Energy comes in six forms:


  1. Chemical energy
  2. Electrical energy
  3. Radiant energy
  4. Mechanical energy
  5. Nuclear energy
  6. Thermal energy

These six forms of energy are all related.  Each form can be converted or changed into the other forms.  For example, when wood burns, its chemical energy changes into thermal (heat) energy and radiant (light) energy.


Energy stored in the bonds between atoms in molecules is chemical energy. For example, in photosynthesis plants take in radiant energy from sunlight.  This solar energy is stored in complex chemical compounds such as starches and sugars.  The stored energy is released when these compounds break down into simpler compounds.

When you eat plant or animal tissue, your digestive system adds chemicals called enzymes that help break down the food.  Digestion converts stored energy in food to other forms of energy that your body can use, such as mechanical energy to walk across the room.

Many energy sources commonly used by humans are forms of chemical energy.  They are usually labeled “fuels.”  The way to use the chemical energy in most fuels is by burning them, as we do with wood, natural gas, gasoline, coal, and others.  When these fuels burn, they give off heat, because the chemical reaction called combustion is an exothermic reaction.  That is, the reaction releases thermal energy in the form of heat.

Some chemicals contain a great deal of energy that can be released all at once.  These chemicals are called explosives.  For example, when dynamite explodes, its chemical energy changes very quickly into thermal and radiant energy and transfers from a potential state to a kinetic state.


Electrical energy is the energy carried by moving electrons in an electric conductor.  It cannot be seen, but it is one of our most useful forms of energy because it is relatively easy to transmit and use.

All matter consists of atoms, and every atom contains one or more electrons, which are always moving.  When electrons are forced along a path in a conducting substance such as a wire, the result is energy called electricity.

Electrical generating plants do not create energy.  They change other forms of energy into electricity.  For example, power plants can convert chemical energy stored in fuels into thermal energy, which evaporates water into steam, which produces mechanical energy as it moves through turbines.  The turbines spin generators, which produce electricity.


Atoms absorb energy from an outside source and release (or “emit”) this energy as electromagnetic radiation.  This radiation can be in the form of waves of many different wavelengths or frequencies.

Many energy sources emit radiant energy.  The sun and other stars are luminous or “light-giving” objects that produce radiant energy from nuclear reactions.  Luminescence may result from biological processes (e.g., fireflies), from chemical reactions like burning kerosene in a lamp, from friction, or from electricity, as in a light bulb

Visible light is electromagnetic energy emitted at wavelengths our eyes can see.  Electromagnetic energy emitted at wavelengths we cannot see may take the form of infrared radiation, ultraviolet radiation, X-rays, gamma rays, and radio waves.  Gamma rays have wavelengths much shorter than visible light.  Radio waves are the opposite.  Their frequencies are far longer than the longest waves our eyes can see.

The electromagnetic spectrum includes radio, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Solar radiation, a term used to describe all electromagnetic radiation emitted by the sun, including visible light, passes through the atmosphere, is absorbed, and re-radiated back out to space as infrared (heat) waves.


Mechanical energy is the most familiar form of energy.  It is the energy a substance or system has because of its motion.  Every moving object has mechanical energy, whether it is a hammer driving a nail, a leaf falling from a tree, or a rocket flying in space.  Mechanical energy pulls, pushes, twists, turns and throws.

Machines use mechanical energy to do work.  Our bodies also use mechanical energy to perform motions such as throwing a ball or moving a pencil to write on paper.


A release of nuclear energy occurs when the nuclei of atoms are changed.  Hydrogen and uranium are two kinds of matter used to produce nuclear energy.  In a nuclear reaction, the tremendous binding energy inside a hydrogen or uranium nucleus is released.

Nuclear energy is released during atomic fission, when uranium nuclei are split.  It is also released during fusion, when hydrogen nuclei combine to form a helium nucleus.  In fission and fusion, nuclear energy produces thermal energy, which is given off as heat.  Fission’s heat is used to generate electric power in hundreds of locations worldwide.  The sun and other stars use fusion to generate radiant and thermal energy.  As stars give off energy, they lose mass.  Someday humans may be able to harness nuclear fusion as well.

Nuclear energy also has other uses.  In medicine, it is used in radiation therapy to treat cancer.  The U.S. Navy uses nuclear energy to power some submarines and large ships.  They can stay at sea for long periods without stopping to refuel, because their nuclear fuel takes up little space.


Thermal energy is the energy a substance or system has related to its temperature, i.e., the energy of moving or vibrating molecules.  Atoms and molecules, the smallest particles of any substance, are always in motion.  The motion of thermal energy is usually not visible, but we can feel or see its effects.  We use thermal energy to cook our food and heat our homes, and we use it to generate electricity.

Thermal energy is not the same as heat.  Heat is energy transferred between substances or systems due to a temperature difference between them.  So it is correct to say that a system contains thermal energy, but not that it “contains” heat, since heat means energy that is transferred from one thing to another.

The amount of heat transferred by a substance depends on the speed and number of atoms or molecules in motion.  The faster the atoms or molecules move, the higher the temperature, and the more atoms or molecules that are in motion, the greater the quantity of heat they transfer.

In solid substances, the particles’ movement is limited, resembling vibration.  Add heat to a solid, and the molecules move faster.  When enough heat is added, the substance melts and becomes a liquid, in which the particles slip and slide past one another.  Adding more heat eventually causes the molecules to bounce around randomly; the substance becomes a gas.  These phase changes occur at different temperatures, depending on the substance.