CBSE Class 7 Physics Notes- Chapter 1 Heat

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                                               CHAPTER 1 HEAT
The measure of hotness or coldness of a body is called temperature. It is measured using a device called thermometer. A thermometer uses a scale to indicate the temperature. The two most common scales used are the Celsius scale represented by oC and the Fahrenheit scale represented by oF.
The normal temperature of a human body is 37oC and 98.6oF
When the transfer of heat takes place from the hotter end of the material to the cooler end, the process is called conduction. The materials which easily allow heat to pass through them are known as conductors whereas the materials which do not allow the heat to pass through them are called insulators.

When the transfer of heat takes place through actual movement of molecules. The heating of water is the best example of convection. When water is heated, the water near the flame is hotter than the rest of the water. The hot water rises up and the cold water moves down towards the flame. When this water gets heated up, water from the sides moves down and gets heated. This process continues till all the water is heated up.

In the coastal areas, the land breeze and sea breeze can be observed. During the day, the land gets heated up faster than the water so the air over the land is hotter than the air over water. As the hot air rises up, the cold air from over the sea rushes in to take its place and the hot air moves towards the sea. During the night, land cools down faster than the water. Now the air over the water rises up and cool air from the land rushes in to take its place. The movement of air from sea to the land is called sea breeze. The movement of air from land to sea is called land breeze.

When the transfer of heat does not require any medium, it is called radiation. For e.g. the sun’s heat reaching the earth.
Every hot body, including human body, radiates heat. Dark coloured objects absorb more heat and radiate heat faster. Light coloured objects absorb less heat and radiate less heat as well. Therefore, we wear light coloured clothes in summers and dark coloured clothes in winters. We wear woollen clothes in winters because wool is a poor conductor of heat as it has air trapped in between its fibres which means that the heat of our body remains trapped. Air being a bad conductor is also a reason as to why it is better to cover yourself with two thin blankets joined together instead of one thick blanket as there is a layer of air between the two blankets joined together which will keep you warm. 

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