Heat (Part 4) class 7



Heat

Q. Explain the method of transfer of heat in detail.
Ans. Transfer of heat takes place when two things are at different temperature.
The transfer of heat takes place through 3 different methods:
i) CONDUCTION
Conduction is a process of transfer of neat in solids where heat passes from one molecule to other in the form of energy. It moves from the hotter end to colder end. Molecules do not leave their place in the conduction process.
For example, a steel spoon kept in hot water becomes hot after sometime due to conduction. On the basis of conduction, we can further classify materials as good thermal conductor and poor thermal conductor (insulators).
The materials which allow heat to pass through them easily are conductors of heat. For examples, aluminum, iron and copper.
The materials which do not allow heat to pass through them easily are poor conductors of heat such as plastic and wood. Poor conductors are known as insulators.
Water and air are also poor conductors of heat.

 ii) CONVECTION
The process of transmission of heat, from one place to the other by the actual movement of the heated particles is known as convection. In fluids (liquids and gases), generally, the heat is transferred by the process of convection.
Molecules at the bottom of the container get heated up first, they become light and move up. Colder and heavier, molecules exchange their places. The process goes on till the time all the liquid or gas gets an equal temperature.
Unlike conduction, molecules leave their place and move up by taking heat along. For example:
  •  Water in a container heats up by convection,
  •  Sea breeze and land breeze are caused due to convection of gas air. 
iii)RADIATION
Radiation is a process of transfer of heat that does not require any medium. The object which is comparatively hotter than the other, it radiates heat into the surroundings till the time both the objects gets equilibrium.
All hot bodies whether solid, liquid or gas radiate heat to colder surroundings in all directions. 
For e.g.: 
  • The heat of Sun reaches Earth by radiation. 
  • When we stand around a bonfire, we feel hot because heat reaches us by radiation. 
  • Ice cream melts when kept in open it receives heat by radiation.
Q. What are the essential conditions needed for heat to be conducted from one body to another?
Ans. Conditions needed for heat to be conducted from one body to another are as follows:
i) They must be in contact with each other
ii) They must be at different temperatures

Q. Where is the freezer compartment put in a fridge and why?
Ans. It is put at the top. It is located there because, as cool air moves down, it cools the food on the way down. It is warmer at the bottom, so this warmer air rises and convection current is set up.












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