In a typical day, the energy you use comes from a variety of different resources. Most importantly, your body gets its energy from the food you eat. Homes, personal technology, creature comforts and transportation all require energy as well; they use resources such as fossil fuels, sunlight and nuclear energy.
Every activity in which you engage, whether it’s napping, doing housework or running a marathon, requires energy. Between the extremes of resting and hard exercise, the human body expends roughly 100 to 1,000 watts of power continually -- all of it coming from food. The sugars, fats and proteins you eat contain energetic chemical bonds built up inside the plants and animals from which they originally came. In your cells, “energy factories” called mitochondria release this energy in a form that’s useful to your body. The calories listed on food packaging are a way to quantify the energy in what you eat: each dietary calorie translates to 4,184 joules, or enough to sustain a runner for about four seconds.
The sun is the ultimate source of most of the energy you use every day. For example, sunlight allows plants to grow and produce food. More directly, however, solar power is a growing part of the renewable energy scene. Large-scale electricity generation from sunlight feeds into the power grid, supplementing traditional nuclear and fossil fuel sources. At a more personal level, many portable gadgets such as watches and calculators feature solar batteries powered by ambient light. Most communications satellites get their energy from solar cells, and solar panels are a fixture on many homes and office buildings, providing heat and electricity.
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Coal, gasoline, natural gas and other fossil fuels meet the majority of the world’s immediate energy needs. Formed deep underground from the decay of plant and animal matter over millions of years, these fuels are low in cost, energy-dense and easily transported. Because of their high energy content, portability and wide distribution network, liquid fuels such as gasoline and kerosene are crucial to modern transportation. Natural gas and coal account for the bulk of heating and industrial-scale power generation. Although fossil fuels are not without their drawbacks, they will continue to be an important energy resource for the foreseeable future.
Currently, electric utilities operate 65 nuclear power plants in the U.S., accounting for over 100,000 megawatts of capacity. Nuclear reactors exploit the energy of radioactive decay in uranium and other elements; heat given off by nuclear reactions is used to boil water into steam, which in turn drives turbines and electric generators. Nuclear power accounts for about 20 percent of the electricity produced in the U.S. and about 8.5 percent of all the country’s energy resources.