THERMAL ENERGY

Introduction

Thermal energy simply means heat energy. The sun is the largest source of thermal energy to the Earth. Virtually all life on the earth rely on the sun’s energy, which reaches us in a vast spectrum of wavelengths – gamma and x-rays (highly dangerous), ultraviolet, visible, infrared, microwave and radio wave. It is the interaction (absorption) of ultraviolet radiation with matter on earth that gives rise to solar thermal energy. Evidently, regions with minimal sunshine experience cold weather, compared with equatorial regions of the earth, where the sunshine is maximum with the attendant high temperatures. Other sources of thermal energy include: electric arc/heating, geothermal energy, biofuel and nuclear energy (highly dangerous).

Thermal energy is applied in many ways such as cooking, sundry work and power generation (steam turbine). Thermal energy can be transmitted through radiation, conduction and convection.

 

Heat Flow

Heat (thermal) energy is the energy that flows from a hot body to a relatively cooler one by virtue of their temperature differential. Like other forms of energy, the unit of heat is the Joules (J). Heat is not to be confused with temperature. Temperature is a measure of how hot or cold a body is; its unit is degree Celsius (°C) or Kelvin (K).

Sources of heat energy include: electric arc, electric heating of a conductor, combustion (burning) of biofuels, nuclear reactions and geothermal sources.

Heat naturally flows from a hot body to a relatively cooler one when the two are sufficiently in contact with each other. This heat flow proceeds automatically until they are thermally at equilibrium.

If there is no heat loss to the environment, the rise in temperature of a body is proportional to the quantity of heat absorbed by the body, the mass of the body and the type of the substance.

 

Heat Transfer (Conduction, Convection and Radiation)

Heat transfer from a hot zone to a relatively cold one can occur in three modes, namely: conduction, convection and radiation.

 

Heat Conduction

Conduction of heat occurs when heat travels through a solid material from one end to another, without actual translational motion of the atoms. When heat is applied to one end of a metallic rod, the kinetic energy of the metal atoms increases. In an attempt to assume the ambient (normal) state, these hot metal atoms pass on the heat to the neighbouring cooler atoms thereby propagating (conducting) the heat from the hot area to the cooler area. Solid materials that easily allow heat to pass through them are called conductors of heat, while those that reluctantly allow passage of heat are called insulators. Heat conductors include all metals (aluminium, copper, zinc, mercury, lead, etc) while insulators include glass, rubber, plastics, water and wood.

 

Applications of Heat Conduction

Below are some of the applications of heat conduction:

1. Because metals are good conductors of heat; aluminium, silver, etc. are employed in fabricating cooking pots and sauce pans. However, the handles of cooking pots are made of insulators of heat such as plastics and wood. This is to allow the pots to be safely handled while still hot.
2. Car radiators are made from aluminium to conduct heat away from (cool) the engine.
3. Woolen dresses and blankets are employed to keep the body warm in cold regions. Wool, being an insulator of heat, prevents heat from being conducted away from the body, thereby, keeping the body warm in a cold region.
4. The ceiling materials (plastic, wood, asbestos, etc.) under the roof of a house prevent (insulate) the heat from the aluminium roofing sheets from reaching the occupants.
5. The outer walls of refrigerators, flasks, etc. are lined with insulators such as fibre glass and polyurethane foam to prevent heat exchange between inside and outside of the system.

 

Convection of Heat

Convection of heat is the process by which heat is transferred in a liquid or gas by the actual movement of atoms or molecules from hotter to cooler regions.

Convection of heat is the only mode of heat transfer in fluids (liquids and gases). Unlike the atoms and molecules of solids which are more or less fixed in position, the atoms and molecules of gases actually undergo translational motion when heated. When heat is applied to one part of a fluid, the atoms or molecules receiving the heat get hotter and lighter, with increased kinetic energy. These hot particles actually move away from the hot region to cooler one. Automatically, the cooler, heavier particles fall into place to replace the escaping hot particles. This cycle continues until the entire fluid gets evenly heated.

 

Applications of Convection

1. The air-conditioning of a room depends on convection current. Notice that the air conditioner (AC) is usually placed at an elevated position in the room. As the AC cools the air, the cooler heavier air falls to the floor of the room. Hotter, lighter air then rises to the upper part of the room where the AC cools it. This cycle continues until the entire room becomes cool.
2. Wind is caused by convection current. During the day, the sun heats up the land mass faster than the oceans, seas, lakes, etc. At night, the land mass becomes cool while the water bodies give up the heat gradually. This heat warms up air above the water body. The hot air now expands and moves away from the water body. The cooler air from the land mass rushes towards the water body. This rushing of the cooler air towards the waters gives rise to breeze, wind, hurricane, etc.

 

Radiation of Heat

Heat wave can be transmitted through vacuum by radiation. The radiant energy from the sun that causes heat sensation is the ultraviolet (UV) wavelengths. Therefore, unlike the conduction and convection, heat transmission by radiation does not require any medium. Hot objects give off heat energy which may be transmitted to neighbouring regions through radiation.

Generally, rough black surfaces absorb and radiate heat energy faster than polished white surfaces.

Heat energy can be detected by some photographic plates, thermopile and thermocouple.

 

Applications of Radiation

1. School uniforms in tropical regions are predominantly white because white colour does not absorb heat energy as fast as black.
2. Any house painted white will keep the interior cool because white colour does not absorb heat fast.
3. Tiles are used to bring about cooling in a room, while rugs keep the room warm.