Temperature > 02 - Heat Transmission

Heat transfer is the transfer of thermal energy from one place to another. This transfer can occur in three different ways: conduction, convection and radiation.

Conduction

This is the process of heat transfer in which thermal energy passes from one place to another through the particles of the medium that separates them. In conduction, the transfer of thermal energy from one region to another occurs as follows: in the region of higher temperature, the particles are more energized, vibrating with greater intensity; thus, these particles transmit energy to neighboring particles, which are less energized, which begin to vibrate with greater intensity; these, in turn, transmit thermal energy to the following ones, and so on.

It should be noted that, if the constituent particles of the medium did not exist, there would be no heat conduction. Therefore:

Heat conduction is a process that requires the presence of a material medium for its realization, and cannot occur in a vacuum (a place free of particles).

Heat propagates through the wall of a pizzeria oven.

Convection

To illustrate convection, imagine a room where we turn on an electric heater that is placed on the floor, in the center of the room.

The air around the heater heats up, becoming less dense than the rest. As a result, it rises and the cold air descends, exchanging the position of the hot air that rises with the cold air that descends. We call this movement of fluid masses convection and the air currents formed are convection currents.

In a room, hot air (less dense) rises, while cold air (more dense) descends.

Therefore, convection consists of movements of fluid masses, changing position. Note that it makes no sense to talk about convection in a vacuum.

Therefore, we can state that convection only occurs in fluids (liquids, gases and vapors), and cannot occur in solids or in a vacuum.

Convection can be natural, when it is caused by differences in density due to the difference in temperature between the fluid masses, or forced, when it is caused by pumps or fans.

Note that in convection there is no transfer of energy from one body to another, but only the latter change position.

Therefore, we conclude that, strictly speaking, convection is not a heat transmission process, since there is no transfer of energy from one body to another.

Examples:

I) Air conditioning unit and electric heater

In the summer, the air conditioning unit introduces cold air into the rooms from the top. Thus, due to its greater density, the cold air descends, causing the circulation of the air contained in the room.

The air conditioning unit should be placed at the top of the room wall.

In the winter, the air heated by the electric heater should be produced at the bottom of the room.

Note that if the opposite were done, the cold air (denser) would remain at the bottom and the hot air (less dense) would remain at the top, with no air circulation.

II) Coastal breezes

At the seaside, the sand, having a much lower specific heat than water, heats up more quickly than water during the day and cools down more quickly at night.

Therefore, we have:

DURING THE DAY:
The air near the sand becomes warmer than the rest and rises, giving rise to a current of air from the water to the land. The wind that, during the day, blows from the sea to the land.

During the day, breezes blow from the sea to the land.

DURING THE NIGHT:
The air near the surface of the water cools less than the rest. As a result, it becomes warmer than the rest and rises, giving rise to a current of air from the land to the water. This is the wind that, during the night, blows from the land to the sea.

During the night, breezes blow from the land to the sea.

III) Domestic refrigerator

In refrigerators, the freezer is always placed at the top, so that the air cools in its presence and descends, giving rise to the warmer air, which rises.

In domestic refrigerators, food is cooled by the cold air, which descends due to convection. The shelves are made of grids (and not solid) to allow air convection inside the refrigerator.

Radiation

This is the process of heat transmission by means of electromagnetic waves (heat waves). The energy emitted by one body (radiant energy) propagates to the other, through the space that separates them.

Since heat transmission is done by electromagnetic waves, radiation does not require the presence of a material medium to occur, that is, radiation occurs in a vacuum and also in material media.

However, not all material media allow the propagation of heat waves through them.

In this way, we can classify material media as:

Diothermal:

These are the media that allow the propagation of heat waves through them (these are the media transparent to heat waves).

Ex.: atmospheric air.

Athermal:

These are the media that do not allow the propagation of heat waves through them (these are the media opaque to heat waves).

Thermal energy comes from the Sun through electromagnetic waves.

Examples of radiation include the solar energy we receive daily, the energy emitted by a fireplace that warms us in winter, the energy emitted by a filament bulb, whose effect we feel effectively when we approach it, etc.

All radiant energy, transported by radio waves, infrared, ultraviolet, visible light, X-rays, gamma rays, etc., can be converted into thermal energy by absorption. However, only infrared radiation is called heat waves or heat radiation.