HEAT AND ITS EFFECTS-CLASS 07-CBSE-SCIENCE-SUPER NOTES-BEST NOTES FOR EXAMS

      

       

    

         HEAT is a form of energy which causes sensation of hotness or coldness.

• Heat is measured in joules or calories .
• S. I. Unit of heat is joule. 
• 1 kilojoule (KJ)  = 1000 joules (J)      
• 1 kilocalorie (KC)  = 1000 calories (C) 
• 1 cal= 4.184 J

Effects of heat 

1. Heat causes expansion.

2. Heat causes change in temperature.

3. Heat causes change in state.

1. Heat causes expansion 

     All substances expand on heating and contract on cooling. 

•  Expansion in solid

Molecules in solids are closely packed together and their positions are fixed . The space between the molecules of solids are almost negligible and the force of attraction between them is very strong . These molecules only vibrate in their mean positions.  While heating, the movement of these particles increases result in an increase in distance between molecule which cause the volume of solid increase.

• Expansion in liquids

The molecules of liquids are loosely packed as compared to solid and their intermolecular space is also more . Also the force of attraction between the molecule is more . So when liquid is heated, the vibration of its molecule increases with increase in  the average distance between them resulting, as increase of volume of liquid.

• Expansion in gases

The intermolecular space in gases are much bigger as compared to liquid and solid . There intermolecular force of attraction is weakest and hence, the  expansion is more in case of gases as compared to solids and liquids.

                     TEMPERATURE

Temperature is the degree of hotness or coldness of an object on some chosen scale. 

• S. I. Unit- kelvin(K)
• Commonly used unit - Celsius and Fahrenheit. 
• Thermometer- It is a used for measuring temperature of body .                 It works on principle of expansion of liquids on heat .                                              It has two standard markings on it known as fixed point.                                          1. Lower fixed point-  Temperature at which ice melts at sea level.                        2. Upper fixed point  - Temperature at which pure water boils at sea level. 
• SCALES FOR MEASUREMENT OF TEMPERATURE-

               1. Celsius - It was named after the greatest scientist and Celsius who discovered it.

       Lower Fixed Point - 0 degree Celsius 

       Upper fixed point -100 degree Celsius 

       Internal divisions- 100

                 2. Fahrenheit -It was named after Daniel Gabriel Fahrenheit.

               Lower Fixed Point -32 degree Celsius 

               Upper fixed point- 212 degree Celsius 

               Internal Divisions-  180

                 3. Kelvin-It is the S. I Unit of measuring temperature. 

                (In detail you will learn in higher classes). 

RELATIONSHIP BETWEEN CELSIUS AND FAHRENHEIT 

 Formula   C/100 = F-32/180.

Q.  Convert 98.6 degree F to Celsius. 

Sol-    We have, 

                              F=98.6 

          And we know that, 

                              C/100=F-32/180

           Therefore, 

                              C/100= 98.6-32/180

                Or,          C= 37 degrees. Ans

Types of thermometer.... 

1. Laboratory  thermometer 

Laboratory thermometer 

• It is used in Laboratories for measuring different temperatures. 
• It consists of very fine capillary tube protected by a thick glass tube called stem of thermometer . 
• The lower end of capillary tube is convert date into glass bulb filled with Mercury and the upper end of this tube is sealed.

Range = -10 degrees Celsius to 110 degrees Celsius. 

                2. Clinical thermometer 

Clinical thermometer 

• It is used in clinics for measuring body temperature . 
• Construction of clinical thermometer is as same as that of laboratory  thermometer but only difference is that there is a slightly bent in the stem just above the bulb .
• Range- 35 degree celsius to 42 degree Celsius .
• Normal human body temperature 98.6 degrees Fahrenheit or 37 degree Celsius.

