# Introduction to Energy

#### By Mark Ciotola

First published on May 16, 2019. Last updated on February 6, 2021.

## Popular Definition of Energy

When most people refer to energy, they mean the amount of energy in a substance such as coal or petroleum that can be transformed into useful work. For example, a liter of gasoline can be combusted to power an automobile to drive uphill for several kilometers. Or, they are referring to electric energy, which can likewise be used to perform work.

## Physics Definition of Energy

The physics definition of energy includes the popular definition, but it is broader. In physics, energy can also refer to such that cannot be used but is nevertheless present. For example, ambient heat contains energy. Likewise, there is tremendous nuclear energy contained in the atoms comprising ordinary household objects such as forks and spoons but which cannot be accessed for useful purposes.

• The  physics definition of energy $$E$$ refers to the quantity of motion in a system, where $$m$$ is the mass of an object and $$v$$ is its velocity.

$$E = \frac{1}{2}mv^2$$

Energy can also refer to stored energy. Energy occurs in several forms:

• Moving objects possess kinetic energy
• Batteries, springs and raised objects store potential energy. For example, the energy stored in a spring, where $$x$$ is the distance a spring is compressed or stretched, and $$k$$ is a constant of proportionality, is:

$$E =kx^2$$

• Randomly-moving molecules represent heat energy
• Molecular bonds can store chemical energy
• Atomic nuclei can store nuclear energy

## Units of Energy

There are several units in which energy can be expressed.

• The standard scientific unit is the Joule, or $$J$$.
• A less standard science unit is the calorie, or $$cal$$. One calorie is the amount of energy required to raise one gram of water by one degree Celsius at standard atmospheric pressure.
• In the USA, the unit for energy in food is also called the Calorie (with a capital C), but the food Calorie $$Cal$$ is equal to 1000 science calories.
• The energy involved in heating systems is often expressed as in terms of the British Thermal Unit, or $$Btu$$. One Btu comprises about 1055 Joules.
• Electric energy, such as that delivered to your home, is often expressed in terms of the kilowatt-hour $$kWh$$. One Watt, $$W$$, is equal to one Joule per second. The Watt is a unit of power. Multiplying power by time results in an expression of energy.

Photovoltaic panels (source: U.S. Dept. of Energy)

## Power

As mentioned, when most people refer to energy, they mean useful energy, for example a quantity of coal or petroleum. However, when people want to use energy, they nearly always mean power, which is how much work (essentially useful energy) can be delivered per unit of time. Watts, KiloWatts, and Horsepower are units of work.

## Production of Power

Power can be produced by many means. Animals such as horses convert foodstuffs such as hay into muscular force that can be used to pull carts and plows. Wood or coal can be turned to create a thermal difference vis-a-vis the atmosphere that can be used to drive steam engines. Petroleum can drive combustion engines. Photovoltaic cells can convert high energy solar photons into electrical power. Wind mills convert the pressure differentials manifested by  wind into electrical motion.