Energy is a state function that is best defined as the capacity to do work or to produce heat. There are many forms of energy (e.g., radiant energy, kinetic energy, potential energy, etc) each of which can be converted into other forms of energy. The fundamental law of thermodynamics states that the total energy of the universe is fixed and that energy can not be created or destroyed—only converted from one form to another.
Energy can be changed, or transformed, from one form into another. Energy transformation is also called energy conversion. The Système International d'Unités (SI) unit for energy is the joule (J), named after James Joule, who demonstrated that work can be converted into heat. The Joule is the fundamental unit of energy for both work and heat and is the work done by a force of one Newton acting through a distance of one meter. The joule is also equal to 1/4.184 of a calorie. Energy is often expressed as the calorie (cal), which is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. One calorie is equal to 4.184 joules. The Calorie (Cal; also called the kilocalorie) that is used to express the energy in food, is equal to 1,000 calories.
Kinetic energy is the energy of an object in motion and is related by the Newtonian formula 1/2mv2 (where m equals mass in Kg and v equals velocity in meters/second). An object in motion can cause another object to do work by colliding with it, causing it to move a particular distance. The colliding objects can be a hammer swinging down on a nail, or two atoms colliding in a chemical reaction. Examples of kinetic energy include mechanical energy (caused by motion of parts) and thermal energy (caused by the random motion of particles of matter). An object that has potential energy has energy by virtue of position and is related by the Newtonian equation PE=mgh (where m equals mass in Kg, g the acceleration due to gravity—approximately 9.8 m/s2 near Earth's surface—and h equals height in meters). At some point, that object had work performed on it, which resulted in energy storage. One example of performing work on an object to give it potential energy is the lifting of a body against the gravitational force of the Earth. As it is lifted, the body gains potential energy that is converted into kinetic energy as the body falls.
Another example of potential energy is water in an elevated tank. If water is allowed to fall on a wheel and the wheel turns, the turning wheel can be used to produce electricity . The water in the tank has gravitational potential energy. The potential energy of the water is transformed into mechanical energy of the wheel, that is then further transformed into electrical energy.
In 1845, Joule performed an experiment that demonstrated energy transformation both qualitatively and quantitatively. The experiment was not complicated—he placed a paddle wheel in a tank of water and measured the temperature of the water. He cranked the wheel in the water for a period of time, then read the temperature again. He found that the temperature of the water rose as he cranked the paddle wheel. Joule quantified this observation and discovered that an equal amount of energy was always required to raise the temperature of the water by one degree. He also discovered that it did not have to be mechanical energy; it could be energy in any form. He obtained the same results with electrical or magnetic energy as he did with mechanical energy. Joule's experiments showed that different forms of energy are equivalent and can be converted from one form to another.
Interestingly, as Joule expressed it, the energy required to increase one pound of water by one degree on the Fahrenheit scale is equal to the amount of energy obtained by a weight of 890 pounds after falling one foot in Earth's gravitational field.
These observations led to what is now called the "Law of conservation of energy." This law states that any time energy is transferred between two objects, or converted from one form into another, no energy is created and none is destroyed. The total amount of energy involved in the process remains the same.
Most chemical reactions involve transformations in energy. A chemical reaction is simply the process whereby bonds are broken between atoms and new ones are made.
The ultimate example of energy transformation is that of the radiant energy of the Sun . All of the energy on Earth originates from the Sun, energy left over from the formation of Earth (usually thermal energy caused by the gravitational collapse of matter), or energy derived from nuclear decay in Earth's interior.) The thermal energy in Earth's interior drives plate tectonics and, at the surface, the Sun's radiant energy is converted by plants into chemical energy through the process of photosynthesis. This chemical energy is stored in the form of sugars and starches. When these plants are eaten by animals (i.e., as part of the food chain), this chemical energy is either transformed into another form of chemical energy (fats or muscle) or used for mechanical or thermal energy. With regard to fossil fuels , the fuels used in the modern era derive from the transformations of solar energy over millions of years.
See also Earth (planet); Earth, interior structure; Earthquakes; K-T event; Landslide; Mass movement; Radioactivity