CBSE – 7th Standard Science

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Chapter 4 – Heat

 

HEAT

  • Some objects are hot some objects are cold around us.
  • A reliable measure of the hotness of an object is its temperature.
  • Temperature is measured by a device called thermometer
  • The thermometer that measures our body temperature is called a clinical thermometer

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  • A clinical thermometer consists of along, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen. Read temperature with a scale on the thermometer

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  • Thermometer should be washed before and after use, preferably with an antiseptic solution.
  • Ensure that before use the mercury level is below 35°C.
  • Read the thermometer keeping the level of mercury along the line of sight.
  • Handle the thermometer with care.
  • Don’t hold the thermometer by the bulb while reading it.
  • The clinical thermometer is designed to measure the temperature of human body only. The temperature of human body normally does not go below 35oCor above 42o That is the reason that this thermometer has the range 35oC to 42oC.
  • The range of a laboratory thermometer is generally from –10°C to 110°C.

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  • Different types of thermometers are used for different purposes.
  • One such thermometer is known as the laboratory thermometer
  • Laboratory thermometer should be kept upright not tilted.Bulb should be surrounded from all sides by the substance of which the temperature is to be measured.
  • The bulb should not touch the surface of the container.
  • The heat is, transferred from a hotter object to a colder object from the end nearest to the flame.
  • The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. In solids,heat is transferred by the process of conduction.

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  • 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.

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  • Poor conductors are known as insulators. The water and air are poor conductors of heat.
  • When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

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  • In the coastal areas land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze.

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  • At night the water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.
  • From the sun the heat comes to earth by another process known as radiation. The transfer of heat by radiation does not require any medium.
  • It is more comfortable to wear white or light-coloured clothes in the summer and dark-colored clothes in the winter
  • Dark surfaces absorb more heat and, therefore, we feel comfortable with dark colored clothes in the winter.
  • Light-colored clothes reflect most of the heat that falls on them and, therefore, we feel more comfortable wearing them in the summer.
  • In the winter, we use woolen clothes. Wool is a poor conductor of heat, air is trapped in between the wool fibres. This air prevents the flow of heat from our body to the cold surroundings. So, we feel warm.
  • Our sense of touch is not always a reliable guide to the degree of hotness of an object.
  • Temperature is a measure of the degree of hotness of an object.

Course Curriculum

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