# What Is Energy? Energy Definition and Examples (Science)

The concept of energy is key to science and engineering. Here is the definition, examples of energy, and a look at the way it is classified.

### Energy Definition

In science, energy is the ability to do work or heat objects. It is a scalar physical quantity, which means it has magnitude, but no direction. Energy is conserved, which means it can change from one form to another, but isn’t created or destroyed. There are many different types of energy, such as kinetic energy, potential energy, light, sound, and nuclear energy.

### Word Origin and Units

The term “energy” comes from the Greek word energeia or from the French words enmeaning in and ergon which means work. The SI unit of energy is the joule (J), where 1 J = 1‎kg⋅m2⋅s−2. Other units include the kilowatt-hour (kW-h), British thermal unit (BTU), calorie (c), kilocalorie (C), electron-volt (EV), erg, and foot-pound (ft-lb).

### What Losing Energy Means

One form of energy may be converted into another without violating a law of thermodynamics. Not all of these forms of energy are equally useful for practical applications. When energy is “lost”, it means the energy can’t be recaptured for use. This usually occurs when heat is produced. Losing energy doesn’t mean there is less of it, only that it has changed forms.

Energy may be either renewable or nonrenewable. Photosynthesis is an example of a process the produces renewable energy. Burning coal is an example of nonrenewable energy. The plant continues to produce chemical energy in the form of sugar, by converting solar energy. Once coal is burned, the ash can’t be used to continue the reaction.

### Kinetic Energy and Potential Energy

The various forms of energy are classified as kinetic energy, potential energy, or a mixture of them. Kinetic energy is energy of motion, while potential energy is stored energy or energy of position. The total of the sum of the kinetic and potential energy of a system is constant, but energy changes from one form to another.

For example, when you hold an apple motionless above the ground, it has potential energy, but no kinetic energy. When you drop the apple, it has both kinetic and potential energy as it falls. Just before it strikes the ground, it has maximum kinetic energy, but no potential energy.

### Renewable and Non-Renewable Energy

Another broad way of classifying energy is as renewable or non-renewable. Renewable energy is energy that replenishes within a human lifetime. Examples include solar energy, wind energy, and biomass. Non-renewable energy either does not regenerate or else takes longer than a human lifespan to do so. Fossil fuels are an example of non-renewable energy.

### Forms of Energy

There are many different forms energy can take. Here are some examples:

• nuclear energy – energy released by changes in the atomic nucleus, such as fission or fusion
• electrical energy – energy based on the attraction, repulsion, and movement of electrical charge, such as electrons, protons, or ions
• chemical energy – energy based on the difference between the amount required to form chemical bonds versus how much is needed to break them
• mechanical energy – the sum of the translational and rotational kinetic and potential energies of a system
• gravitational energy – energy stored in gravitational fields
• ionization energy – energy that binds an electron to its atom or molecule
• magnetic energy – energy stored within magnetic fields
• elastic energy – energy of a material that causes it to return to its original shape if it’s deformed
• radiant energy – electromagnetic radiation, such as light from the sun or heat from a stove
• thermal energy – kinetic energy due to the motion of subatomic particles, atoms, and molecules

### Examples of Energy

Here are some everyday examples of energy and a look at the types of energy:

• Throwing a ball: Throwing a ball is an example of kinetic energy, potential energy, and mechanical energy
• Fire: Fire is thermal energy, chemical energy, and radiant energy. Its source may be either renewable (wood) or non-renewable (coal).
• Charging a phone battery: Charging a phone involves electrical energy, chemical energy (for the battery), and both kinetic and potential energy. The stored electrical charge is potential energy, while moving charge is kinetic energy.

### References

• Harper, Douglas. “Energy”. Online Etymology Dictionary.
• Smith, Crosbie (1998). The Science of Energy – a Cultural History of Energy Physics in Victorian Britain. The University of Chicago Press. ISBN 978-0-226-76420-7.