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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 energy 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.

Energy is often defined as the ability to do work. However, to be useful, we need to be more specific. Ultimately, in physics, the  term energy \(E\)ultimately refers to kinetic energy involving  the motion in a system, where \(m\) is the mass of an object and \(v\) is its velocity:

\(E = \frac{1}{2}mv^2\).

An important point is that motion can be on a visible, macroscopic level, such as that of a rushing train, or on a microscopic level, such as the motion of molecules in the atmosphere. Randomly-moving molecules are represent thermal energy or heat.

Energy can also refer to stored energy that can be transformed into motion. Energy occurs in several forms:

• 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:

\(E =kx^2\)

• 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.

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.

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