The atoms and molecules that make up a substance are in constant motion. This movement is what gives a material its temperature: the more the molecules are moving about, the higher the temperature. Thermal energy is simply the energy possessed by a substance due to the movement of its atoms or molecules. It is important not to confuse this with heat, which is energy that is transferred from one place to another. Thermal energy has long been exploited by humans for cooking, heating, power generation and industry.
Radiation, Conduction and Convection
Heat is defined as energy that is transferred from one region to another, always flowing from a higher to a lower temperature area. It can be transferred by infrared radiation, a form of electromagnetic radiation with a range of wavelengths that lie between radio waves and visible light. This interacts with matter to cause molecules to move about at greater speeds, which is seen as a rise in temperature: the material becomes warm or hot, because energy has been transferred from the source of the radiation to the material that absorbs it. For example, if an object if held near a flame, it becomes hot due to infrared radiation from the flame.
Transfer can also take place by two other methods. In conduction, the movement of molecules in a hot region causes increased movement in cooler areas as the molecules bump into one another. For example, if a metal spoon is held in a flame, the other end of the spoon will eventually become hot.
Convection involves the movement of a region of gas or liquid that is warmer than its surroundings. For example, the Sun heats the ground, which in turn heats the air, which then moves upwards, as the warm air is less dense than the cooler air above. It is convection that drives the world’s weather systems, as warm air from the tropics rises and flows outwards.
Thermal Energy and Matter
Thermal energy can bring about a change in the state of matter. If the molecules in a solid increase their motion sufficiently, it will melt to become a liquid. A further increase will cause the liquid to boil and become a gas, although liquids also tend to evaporate at temperatures well below their boiling points, as some molecules will be moving fast enough to escape the liquid. Since the molecules in a gas are moving more quickly than those in a liquid, a gas has more energy. This is why sweating cools people down: as sweat evaporates, it takes heat away from the body.
Exploiting Thermal Energy
The use of thermal energy, either directly or to generate electricity, drove the industrial revolution. It allowed the large-scale production of iron and steel and produced the steam to drive the turbines used to generate electricity. Humanity has for some time been heavily dependent on the combustion of fossil fuels, such as oil, coal and natural gas, as sources of thermal energy. Concerns about pollution, climate change and non-renewability, however, have generated a lot of interest in alternatives.
One source being exploited is geothermal energy. The Earth has a molten core, which is thought to have a temperature of 5,432-9,032°F (3,000-5,000°C). This high temperature comes partly from heat left over from the formation of the Earth, trapped beneath insulating layers of rock in the crust, and partly from the decay of radioactive elements. Between the core and the crust lies the mantle, a hot, semi-liquid region that drives plate tectonics and volcanic eruptions. There are many “hot spots” on the Earth’s crust where this heat is close to the surface and can be harnessed in various ways.
Geothermal heat can be used either directly, to provide heating for homes, or to generate electricity. Geysers are a ready-made source of hot water, but most geothermal projects involve drilling holes and pumping water into them. The water is heated below the surface and pumped out again to provide energy. Geothermal energy is not, strictly speaking, a renewable energy source, but there is such a huge supply of heat from the core that it will not run out in the foreseeable future.
Although solar energy can be exploited to produce electricity directly through solar panels, another area being explored is solar thermal energy. This involves capturing heat from the Sun to provide power or hot water. This may be used for central heating in homes, where water, or another fluid, is pumped through a container that receives sunlight, raising its temperature. Alternatively, heat from the Sun can be used to drive mechanical devices that generate electricity, or may be focused by convex mirrors in such a way as to provide heat for cooking or other purposes. This idea can also be adopted on a larger scale to boil water that drives a turbine or to build a “solar furnace” that can achieve the extreme temperatures required for some industrial processes.