Heat Science(Chap-4) NCERT(Class-7)

Science(Chap-4)                                                      Heat

Ø  We identify  the degree of hot or coldness  of any object by touching it. As the sense of touch is not always reliable, we need a reliable measurement device.

Ø  Temperature is a reliable measure of the degree of hotness of an object.

Ø   Thermometer is a device used for measuring temperature .

Ø  The thermometer that measures human body temperature is called a Clinical Thermometer. It consists of a long, narrow, uniform glass tube and has a bulb at one end which contains mercury. Outside the bulb, a small shining thread of mercury is seen.

Ø  There is a scale on thermometer known as Celsius scale, indicated by °C. A Clinical thermometer  reads temperature from  35°C to 42°C.

Ø  Thermometer should be washed before and after use preferably with antiseptic solution. Before using a thermometer, it should be ensured that the mercury level is at 35°C. it should not be hold near any flame or sun light.

Ø  Mercury is a toxic substance and if the thermometer breaks, it is very difficult to dispose. That’s why, now-a-days, digital thermometers are available which do not use mercury,

Ø  The normal temperature of human body is 37°C. it does not go below 35°C or above 42°C.

Ø  Laboratory Thermometer is used to measure the temperature of objects other than human body. Its range is usually from -10°C to 110°C.

Ø  Maximum-Minimum Thermometer is used to measure weather reports, maximum and minimum temperature of the previous day.

Ø  The heat flows from a body at a higher temperature to a body at a lower temperature. There are three ways in which heat can flow from one object to another. These are Conduction, Convection and Radiation.

Ø  The process by which heat is transferred from the hotter end to the colder end of an object is known as Conduction. In solids, generally the heat is transferred by this process.

Ø  The materials like Aluminum, Iron Copper etc.  allow heat to pass through them easily. so, these are called conductors of heat.  

Ø  The materials which do not allow heat to pass through them easily are poor conductors of heat, such as plastic and wood. These poor conductors of heat are called Insulators.

Ø  The water and air are poor conductors of heat.

Ø  In liquids and gases, the heat is transferred by the process of Convection.

Ø  When water or oil is heated, the water or oil near the flame gets hot and rises up. The cold portion from the side moves down towards the source of heat. This process continues till the whole water/oil  gets heated . This mode of heat transfer is called Convection.

Ø  No medium is required for transfer of heat by radiation.

Ø  The people living in the coastal areas experience an interesting phenomenon. During the day, the land gets heat faster than the water. So, air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place and the warm air from the land  moves towards the sea to complete the cycle. The flowing of sea air towards the land is called ‘sea breeze’.

Ø  At night,  the water in the sea cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.

Ø  Sun light can not reach earth by conduction or convection as there is no medium such (there is air in most part of the space between the earth and the sun). Sun heat comes to earth by radiation.

Ø  Dark coloured objects absorb more heat than the light-coloured objects. This is the reason we feel more comfortable in light-coloured clothes in the summer.

Ø  Woolen clothes keep us warm during winter. It is because wool is a poor conductor of heat and it has air trapped in between the fibres.