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Heat-Thermal energy.

posted Nov 20, 2015, 4:24 PM by Upali Salpadoru   [ updated Nov 23, 2022, 9:12 PM ]

Fig. We receive heat energy in 3 different ways.

තාප ශක්තිය පැතිරෙන අයුරු

 There are three main ways by which heat may be transferred, They are depicted in the above figure. තාපය සංක්‍රමණය විය හැකි ක්‍රම තුනක් ඉහත රූපයෙන් පෙන්වය්
A better name for heat is ‘internal energy’ or ‘thermal energy’ because the energy arises from the motion of atoms and molecules.

In solids the molecules do not move from place to place but they vibrate. Faster they vibrate hotter they would be.  

Temperature is a device we use to get an idea of the vibrational energy in molecules.




In making a thermometer it is necessary to use a large volume of the liquid and allow the expanding column to rise in a capillary  tube.

( hair thin)  


Fig. Principle of the thermometer.


 H2O molecules are at a very high state of vibration. This is why gas bubbles form inside the liquid.

Kinetic energy from these particles make the glass particles vibrate faster causing the the mercury particles also to vibrate faster.

As mercury expands much more than solid glass, mercury level rises.

Temperature is a very good measure to get an idea of how hot a substance is but it is not a measure of energy content.

Heat flows from a hot body to a cooler body; that means from a high temperature to a lower temperature.

( It is not from one having a lot of energy to another having less energy)


Generating heat

 We can get heat energy in various ways. The easiest way to get heat is by burning, burning wood, coal or gas. This chemical change produces a lot of heat and light. In the tropics, people use wood fires to cook and in the cold countries wood burners heat the houses.
  Nowadays people mostly use electricity to heat the homes and even for cooking. When an electric current passes through a metal of high resistance its molecules get the energy to vibrate faster. This energy can be used in various ways. Micro wave cooker is a special way to convert electricity to heat. heater.jpg
 Earliest men obtained heat by rubbing together sticks.
When motor cars apply brakes the wheels develop a lot of energy.
In some modern cars this energy is converted back into electricity.
 fire 3.jpg 
  The ultimate source of heat energy for our planet is the sun. 

The earth gets 99% of the energy from solar radiation.

Just a little heat comes from inside the earth due to radioactivity. This is called geo thermal energy.

 Solar energy.jpg

1. Conduction.

 Conduction This can occur only in solids as the particles are closely packed. Faster vibrating particles influence the neighboring molecules also to vibrate faster. All materials do not conduct very well. Some do not conduct at all and then they are called insulators. 
An experiment to select good and bad conductors.

  Nelli and Ali did an experiment to classify substances as conductors and insulators
They selected four materials of approximately the same length and thickness.
They held one end of the 4 sticks and heated the other end.

Ht 1.jpg


This is their record sheet.





Hand received little heat.

Not a good conductor


Hand felt the heat quickly.

A Good conductor.


Hand did not get the heat even when the wood was burning.

An insulator,


Ali had to drop quickly.

Very good conductor

Home  experiment. 1.

 Aim:- To test the conductivity of a substance. You need: A candle, a few match sticks. Rods of different materials.       

Warning:  An adult is essential to help you. HT 2.jpg  
  Method: Select a rod of about 30 cm.
Attach some match sticks on to that using candle wax. Then hold it with one hand and heat the other end by a flame. 
Count the number of matches that drop during a definite time.
  Repeat the experiment with different rods.

Tabulate the results in a chart like this.


Time taken

No. of matches dropped

2.   Convection

 The shape of the candle flame depends on the convection current.
In gases heated particles move faster. Due to the expansion , they become less dense. Then the hot molecules rise forming an upward draft. As this reduces the pressure cool air is drawn in. 
 Convection currents in a liquid. Heated water in red. Unheated water in blue. 


Home Experiment.2.

  Aim:-  To observe convection in air. Warning:- An adult is essential to help you.
You need:-
A box of matches
 Two tongs or tweezers
     A candle

Hold two match sticks equidistant from a flame. 

