Heat
Science Class Seven
Transfer of Heat
Heat can be transferred from hotter body or place to the colder body or place.
Conductor and Insulator
Conductor
Bodies or objects which allow the transfer of heat through them are called conductors.
Rather it would be better to say the good conductor of heat instead of conductor.
All metals are conductors of heat such as iron, aluminium, copper, silver, gold, etc.
This is the cause that cooking utensils are made of metals, such as aluminium, copper, stainless steel, iron, etc.
Insulator
Bodies or objects which do not allow the transfer of heat through them are called insulators.
All non-metals are insulator of heat, for example rubber, plastic, wood, etc.
Insulator is known as bad conductor of heat.
This is the cause that the handles of cooking utensils are made of wood or plastic.
Processes of Transfer of Heat
There are three process by which heat is transferred. These processes are conduction, convection and radiation.
Conduction
The process of transfer of heat in solids is called conduction.
In other words, in solids, heat is transferred from one end to other end by the process of conduction.
Or Conduction is a process by which heat is transferred from one end to other end in a solids.
Heat does not transfer from one end to other end in a solid at once, rather it takes some time i.e. heat is transferred from one end to other end in a solid gradually.
The end of object which is placed on source of heat becomes hot first, then heat is transferred to place which is just after that part and so on. And gradually heat is transferred to whole of the object.
Example:
(1) A cooking utensil is kept on the flame of gas stove starts heating gradually. First of all, the lower part which is in direct contact of flame is get heated and gradually heat is transferred to the whole of the utensil. In this the process of conduction takes place.
The raw food kept in the cooking utensil also starts getting heat by transfer, i.e. by the process of conduction. And after some time, food kept in the utensil gets cooked.
(2) When one end of an iron rod is put over the flame, heat is transferred gradually to the other end. If you hold the one end of an iron rod by placing its one end over the flame, initially you will not feel hotness at all, but after some time, i.e. after few minutes, the other end of the rod starts heating gradually. You can feel it easily.
Caution: Do not do it without the supervision of elders.
Substances: Matter
Anything which occupy a space is called matter.
Example: Air, Water, table lamp, mobile, tree, a person, an animal, brick, soil, sand, etc. In fact all things present in the universe are matter, because every things occupy a space.
All matter are made of small particles. This all solid, liquid and gas, everything is made of small particles.
Convection
The process of transfer of heat in liquids is called convection.
When water is heated, particles of water which are got heated, rise up, and colder particles of water move down to fill up that place. This process continues and gradually whole water gets heated.
This continuous process of rising up hotter particles of water and replace by colder particles is known as convection.
Air also gets heated by the process of convection.
Storm
When air starts blowing with high speed, the situation is called storm.
When a particular area gets more heat from sun, the air present at that place gets heated and rises up and creates a gap at that place, the colder air from surrounding rushes to fill up the gap. This process continues and air starts blowing with a speed. This happens because of the process of convection.
Air moves from one place to other places because uneven heating. Air moves from one place to other place because of rising up of air at a particular place.
Radiation
We can feel heat without touching a flame apart from a short distance. Similarly, when we sit near a room heater, we feel its heat from a short distance.
This happens because a hot object transmit its head to the surrounding. The heat from sun reaches to us by the process of transmission of heat.
There is no air or a medium present between sun and earth. In spite of absence of any medium heat reaches to earth. This happens because heat is transmitted from sun.
The process of transmission of heat to its surrounding by a hot body is known as Radiation. The process of radiation of heat does not require any medium.
When a hot utensil or is left for a while, it gradually cools down. This happens because a hot object transmits heat to its surrounding by the process of radiation.
Sea Breeze and Land Breeze
Land gets heated more quickly than water. People lives near the coastal area or near big water body feels colder breeze in summer and hotter breeze in winter.
Sea Breeze
In summer land heated more quickly. Because of this air just above those lands gets heated and rises up leaving a gap. To fill up this gap comparatively colder air above the sea water surface rushes towards the land leaving a gap above the water. To fill this gap above the water the air above the land rushes to fill the gap. This cycle continues and a continuous flow of colder air starts blowing from sea surface towards land surface. Because of this people lives near the coastal areas feel colder air. This flow of air from above the sea towards the land is called Sea Breeze.
Land Breeze
In winter, the land surface is cooled down more quickly than water. Because of this air above the water surface rises up and creates a gap, to fulfil this gap air above the land surface rushes towards the sea. This also creates a gap above the land area and hotter air above the water surfaces rushes towards the land to fulfil the gap. This process continues and a cycle of hotter air from water surface of sea water towards land surface starts blowing. This is called Land Breeze.
Absorption of heat by Black surface and Light Surface
When heat falls over an object, some of the heat is absorbed and some of the heat is reflected back by the object.
Black surface absorbs more heat than light coloured objects.
Because of this nowadays cooking utensils are painted in black colour so that they can absorb more heat which will cook food faster.
Kinds of Cloths We Wear in Summer and Winter
Dark cloths absorbs more heat than light coloured cloths.
In summer, people prefers wearing light clothes. Because light coloured clothes reflect most of the heat and keeps people comfortable in summer.
While in winter, people prefers wearing dark coloured clothes. Because dark coloured clothes absorbs more heat than light coloured clothes. Because of absorbing of more heat, people who wears such dark coloured clothes feels comfortable.
Woollen cloths keep us warm in winter
Woollen clothes are more porous and thick compare to simple cloths. Pores found in thick woollen clothes trap air bubbles within them, which prevents getting coldness in and passing the body heat to outside. It acts as an insulator and keeps oneself warm.
These are the cause that people wear woollen clothes in winter which keep them warm.
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