The sun is the source of most of the energy on the Earth. Solar energy lights and warms the Earth, and plants use this energy to photosynthesize and grow. Many processes on Earth convert solar energy into different forms of energy. These include a plant's taking up sunlight, decomposing and becoming convertible to biofuel. Wind currents, another energy source, arise because the heat from the sun varies and alters the air pressure over certain areas of the planet.
Energy stored in atoms is the only other original source of energy in science apart from solar energy. This energy can be harnessed and used to create nuclear reactions to power electrical plants. Some mechanical energy sources in physics are kinetic energy, or energy of movement; and gravitational energy, found in falling objects. Chemical energy is the energy stored in molecules, thermal energy is the energy of heat, radiant energy is energy found in beams such as light, and electromagnetic energy results from electrical and magnetic slopes in an object.
There are three sources of energy in biological systems. These are fats, carbohydrates and proteins. One gram of fat delivers nine calories of energy, while a gram of carbohydrate or protein produces roughly four calories of energy. The body takes in this energy through food, but can also store the energy. The body stores carbohydrate in muscle and liver, protein is stored in muscle, and fat stays under the skin or on the abdomen.
Primary energy is energy that is present in natural conditions before humans alter the energy source. This type of energy includes solar energy, wind energy, geothermal energy -- heat energy stored under the ground and produced by radioactive decay -- running water, crude oil, plants and elements used in nuclear reactions.
Secondary energy sources are those sources altered by people. These include turning the wind energy into kinetic energy through turbines and then into electrical energy, splitting up crude oil into different fractions, and splitting atoms of uranium. Important secondary fossil fuels are gasoline, oil, diesel and the electricity produced from coal and nuclear material.