) gain thermal  
energy, they move more quickly, have more collisions, and spread farther apart.  
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This decreases the density of the particles, so they rise up through the fluid. As they rise, they  
transfer their thermal energy to other particles of the fluid and cool off in the process. With less  
energy, the particles move more slowly, have fewer collisions, and move closer together. This  
increases their density, so they sink back down through the fluid. When they reach the bottom of  
the fluid, the cycle repeats. The result is a loop of moving particles called a convectional  
current  
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How can we minimize Convection?  
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It can be minimized by keeping the place vacuum,  
Example, vacuum flask minimizes heat loss by convection.  
It can also be reduced by filling air cavities with insulating materials  
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Application of Convection  
(i)  
Chimneys  
Smoke and gases from fires in houses and factories rise up chimneys and the flow is  
convection current. Gases pass up a tall chimney faster than up a short one, this is  
because high chimneys have greater pressure difference  
(ii)  
Ventilation  
Damp, warm, breathed – out air is less dense than ordinary air; it rises and can escape  
through openings near the roof. Houses and cinemas have openings in or near their  
ceilings.  
(iii)  
Air condition  
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On a hot summer day, air-conditioners are used constantly. The process of cooling air in air-  
conditioners employs the principle of convection. The air which is cold is released by the air-  
conditioners. Now, this cold air is denser than the warm air, and, hence, it sinks. The warm  
air, being less dense, rises and is drawn in by the air-conditioner. As a result, a convection  
current is set up and the room is cooled.  
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When it is cold, the heated air is turned on heating the surrounding air. The heated air rises  
up and cold air moves down to take the place of the rising warm air. This forms convectional  
current which continue to circulate until the air in the room is at the desired temperature  
Domestic hot water supply system  
(iv)  
This system works on the principle of Convection Current. Hot water moves from the boiler to  
where it is used under convection current. Inside the boiler water is heated making it less  
dense, water rises and flow into the hot water reservoir where it is stored until needed for use  
main water supply.See the figure below  
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(v)  
Winds  
Air flows from area of high pressure to area of low pressure. The warm air molecules over  
hot surface rise and create low pressure. So, cooler air with high pressure flows towards  
low pressure area. This causes wind flow.  
(vi)  
Land and sea breeze  
Land and sea breezes are a result of expansion of air caused by unequal heating and  
cooling of adjacent land and sea surfaces  
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During the day time the land is warmer than the sea thus the air over the land surface  
becomes less dense and so it rises. The space left is occupied by cooler air from above the  
sea surface. Thus sea breeze occur  
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During the night time the sea is hotter than the land thus the air over the sea becomes less  
denser and therefore replaced by cooler, denser air from the land . Thus land breezes  
occur