Heat (Part 5) class 7


Heat

Q. What are sea breeze and land breeze?
Ans. The people living in the coastal areas experience an interesting phenomenon.
Sea breeze:
During the daytime, the land gets heated faster than the water. The air over the land becomes hotter and rises up to create a low pressure of air. The colder air from the sea which is at higher pressure rushes towards the land to balance the pressure. The warm air from the land moves towards the sea to complete the cycle. This causes air to move from sea towards the land and is called sea breeze.           
To receive the cooler sea breeze, the windows of the houses in coastal areas are made to face the sea.               
Land breeze:
During night, the land cools up quickly than water. On cooling, a low pressure of air is created on sea (because sea is still hotter than land) while a high pressure of air is created on land. So air moves from land to sea. This is called land breeze.
   
Q. Write a short note on role of colours in context of heat.
Ans. When this heat falls on some object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.
Colours play an important role in context of heat.
  • Dark surfaces absorb most of the heat and light coloured surfaces reflect most of the heat that falls on them. For example: in summer we prefer light-coloured clothes and in winter we usually wear dark-coloured clothes. 
  • A good absorber is a good radiator, and a poor absorber is a poor radiator. For example: radiators of cars, A.C., freeze etc are painted with black colours.
Q. Trapping of air between two objects can prevent flow of heat between the object and surroundings. Give reason with examples.
Ans. A layer of air in between two objects prevents the flow of heat because air is a poor conductor of heat. For example:
  • Wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
  • To construct buildings, that are not affected much by heat and cold outside, we can construct outer walls of buildings so that they have trapped layers of air. One way of doing this is to use hollow bricks, which are available these days.
  • False ceilings keeps the room warm during winters and cool during summers.

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