Sources of Thermal Energy

Sources of heat, or thermal energy, surround us. Examples are numerous, but can be understood in terms of a few broad categories. Thermal energy is the disordered vibration of molecules. So for example, sunlight is not thermal energy, but converts to thermal energy when it strikes our skin. Energy is conserved and cannot be created or destroyed, so thermal energy always comes from some other energy form, such as chemical energy, kinetic energy, or electromagnetic energy.

Underground Heat

Earth has not fully cooled off yet from its initial formation, as have smaller bodies like Mars and the Moon. Bombardment during its youth by the planetoids of the early solar system transferred kinetic energy to the Earth as thermal energy. Loss of this heat by convection of its dense insides over the subsequent billions of years has been very slow, and the insulating blanket of the Earth’s outer layers of the Earth only slows it more. Radioactive decay also serves as a heat source within the Earth. As a result, the Earth stays between 40 and 70 degrees Fahrenheit a few feet below ground over most of the Earth, regardless of the time of year. For several miles down, Earth’s temperature increases at a rate of about one degree Fahrenheit per 60 feet of depth.

Solar Fusion

Elements lighter than iron fuse inside stars, including the sun. This produces sunlight, which warms half the Earth at any given time. Scientists have created nuclear fusion here on Earth, but as of 2009 the reactions have not returned more energy than the scientists had to put in to cause the fusing in the first place.

Exothermic Reactions

Exothermic chemical reactions involve molecules reacting in such a way to reduce their internal energy. The lost energy manifests as heat. The molecules then lie at a lower energy level and (usually) in a more stable state than before. Exothermic reactions may need a little “nudge” to get started, but their net energy release is positive. For example, electrical energy input causes the dissociation of water 2H2O->2H2+O2, while the reverse reaction 2H2+O2->2H2O is spontaneous enough to power rockets. Exothermic reactions include combustion of fossil fuels.

Friction

A mechanical source of energy can also be a thermal source of energy if it produces friction. Examples include rubbing your hands to keep warm, rubbing sticks together to make fire, and auto engine parts rubbing together and needing a radiator to avoid overheating. Even convection in Earth’s dense inner layers creates significant heat under its tremendous pressures.

References

• Utah Schools Department: Types of Chemical Reactions
• Argonne National Lab: Temperature of the Earth’s Crust
• “The Dynamic Earth”; Brian Skinner; 1992

Resources

• Cornell U.: Earth’s Dynamism Creates Heat
• Straight.com: Homeowners Tap Earth for Geothermal Energy