Heat Conduction class 7 Types of heat transfer.pptx

Class 7 Heat
Types of Heat Transfer

1. Conduction Conduction is the process of transfer of heat from the hot end to the cold end from molecule to molecule of the medium Thus, a medium is required for the transfer of heat by conduction In conduction, the particles of the medium only vibrate about their mean position.

2. The transfer of heat by conduction takes place mainly in solids. It also occurs in liquids and gases but only to some extent

3. Why does conduction happen best in solids????

4. Transfer of heat by conduction

5. Conductors The substances which allow heat to pass through them easily are called conductors of heat. Examples: silver, copper, iron, aluminium , mercury etc. All metals are good conductors of heat because their particles are very closely packed so the vibrations are passed on very quickly.

6. Insulators The substances which do not allow heat to pass through them are called insulators (or bad conductors) of heat. Examples: wood, plastic, mud, cork, cotton, wool, asbestos, still air All pure liquids (except mercury) and gases are bad conductors of heat

7. Why mud houses are cool in summer??? Mud is a bad conductor of heat. During summer season,  the outside temperature is more than the temperature inside the mud  so we feel cooler in mud house. Whereas, in winter season the outside temperature is less than the temperature inside the mud. So, we feel warmer in mud house.

8. Note: The flowing air is a conductor of heat, but still air (i.e. the trapped air) is an insulator

9. Water is a bad conductor of heat Air is also a poor conductor of heat

10. Applications of conductors and insulators in daily life An oven is made of double walls and the space between them is filled with wool, cork etc. The wool and cork are insulators of heat. They prevent the heat of the oven to escape.

11. Cooking utensils and pans are made of metals such as copper, brass, steel etc. The reason is that metals are conductors of heat and so they heat up rapidly.

12. Cooking utensils, pans and tea kettles are provided with wooden or ebonite handles The wood and ebonite being insulators of heat, does not pass heat from the utensils to our hand

13. In summer, ice kept wrapped in a gunny bag or it is covered with saw dust. The air filled in the fine pores of the gunny bag or saw dust, is insulator of heat. The air does not allow the heat from outside to pass through it to the ice. Thus, ice is prevented f rom melting rapidly

14. We use woollen clothes in winter. Woollen clothes have fine pores which are filled with air. Wool and air are insulators of heat. Therefore, heat from our body does not escape through them and they keep us warm

15. During very cold weather, water pipes are covered with cotton. Cotton has air trapped in its fine pores. Cotton and air are insulators of heat. Thus, cotton prevents the water in the pipes from freezing

16. Quilts are filled with fluffy cotton. Air is trapped in the fine pores of cotton. Cotton and air are insulators of heat. They prevent heat from our body to escape and thus keep us warm Newly made quilts are warmer than old ones because in old quilts, there is less air trapped in the cotton