Conservation of Energy

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Conservation of Energy (The First Law of Thermodynamics) The principal of conservation of energy, or the first law of thermodynamics, implies that the energy of a system does not change as it goes from one state to another; only its form changes.

The First Law of Thermodynamics:

Energy can be neither created out of nothing nor destroyed into nothing, but it can be changed from one form to another.

For example, a glass resting on the edge of a table has a certain potential energy. If the glass is knocked off the edge, its potential energy is converted to kinetic energy as it accelerates towards the ground. When the glass hits the ground the kinetic energy is converted to light energy (sparks), sound energy (a bang), thermal energy (heat), and chemical energy (the glass breaks). The first law of thermodynamics is the basis of all energy conversions from one form to another. Many of our biological activities are geared to perform these energy conversions. For example, during digestion, food molecules are broken down into progressively simpler molecules (chemical-chemical conversion); in the process some of the chemical energy stored in those molecules is converted to the thermal energy necessary to maintain our body temperature. Numerous practical devices have been designed that accomplish useful tasks through the conversion of energy from one form to another. Table 1 gives examples of different kinds of energy conversions.

 Examples of Different Kinds of Energy Conversions
Table 1: Examples of Different Kinds of Energy Conversions

Question: If energy can be neither created nor destroyed, how can people claim that there is an “energy shortage”?

Answer: Terms such as “energy shortage” and “energy waste” are misnomers. While total energy must remain constant, useful energy — that which can be used as fuel or perform work— may be in short supply. According to the first law, energy can never be wasted; it may only be converted to a form not readily usable to us. This is explained more when we talk about the second law of thermodynamics.

Question: If energy can be transformed, what is it transformed into?

Answer: Although we often talk about transformation of energy, we should note that the nature of energy has not changed. We are only talking about its manifestation from one form to another.


(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

El-Sayed, Y., The Thermodynamics of Energy Conversions, Elsevier Direct Science, 2003.

Cengel, Y. A., Heat Transfer: A Practical Approach, McGraw-Hill, Inc., 1998.

Rifkin, J., Entropy, The Viking Press, 1980.

El-Wakil, M/ M., Power Plant Technology, McGraw-Hill, Inc., 1984.

Energy and Buildings, Science Direct Elsevier Publishing Company. An international journal publishing articles about energy use in buildings and indoor environment quality.

Energy Conversion and Management, Science Direct Elsevier Publishing Company. This journal focuses on energy efficiency and management; heat pipes; space and terrestrial power systems; hydrogen production and storage; renewable energy; nuclear power; fuel cells and advanced batteries.

Energy and Buildings, Science Direct Elsevier Publishing Company, An international journal dedicated to investigations of energy use and efficiency in buildings.

External Links

How Things Work (

How Stuff Works (

California Energy Commission Consumer Energy Center (