Energy is the ability to do work
Energy comes in different forms:
- Heat (thermal)
- Light (radiant)
- Motion (kinetic)
- Nuclear energy
People use energy for everything from walking to sending astronauts into space.
There are two types of energy:
- Stored (potential) energy
- Working (kinetic) energy
For example, the food a person eats contains chemical energy, and a person's body stores this energy until he or she uses it as kinetic energy during work or play.
Energy sources can be categorized as renewable or nonrenewable
When people use electricity in their homes, the electrical power is probably generated by burning coal or natural gas, by a nuclear reaction, or by a hydroelectric plant on a river, to name just a few sources. When people fill up a car’s gasoline tank, the energy source is petroleum (gasoline) refined from crude oil and may include fuel ethanol made by growing and processing corn. Coal, natural gas, nuclear, hydropower, petroleum, and ethanol are called energy sources.
Energy sources are divided into two groups:
- Renewable (an energy source that can be easily replenished)
- Nonrenewable (an energy source that cannot be easily replenished)
There are five main renewable energy sources:
- Solar energy from the sun
- Geothermal energy from heat inside the earth
- Wind energy
- Biomass from plants
- Hydropower from flowing water
Most of the energy consumed in the United States is from nonrenewable energy sources:
Crude oil, natural gas, and coal are called fossil fuels because they were formed over millions of years by the action of heat from the earth's core and pressure from rock and soil on the remains (or fossils) of dead plants and creatures such as microscopic diatoms. Most of the petroleum products consumed in the United States are made from crude oil, but petroleum liquids can also be made from natural gas and coal.
Nuclear energy is produced from uranium, a nonrenewable energy source whose atoms are split (through a process called nuclear fission) to create heat and, eventually, electricity.
The chart below shows the energy sources used in the United States. In 2017, nonrenewable energy sources accounted for about 90% of U.S. energy consumption in 2017. Biomass, which includes wood, biofuels, and biomass waste, is the largest renewable energy source, and it accounted for nearly half of all renewable energy consumption and about 5% of total U.S. energy consumption.