Скачать или смотреть Prep.2 | Methods of Heat Transfer | Unit 1 - Lesson 4 | Science

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شرح ساينس تانية اعدادي - ترم اول

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in this lesson:
Heat
Is the energy that is transferred from one system to another or to the surroundings due to difference in their temperatures.

When two non-isolated systems with different temperatures come into contact (Cause), thermal energy flows (transfers) from the system with the higher temperature to the system with the lower temperature (Effect).
Heat continues to transfer (flow) between the two systems till they reach the same
temperature, which means that they reach thermal equilibrium.

Thermal equilibrium
The state reached by two systems with different temperatures after they come into contact and their temperature become equal.

First: Heat transfer by conduction
Explanation of heat transfer by conduction
When one end of a metal rod is heated, the average kinetic energy of its particles increases, causing them to vibrate to a greater extent.
Some of the energy of these particles is transferred when they collide with neighboring particles, which increases the kinetic energy of these neighboring particles, in the same manner, part of the energy is transferred to the next particles, without the particles moving from their positions.

Thermal Conduction
The transfer of thermal energy through solid objects from one point to another without the particles moving from their positions.

Thermal conductivity
A measure for the extent to which a substance can conduct heat through it.

Second: Heat transfer by convection currents
Thermal convection
The transfer of thermal energy in fluids with the movement of their particles.

GR. The space heater is placed on the floor of the room.
To ensure heat distribution throughout the room, as heat is transferred from the space heater to the air around it, causing the warm lighter air (with lower density) to rise to the top and the colder denser air (with higher density) to sink.

GR. The freezer is installed at the top of the refrigerator.
To ensure heat distribution throughout the refrigerator, as the denser cold air sinks to the bottom and allowing the lighter warm air to rise to the top.

How it happens:
(1) When the sun rays fall during the daytime, the air in contact with the land heats up more (becomes hotter) than the air above the surface of the sea.
(2) The hot air above the land rises and is replaced by the cool air (breeze) from the sea.
Its effect: The effect of this phenomenon is more perceptible (can be felt) in summer than in spring or autumn.


Third: Heat transfer by radiation
Thermal Radiation
The transfer of infrared radiation from the surfaces of objects, especially the hot objects, without the need for material particles.

Thermography
A technique that uses a camera to detect the thermal radiation (infrared rays) emitted from objects and convert it into coloured images, the colours in these images depend on the changes in the object's temperature.


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