If they do not light bring them closer to the flame very slowly.

Which one will ignite first?

 Which would ignite first?Ht 3.jpg

Home Experiment. 3.

  Aim:-   To observe convection currents in air. You need:- A glass jar or a bottle. A paper partition. Box of m 1.atches.

1. Smoke producer
2. Glass or plastic jar (transparent
3. Partition.
4. Path of smoke.
5. Birthday candle.
Partition the jar with  a card. Roll some paper and ignite so as to produce smoke. Smoke becomes a tracer to know the movements of air. Hold the source of smoke as shown and observe the smoke trail.

As the hot air rises above the candle air is drawn in on the other side.
These is due to the convection current.
Hot air rises as they are less dense (Lighter)


3.   Radiation.

  This is the spreading of energy without the help of molecules. This mode may be considered as transmission of energy as waves. So they posses a frequency and wave length. 
Wave formula
Velocity = frequency x wave length. 
These are called "Electro magnetic waves" They all have the same velocity in space.


Common properties of these .

a. They travel at the highest velocity attainable. 300.000 km /second. (3 x10 8 ms-1)

b. They can travel in a vacuum.  

Preparation for the next part.

 Ali:: Have you ever wondered why a a steel pan is heavier than a aluminium pan of the same size?
 Nelli : Actually why is that ? Ali:- Lets ask the teacher.

Every known material is made up of Atoms and molecules. In some substances the molecules are tightly packed, while in others they are loosely packed. So the density depends on the amount of " inter molecular spaces in a substance' Do not assume that these spaces are filled with air. There is nothing inside those spaces.



 This is another important effect of heating. Except for a very few cases, all materials increase in size as the temperature rises.
 This has been termed as ‘expansion’ This effect is mainly responsible for the changes in state also.

Why do solids, liquids and gases expand when heated?


Vib 2.jpg

Why do molecules need more space at a higher temperature.

  The above diagram shows why materials expand when heated. The thick circle shows the molecule and the thin circle shows how far it can move on a vibration. Actually it is the amplitude of vibrations. As the energy increases molecules need more space. So they become far apart and the object less dense.     
Comparing the expansion of some solids.

Exp 4.jpg

 An experiment to observe the expansion of air due to body heat.



An experiment to observe the lineear expansion of a metal wire.



 Change of state.

What happens to the attraction between molecules as they become far apart?
It has to decrease. This is why a blacksmith will heat a rod of iron before hammering it into a blade of a knife
Further heating will decrease the attractions to such an extent that the molecules will crumble down. This what we call melting
What happens if you raise the temperature a little more?

With the complete loss of attractions the molecules will move faster and faster and some of them will escape and spread away in all directions. This the next change of state that we call evaporation.

 Quantity of heat energy
Just like any other type of energy heat has to be measured in Joules. One way to measure heat is to note the increase in temperature of a pure substance such as water. 
Water needs 4.18 Joules of energy to increase the temperature of 1 gram by 1°C .

 This is known as the specific heat of water.


Latent Heat

  When you heat water, the temperature goes up but when you continue to heat the boiling water temperature remains the same? Isn't that strange? When you boil water in an open vessel, molecules escape into the air vibrating at a very high rate. They need extra energy. This heat that is gained by water for the transformation is called latent heat. Latent heat is also required for melting.  

Some thermal properties of water.

SubstanceLatent Heat Fusion kJ/kgMelting Point °CLatent Heat Vaporization kJ/kgBoiling Point °C

A Sensitive Thermometer

 A Thermometer is an instrument which can detect the change in temperature. Most of them  work due to the expansion of a substance.
The substance usually used is either mercury or alcohol.

The solids and liquids expand very little even for a high change in temperature. But all gases expand much more.


 Air Thermometer

If the air gets even slightly heated , it will expand increasing the pressure. Due to this pressure the liquid will go up in the thin tube.

