Tranfer of heat. (short note) 7std
Answers
answer:
The mode of transfer of heat from hotter part of a material to its colder part or from a hot material to a cold material in contact with it, without the movement of material as a whole, is known as conduction. In all the solids, heat is transferred by the process of conduction.
explanation:
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The Flow Of Heat
as we know heat is a form of energy that makes an object hot. the heat always flows from a hotter object to a colder object or we can say from a body at higher temperature to a body at lower temperature, whenever two bodies at different temperatures come in contact.
therefore, when a steaming cup of coffee is left lying, it becomes cold because the heat flows from the coffee to the surrounding atmosphere.
Modes Of Heat Flow
there are three different ways by which heat flows from one body to another. these three modes of transfer of heat conduction, Convection and radiation.
Conduction
the mode of transfer of heat in solids by which heat flows from the hotter and to the colder and of a solid object is called conduction.
how heat flows by conduction
the particles of solids are very closely packed with little spaces between them. all particles vibrate at their fixed positions. when a material is heated, its particles vibrate fast and Bang into the particles next to them. thus, the heat is carried from one particle to another just like a relay race.
conduction of heat through different solid substances
the materials which conduct heat through them easily are known as conductors or good conductors of heat. all metals like copper gold silver aluminium iron etc. are good conductors of heat. the materials which do not conduct heat through them easily are known as insulators or bad conductors of heat. would break plastic glass wool cotton ice snow air and water are some poor conductors of heat.
good and poor conductors of heat in everyday life
★good conductors ★
- steel, copper and aluminium are used to make cooking utensils, so that they can easily transfer the heat from the source to the food being cooked.
- coppercopper iron and aluminium are used to make the base of an electric iron solar heating pipes and boilers in chemical and textile industries.
★ poor conductors of heat ★
- the handles and utensils are made of wood plastic and Bakelite. these are poor conductors of heat. water and ice are called poor conductors of heat because they do not allow it to pass through them by conduction. it does not mean that they do not allow heat to pass through them. the heat flows through them by another mode or method called convection.
Convection
the mode of transfer of heat in fluids by which heat flows from one part of a fluid to another by the movement of fluid particles itself is called connection.
how heat flows by Convection
when a liquid or a gas is heated its particles near the source of heat get heated and move upwards. the space left by them is quickly filled by the colder particles from the surroundings. now, these these particles get heated and rise up words. gradually, more and more particles keep on getting heated. the process continues till the whole fluid gets heated. Thus, in in the process of convection, the the heat gets transferred by the actual movement of particles within the substance.
Convection currents in nature
Convection currents takes place in air and ocean water, resulting in a number of natural phenomena i.e., ocean currents.
★ air and water are poor conductors of heat but they are good conductors of heat★
Radiation
we feel the warmth of the sun when we are outside because we get energy from the sun in the form of heat it is not possible by conduction because this process takes place in solids only. also the Convection process is not possible because there is no medium in some parts of the space between the Earth and the sun. there is another process of transfer of heat which can take place without any medium. this process is called radiation.
the mode of transfer of heat from distant services in the environment without requiring any medium or contact is called radiation. in this process heat flows in the form of waves. so we can also define radiation as the spread of flow of heat in the form of heat waves.
★radiation spreads heat in the environment without direct contact★
the radiation carrying heat is called thermal radiation.
when thermal radiation falls on an object, a part of it gets reflected, a part is absorbed and some of the path may be transmitted. the temperature of the object increases and it becomes hot due to absorbed part of the radiation. but but the amount of heat absorbed by the different objects is different. the the amount of heat absorbed by an object depends upon:
- the the nature of the material
- its its distance from the source of heat
- the the colour of the object
choicechoice of colours in our life
colours play an important role in our life. sincesince the amount of heat absorbed or radiated out depends upon the colour of the object the choice of colours of clothing, umbrellas, walls of the buildings, cooking utensils, etc. should be made intelligently.