Changes

Wood and other plant materials were first used by early humans as fuel for fire. Many centuries later, they added wind and running water as new sources of energy. The use of coal as a fuel, limited until the mid-19th century, grew dramatically during the [http://www.history.com/topics/industrial-revolution Industrial Revolution], with coal replacing wood as the main source of energy in many regions of the world. Coal, in turn, was progressively replaced by oil during the first have of the 20th century, and by the mid-1960s, oil became the main source of energy. During the last decade of the 20th century, oil has provided 37% of the world's energy supply. At the same time, the contribution of natural gas to the total world supply of energy, which was minor in the first half of the 20th century, increased consistently during the second half, and as of 2005, gas provided about 22% of the world's consumption of energy. The energy supplied by running water has been used for many centuries but has never provided more than 6 or 7% of the total energy consumed in the world. The contribution of nuclear power, first used in the 1960s, reached 6-7% about 1990 but remained at that level during the 1990s.

Wood and other plant materials were first used by early humans as fuel for fire. Many centuries later, they added wind and running water as new sources of energy. The use of coal as a fuel, limited until the mid-19th century, grew dramatically during the [http://www.history.com/topics/industrial-revolution Industrial Revolution], with coal replacing wood as the main source of energy in many regions of the world. Coal, in turn, was progressively replaced by oil during the first have of the 20th century, and by the mid-1960s, oil became the main source of energy. During the last decade of the 20th century, oil has provided 37% of the world's energy supply. At the same time, the contribution of natural gas to the total world supply of energy, which was minor in the first half of the 20th century, increased consistently during the second half, and as of 2005, gas provided about 22% of the world's consumption of energy. The energy supplied by running water has been used for many centuries but has never provided more than 6 or 7% of the total energy consumed in the world. The contribution of nuclear power, first used in the 1960s, reached 6-7% about 1990 but remained at that level during the 1990s.

Fossil fuels (oil, natural gas, and coal) can be the source of transportation fuels and be used to generate electricity. Other primary sources of energy (nuclear, hydroelectric, and geothermal power, sunshine, and wind) can only be used to generate electricity. Biomass can fuel electricity generation and be the source of gaseous and liquid fuels but only in very small amounts. Hydrogen is not a primary source of energy; it needs to be generated by some process.

Fossil fuels (oil, natural gas, and coal) can be the source of transportation fuels and be used to generate electricity. Other primary sources of energy (nuclear, hydroelectric, and geothermal power, sunshine, and wind) can only be used to generate electricity. [http://www.nrel.gov/learning/re_biomass.html Biomass] can fuel electricity generation and be the source of gaseous and liquid fuels but only in very small amounts. Hydrogen is not a primary source of energy; it needs to be generated by some process.

==See also==

==See also==