Although this is very sensitive, there is a problem. Changes in atmospheric pressure can make your results in accurate.


.Glass-Mercury Clinical thermometer.

Almost every household in the developed world has a clinical thermometer.The mercury column rises according to the part of the body where it is placed and remains there. It is necessary to shake it several times to get it down. There is a device to magnify this column. 

This works only for a limited range of temperatures  may be from 35°C to 42°C 

" You cannot use a clinical thermometer to get the reading of ice or hot water"


Answers to the question.
Select the correct no for each question.
  metal cover

  Prevents radiation,   

Prevents conduction.

For drinking,  

Prevents convection,

  Protection, ,   

 Insulating lid,

Double walled glass bottle.  

  Metal cup ,

   Give the appropriate letters for the answers.
 Name of part  Function
 1. J
 2. H
 3. A
 4. I
 5. B

The picture shows a Dewar (Thermos) flask.   Name the parts numbered and give the function of each part.

HELP - Select the answers from these jumbled words and phrases.

a. Metal cover,  ,   c.  Prevents radiation,   d. Prevents conduction.

e. For drinking,  f. Prevents convection,  g.  Protection, ,   h. Insulating lid,

i. Double walled glass bottle.   j.Metal cup ,

(2x10 = 20 Marks)

2.0  State the main method of heat transfer according to the pictures..

A- conduction,   B-  convection, C-  radiation.    D-   Conduction.







Cook 1.jpg



3.0  Divide these into three groups, Good conductors, Poor conductors and insulators.

Copper, glass ,  plastic, pyrex, ceramics, Brass, Stones (granite),water,wool

Aluminium, lead, Concrete, wood, air, rubber. Iron.

( 5x4 = 20 marks)


      Test tube containing some water and ice is heated        as shown.

.1   By what process does the heat reach the water?         ( 4 marks)

 2   What would happen to the water at the top and the        ice at the bottom after a short time?                           ( 4 marks)

3  Give two important conclusions possible from the results of this experiment. 

                                ( 4 marks)

4  Name one liquid that may conduct heat well.      ( 3 marks)

                                                                                           (15 marks)

  5.0   What could be having the nearest connection over these phenomenon.

A – Conduction of heat   B – Convection of heat   C – Radiation   D – Latent heat.

 B A (B)

.1 Sea breezes and land breezes.                          

2.      If you touch wood and iron at the same temperature iron feels cold.

3.       Sun heats the land without heating the air.                         

4.      When the humidity is low we feel cooler..                         

5.     Even when you tilt a candle, the flame remains vertical.                       

6.      Alcohol is cool when taken to a finger.                      

7.      The temperature of Ice and water remains the same until all the ice melts.      

8.      What property of iron makes it unsuitable for a cooking pot handle.            

9.      People near a fire feels hot.             

10.  Double glazing of windows reduces heat loss.   

(3 x 10 = 30 Marks)

                                                 (8 marks)

2  Mention one merit of using each of these as liquid in thermometers.

  1.  mercury        b.  alconol                          (2 marks)

3  Which is the correct reason for a liquid column to go up in a thermometer when warm.

A .  All materials expand when heated.

B - Liquids expand more than solids.

C - Solids are good conductors while liquids are poor conductors.

D - Convection currents cannot form in solids.              (5 marks)
For the solutions Click - Answer page - Science.



As there are no molecules in space how does the vibrational energy come from the sun?
What radiates, really are "Electro magnetic waves". As the waves of these frequencies, 10 15 Hz to 10 13Hz heat the molecules and also hot molecules give out these rays they are also called heat waves. But the more appropriate name for them is "Infra red radiation".
 What is the difference between heat and fire? Heat is energy while fire is matter.  Fire or a flame consists of molecules at a very high temperature. The blue outer region of the flame has a temperature of 1400 °C. The molecules are in an excited state. Some of their electrons have gone to higher orbits. As they come down they emit the energy as electromagnetic waves which include visible light and infrared rays.