What is Energy?
Forms of Energy
Law of Conservation of Energy
Energy Efficiency
Sources of Energy
What is energy?
Energy makes change; it does things for us. It moves cars along the road
and boats over the water. It bakes a cake in the oven and keeps ice frozen
in the freezer. It plays our favorite songs on the radio and lights our
homes. Energy makes our bodies grow and allows our minds to think. Scientists
define energy as the ability to do work. People have learned how to change energy from one form to another so that we can do work more easily and live more comfortably.
Forms of Energy
Energy is found in different forms, such as light, heat, sound and motion.
There are many forms of energy, but they can all be put into two categories:
kinetic and potential.
KINETIC ENERGYKinetic energy is motion––of waves, electrons, atoms, molecules, substances, and objects. |
POTENTIAL ENERGYPotential energy is stored energy and the energy of position––gravitational energy. There are several forms of potential energy. |
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Law of Conservation of Energy
To
scientists, conservation of energy is not saving
energy. The law of conservation of energy says that energy is neither created
nor destroyed. When we use energy, it doesn’t disappear. We change it from one
form of energy into another.
A car engine burns gasoline, converting the chemical energy in gasoline into mechanical energy. Solar cells change radiant energy into electrical energy. Energy changes form, but the total amount of energy in the universe stays the same. Scientists at the Department of Energy think they have discovered a mysterious new form of energy called "dark energy" that is actually causing the universe to grow!
Energy Efficiency
Energy efficiency is the amount of useful energy
you get from a system. A perfect, energy-efficient machine would change all
the energy put in it into useful work—an impossible dream. Converting one form
of energy into another form always involves a loss of usable energy.
In fact, most energy transformations are not very efficient. The human body is a good example.
Your body is like a machine, and the fuel for your machine is food. Food gives you the energy to move, breathe, and think. But your body isn’t very efficient at converting food into useful work. Your body is less than five percent efficient most of the time. The rest of the energy is lost as heat. You can really feel that heat when you exercise!
We use many different energy sources to do work for us. Energy sources are classified into two groups—renewable and nonrenewable. Renewable and nonrenewable energy can be converted into secondary energy sources like electricity and hydrogen.
In the United States, most of our energy comes from nonrenewable energy sources. Coal, petroleum, natural gas, propane, and uranium are nonrenewable energy sources. They are used to make electricity, to heat our homes, to move our cars, and to manufacture all kinds of products.
These energy sources are called nonrenewable because their supplies are limited. Petroleum, for example, was formed millions of years ago from the remains of ancient sea plants and animals. We can’t make more petroleum in a short time.
Renewable energy sources include biomass, geothermal energy, hydropower, solar energy, and wind energy. They are called renewable energy sources because they are replenished in a short time. Day after day, the sun shines, the wind blows, and the rivers flow. We use renewable energy sources mainly to make electricity.
Electricity and hydrogen are different from the other energy sources because they are secondary sources of energy. Secondary sources of energy—energy carriers— are used to store, move, and deliver energy in easily usable form. We have to use another energy source to make electricity or hydrogen. In the United States, coal is the number one energy source for generating electricity. Today the cheapest way to get hydrogen is by separating it from natural gas, a nonrenewable energy source. Hydrogen can also be separated from water and from renewables but hydrogen made from these sources is currently too expensive to compete with other fuels. Scientists are working on ways to make hydrogen from water and renewables more affordable.
Last Revised: October 2008
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