What is a Joule? The History and Applications of Joules, Watts, and Coulombs

Learn about the origin, definition, and practical applications of the Joule, a key energy unit in physics and engineering.

September 15, 2020By Edmund Isakov, Ph.D.

Open Flipbook

On Oct. 21, 1948, the ninth meeting of the General Conference on Weights and Measures (CGPM) adopted the joule as a unit of energy, work, and heat. A joule is defined as the energy transferred when the force of one newton moves an object through a distance of one meter in the direction of the force.

Understanding the Joule

The mathematical expression for the joule is:

J = N × m = (kg × m/s²) × m = kg × m²/s²

In terms of SI base units, it is expressed as m² × kg × s^-2. Another way to express the joule is N × m.

Origins of the Joule

James Prescott Joule (1818–1889), an English physicist, is the namesake of this unit. His groundbreaking experiments and measurements estimated the mechanical equivalent of heat, paving the way for modern energy calculations.

Conversion of Units

To better understand energy conversions between metric and U.S. customary systems, the following chart highlights key equivalences:

1 J = 0.73756 ft-lbf	1 ft-lbf = 1.35582 J
1 J = 0.23892 cal₁₅ (15° C)	1 cal₁₅ = 4.1855 J

This conversion table provides practical insights for engineers, scientists, and educators working across different systems of measurement.

Comparison with the Calorie

The calorie is a pre-SI metric unit of energy. For example, Cal₁₅ is the amount of energy needed to warm 1 g of water from 14.5° C to 15.5° C at the standard atmospheric pressure of 760 mm of mercury.

Today, the joule has largely replaced the calorie in scientific contexts, further solidifying its role as a standard unit of energy.