class 7th heat chapter notes

GRADE VII

SCIENCE

CHAPTER: HEAT (PART-II)

Students, today we will discuss the following topics of the Chapter: Heat.

Ø  Flow of heat

Ø  Conduction

Ø  Conductors & Insulators

Ø  Convection

Ø  Sea breeze

Ø  Land breeze

Ø  Radiation

You will be provided with a video link of YouTube that will help you to understand it better. Most of the topics of this chapter are explained in the written form, as follow:

 

FLOW OF HEAT

Students, do you know why the mercury in a thermometer rise when you place it under your tongue? This is because it gets heated and expands. The heat from your mouth flows to the glass bulb to the mercury.

Heat can flow or travel through solids, liquids, gases and even through space. However it travels differently in different materials. There are three different ways in which heat travels:

a)    Conduction

b)    Convection

c)    Radiation

 

CONDUCTION

Conduction: It is the flow of heat through a substance without the movement of the substance itself.

Ex: When you stir a metal spoon in a pan of hot water, the end you are holding becomes too hot to handle. This is because heat travels from one end of the spoon to the other through conduction.

Note: Heat stops flowing when the two bodies are at the same temperature.

This is exactly what happens when you place a thermometer in your mouth. Heat flows from your mouth to the bulb of thermometer, until the two are at the same temperature.

Important Points:

Heat can flow from one body to another, when:

1.    They are in contact with each other.

2.    They are at different temperature.

 

 

CONDUCTORS & INSULATORS

All substances do not conduct heat with the same ease. So the substances are classified into two categories as under:

a)    Conductors

b)    Insulators

Conductors: Substances through which heat flows easily are called conductors. Ex: most metals are good conductors of heat, silver is the best. That is why cooking utensils are made of metals.

Insulators: Substances through which heat does not flow easily are called insulators or bad conductors. Ex: Paper, Glass, Wood, Plastic and Rubber. We can hold one end of a piece of paper in our hand while other end is burning, because paper is an insulator. Also the handles of cooking utensils are made of plastic, which is an insulator.

CONVECTION

Convection: It is the process of transfer of heat in a liquid or a gas by the movement of the liquid or gas.


How heat flows in liquids & gases?

Take a beaker of water. When the water at the bottom of the beaker is heated, it expands and becomes lighter. The lighter warmer water rises up in the beaker, it comes in contact with the colder water above. It passes on some of the heat to the cold water. In this process, it cools down, contracts and becomes heavy. The heavy water then sinks to the bottom, there it is again heated and this process is repeated. This sets up a continuous current of water – the hot water rises, loses heat and come down, to be heated again. Thus, heat is transferred from one part of the water to another by the movement of the water itself. This is called convection current.

Importance of convection current:

1.    Ventilators and exhaust fans are usually near the ceiling of a room because the air we breathe out is warmer and lighter, than ordinary air. It rises and escapes through the ventilator or exhaust fan, and cold, fresh air enters through the windows.

2.    Hot smoke and gases from industrial furnaces rise and escape through chimneys.

3.    Heater, blowers and heat convertors heat a room by setting convection currents.

4.    The sea breeze and the land breeze which blow in coastal areas are actually convection currents.

SEA BREEZE

The sea takes longer to get heated than does the land. So the land is hotter than the sea during the day. The air in contact with the land becomes hot and rises, and the cooler air above the sea rushes in towards the land to take its place. This sets up a convection current, which we call a sea breeze.

LAND BREEZE

After sunset, the land cools much faster. The air above the sea is warmer than that above the land. It rises, and the cooler air above the land moves out towards the sea. We call this a land breeze.

RADIATION

Heat foes not require a solid, liquid or gas to be able to travel. It can travel through space or vacuum. This is how heat from the sun can reach us.

Radiation: It is the process by which heat travels without the help of a material medium. It is much the same as the process by which light travels, Light and heat are very similar. Both travel in straight lines, in all directions.


All the objects radiate heat. The hotter something is, the more heat is radiates. In other words, the heat radiated by an object depends on its temperature. This heat is called radiant heat or radiant energy.

When radiant energy falls on an object, some of it is absorbed by the object and some is reflected by it.

Absorption of radiant heat by a body depends upon:

a)    The colour of the object

b)    The nature of its surface

 

Shiny, smooth surfaces are better reflectors, while dull, rough surfaces are better absorber. Black objects are the best absorber of radiant heat.

In general, darker colours absorb heat better than lighter colours. This is why it is advisable to wear dark coloured clothes in winter and lighter shades in summer.


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