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Heat-Thermal energy.

posted Nov 20, 2015, 4:24 PM by Upali Salpadoru   [ updated Oct 17, 2016, 11:51 AM ]

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Fig.1. We receive heat energy in 3 different ways.





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.

Thermometer.

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Fig.2. Principle of the thermometer.

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 ( hair thin)  tube.


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


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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.

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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.

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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.

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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.


 Question

 Answer
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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.



1.   Conduction

This can occur only in solids as the particles are closely packed. Faster vibrating particles influence the neighbouring molecules also to vibrate faster. All materials do not conduct very wee. Some do not conduct at all and then they are called insulators.


            An Experiment.


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Fig.3.Testing for conductivity.


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

                                                                                      





This is their record sheet.


Material

Observation

Conclusion

Glass

Hand received little heat.

Not a good conductor

Iron

Hand felt the heat quickly.

A Good conductor.

Wood

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

An insulator,

Copper

Ali had to drop quickly.

Very good conductor

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Fig.4 Matchstick experiment.

A home  experiment.

Aim:- To select good conductors and bad conductors

You need:

A candle, a few match sticks.

Rods of different materials.  

Warning: An adult is essential to help you.

Method:

Fig.2 .As the wax melt matches drop

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

.Tabulate the results in a chart like this.


Substance

Time taken

No. of matches dropped




1.   Convection


Fig.5. The shape of the candle flame depends on the convection current.


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Fig.5b. Convection currents in a liquid.

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. This can happen in liquids too.









A home Experiment.


Aim:- Ht 3.jpgTo observe convection.

Warning:- An adult is essential to help you.

You need:- A box of matches

Two tongs or tweezers

A candle

Fig.6 Which would ignite first.

These two are holding two match sticks equidistant from a flame. If they do not light bring them closer to the flame very slowly.

Observe which one will ignite first.


Fig.3 Which will ignite first?

Home Experiment 3.

Aim:-  To observe convection current.

You need:- A glass jar

                 Some thick paper

                 Box of matches.

Method:

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Partition the jar with  a card. Roll some paper and ignite so as to produce smoke. Hold the source of smoke as sh1own and observe the smoke trail. Smoke becomes a tracer to know the movements of air.
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Fig.7. Observing convection in air.





1.   Radiation.

All bodies above -273 °C radiate energy in the form of waves. These are known as electromagnetic waves. The quantity and the nature , (Wave length and frequency) depending on the temperature.

Here are some waves of the electromagnetic spectrum.

Wave type

Frequency Hz

Wavelength. in m.

Microwaves

10 13 to 10 12

10 -4 t0 10 -1

Infrared:(Heat)

10 15 to 10 13

10 -6 to 10 -4


Light:



10 16   to  10 15

Ultraviolet:

10 17 to 10 16

10 -9 to 10 -7

Expansion

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?

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Fig. 5 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 amplitude increases molecules need more space. So they become far apart and less dense.   

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   Fig.6  Comparing the expansion of some solids.

Fig. 7 An experiment to observe the expansion of a liquid.











Fig. 8 An experiment to observe the expansion of air due to body heat.

  







 Fig.9.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 aturto measure it 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 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.
  



SubstanceLatent Heat Fusion kJ/kgMelting Point °CLatent Heat Vaporization kJ/kgBoiling Point °C
Water33402264.76100

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. 
Fig.10. Air Thermometer. 

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

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.

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 .   

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


Fig.11 Glass-Mercury Clinical thermometer.

Making an Air Thermometer. 
This thermometer, which you can make depends on the expansion of air. All gases, including air expand very much more than solids and liquids. So even for the slightest change there will be a considerable change in volume. The other advantage is that you can build this from scrap without spending a cent.

What U need.

1. An empty paper milk carton with drinking plastic tube..

  (An Empty aluminum can would be better as shown in Fig 1, but making it air tight would be difficult.).  

2. Some coloured water or oil.

What you need is a metal can that can be tightly corked.   A hole has to be made in the cork to insert a tube. The tube can be a discarded ball point pen. There is some coloured water (or oil which will not evaporate soon), at the bottom. The substance that will expand will be the air trapped above the liquid. This gas will press the liquid down. Due to this pressure a liquid column will up along the thin tube of the ball point pen.

A paper scale is attached to the tube to show the changes in temperature. In countries where the temperature does  not change much you can dip the can into a bucket containing ice and go on adding hot water . 
Zero Celsius point can be marked using melting ice. The other temperatures may be calibrated using a wall thermometer. You will not be able to obtain very high temperatures as the liquid will overflow . If you wish to get radiant energy from the sun you must paint the can black.

If you are using a paper carton it would be much simpler. But as paper is a bad conductor of heat it wll take some time to register a temperature.

A very stable thermometer can be made with a fused electric bulb. There are adults who know how to open them up and remove the fused filament.  Then make the thermometer as shown here.  You can use this up  to  the boiling point of water from  freezing point. Provided the liquid will not spill.  Hold this firmly with your two palms and the level will rise quickly stopping at your body temperature.


Č






1.0

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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.


Metal cover,   Vacuum,    Prevents radiation,   Prevents conduction.

For drinking,  Prevents convection,    Protection, ,    Insulating lid,

Double walled glass bottle.   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.

 

.1

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.2

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.3

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.4

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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)


4.0  


      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.

 110 
 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)
6.0

                                                 (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.

 




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