HOW TO USE A CLINICAL THERMOMETER -

1. What's the thermometer with an antiseptic and try it using a cotton swab.

2. Hold and give the thermometer a future to bring the Mercury level below 35 degree celsius.

3. Place the bulb of thermometer under the tongue for 2 minutes, in case of child put it below the armpit.

 4. After 2 minutes take out the thermometer and read the reading.

PRECAUTIONS TO OBSERVE WHILE READING A THERMOMETER -

1. Thermometer should be washed before and after the use. 

2. Don't hold thermometer at bulb while reading . 3. Don't steralise the thermometer in boiling water as it will break the bulb .

 4. Avoid keeping thermometer near flame or in sunlight as it may break . 

5. Hold  the thermometer carefully because if it had any hard object it may break.

NOTE-  Things or substances  which allows heat  to pass through them are called good conductors of heat and objects which does not allow to heat to pass through them are called bad conductor or insulator of heat.

TRANSFER OF HEAT 

The flow of heat from a body at Higher temperature to another body or to another part of some body at a lower temperature is for transfer of heat.

Heat is transferred in three ways conduction, Convection and radiation.

1. CONDUCTION - It is the process of transfer of                                   heat in the body from water                                     into colder and without any      actual movement of molecules of material .                                         The transfer of heat by this process only takes place in solids.

      

 Condition required

It can be conducted from one body to another when - a.  it is contact with each other

             b.  at different temperature. 

2. CONVECTION-  The process of transfer of                                         heat in a liquid or gas by actual movement of particles of the liquid or gas is called convection.

                                 Liquid and gases are heated by this process.

3. RADIATION-  It is the process of movement                                 of  heat transfer of heat from one place to another without any medium. 

 Applications of radiations

1. We wear white or light coloured clothes during summer because white or light colour cloth absorb less heat and keep us cool , on the other hand,  we wear black or dark colour clothes during winters because black Or dark colour clothes absorb more heat and keep us warm . 

2. The houses in the hot countries are usually painted white or light colour . This is done so that the houses absorb less heat from the sun and keep cool during summer . 

3. The containers of solar cooker are painted black to absorb more heat from the Sun.

THERMOFLASK

Thermoflask is a special type of vessel which is hot liquid' Hot' and Cold liquid 'cold' for a long time .

 It is made in such a way that it reduces the loss of heat by conduction Convection and radiation to the minimum level in the following manner-

1. Thermoflask consists of a double wall glass bottle, glass is chosen as a material for making thermos flask because it is a bad conductor of heat. 

2. All the air is evacuated from the space between the two walls of the glass bottle to prevent the transfer of heat by conduction and convection.

3. The surface of double walled glass bottle is silvered like a mirror . This is done to reduce the transfer of heat by radiation. 

4.  A plastic stopper is fitted in the mouth of the flat can a plastic cap is screwed over this topic . This is done to reduce the conduction of heat has plastic is a bad conductor of heat.

5. The glass bottle is held inside the metal case with the help of cork pieces . This is done to prevent transfer of heat by conduction is cork is a bad conductor of heat.

   Thus,  the  transfer of heat into or out of the  thermoflask is very less . So hot liquid remains 'hot' and cold liquid remains 'cold' inside the thermo flask for a long time.

Thermoflask 

LAND BREEZE AND SEA BREEZE 

The cool air from the sea towards the land is called sea breeze ,whereas ,the cool air from the land moves towards the sea is called land breeze .

Land breeze 

Sea breeze 

      QUESTIONS FOR PRACTICE 

 1.  Define temperature .

 2. Name the device which is used to measure temperature .

 3. Name three ways in heat  can be flow or transferred. 

 4. what are insulators ?

 5.what is the range of Celsius scale ?            

 6. which is the fastest mode of heat transfer -   conduction ,Convection or radiation? 

 7. Name the thermometer used to measure.        the temperature of boiling water. 

8. Which property of matter is used in measurement of temperature.

 9. what is the usual temperature range of a laboratory thermometer? 

10. what are the two conditions necessary for the conduction of heat 

11. How does the heat from the sun reaches the earth ?

 12.  How do we measure heat? 

13. Why is mercury used in  thermometer? 

14.  Convert 77 degree fahrenheit into degree celsius.

15.  Explain why a clinical thermometer cannot be used to measure high temperature?