Joule
The joule is the standard unit of energy in the International System of Units (SI), defined as the energy transferred by a force of one newton moving over a distance of one meter. This measurement is roughly equivalent to the energy required to lift an apple one meter. The joule is named after James Prescott Joule, a 19th-century British physicist and brewer who investigated the relationship between heat, energy, and work, contributing significantly to the formulation of the first law of thermodynamics. One British thermal unit (BTU), a nonstandard unit of energy, is approximately equal to 1,055 joules.
In addition to its role as a fundamental measure of energy, the joule is integral to the watt, which is defined as one joule per second, and is commonly used in various contexts. Other units of energy, such as the electron volt (for atomic-scale measurements) and the kilocalorie (used in nutrition), can also be converted to joules. Despite the prominence of the joule in scientific discourse, nonstandard units like BTUs and horsepower hours are still in use, particularly in practical applications involving machinery and nutrition. Understanding the joule and its relations to other units is essential for grasping concepts in physics, engineering, and everyday energy consumption.
Joule
FIELDS OF STUDY: Classic Mechanics; Thermodynamics
ABSTRACT: The joule (J) is the standard unit of energy in the International System of Units. It measures the ability of a system to do work. This can include the chemical energy in the food that one’s body digests to power itself or the thermal and electromagnetic energy radiated by the sun. Various nonstandard units are used in certain contexts, but all can be expressed in terms of joules.
PRINCIPAL TERMS
- British thermal unit (BTU): a nonstandard unit of energy measurement supposedly equivalent to the heat needed to raise the temperature of one pound of water by one degree Fahrenheit.
- electron volts: units of energy equal to the energy carried by electrons from an electric potential to one higher by one volt.
- energy: the ability of a system to perform work.
- horsepower hour: a nonstandard unit of power supposedly equivalent to the amount of work a horse does over an hour.
- International System of Units (SI): also known as the "metric system"; a widely used standardized system of units for measuring natural phenomena.
- kilocalorie: a nonstandard unit commonly used to measure energy content in food.
- watt: the SI unit of power (i.e., energy transfer over time).
Measuring Energy
Questions such as how to quantify the amount of heat given off by a candle led to the first measurements of heat. Heat is recognized as one of the many forms of energy. The joule is the standard unit within the International System of Units (SI) for measuring energy. It is defined as the energy transferred (or "work done") by a force of one newton over a distance of one meter. In everyday terms, this is approximately the amount energy require to lift an apple one meter.
James Prescott Joule and BTUs
James Prescott Joule was a brewer and a physicist in England in the nineteenth century. He studied heat and its relationship to energy and work. He measured the energy output of various systems using the British thermal unit (BTU). His work helped lay the groundwork for the first law of thermodynamics: energy in a system is neither created nor destroyed. In recognition, the SI unit of energy that eventually replaced the BTU was named after him. One BTU is equal to approximately 1,055 joules.
The Joule Today
The joule is an important component of the more familiar unit, the watt. The watt is equivalent to one joule per second. Although the joule is the standard unit, many nonstandard units find usage in certain contexts. The electron volt (abbreviated eV) is popular among physicists working at the atomic scale. It is equal to approximately 1.6 x 10−19 joules. The kilocalorie ("food calorie") is the unit used on nutrition labels to quantify the amount of energy in a given food item. It is equal to approximately 4,184 joules. Just as horsepower is still used to describe the power of large everyday machines such as cars, the horsepower hour is sometimes used to measure the power output of large machines over time. It is equal to 2,684,519.54 joules. However, each of these nonstandard measures can be expressed using the joule.
Bibliography
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"This Month Physics History: December 1840—Joule’s Abstract on Converting Mechanical Power into Heat." APS News. American Physical Soc., Dec. 2009. Web. 3 Apr. 2015.