What is a Joule?

The joule (J) is the unit of measure for energy used in the International System of Units (SI), and the one accepted in scientific circles. The unit is used in all equations and calculations involving energy. Following SI conventions, joules are abbreviated with a capital J because the term is derived from a proper name, though if spelled out, it is not capitalized. One J is defined as the amount of energy expended by a force of 1 newton moving an object 1 meter in the same direction as the force. A newton is the force required to accelerate a mass of 1 kilogram (2.2 pounds) at a rate of 1 meter (3.3 feet) per second per second; therefore, the acceleration is such that the speed of the mass increases by 1 meter per second with each second that passes.

The above definition of a joule may seem complicated, but the advantage about SI units is that they enable all the many things that scientists might want to measure to be quantified in terms of a small number of base units. It also means that, by sticking with these units, all scientists are speaking the same language. The joule is said to be a derived unit, in that it is obtained from the base units kilograms, meters, and seconds.

It can be regarded as a newton-meter — a force of 1 newton exerted over a distance of 1 meter (3.3 feet). Since the newton is itself a derived unit, a joule can be further broken down as meters squared times kilograms times meters per second per second:

Although a joule is 1 newton-meter, energy is not normally referred to using the latter unit, to avoid confusion with the unit of measure for torque, or moment of force, which has to do with rotational motion.

There is often some confusion between these terms, and the units used to measure them. A force is an influence that causes a change in the motion of an object; in other words, acceleration. Work is the exertion of force to produce movement, while energy is the potential to do work, and is expended when work is done. For example, pushing an object in a certain direction involves applying a force, which does work, and energy must be used up. Work and energy can both be measured in joules.

Power is the rate at which energy is used up and is measured in watts. Using up 1 J of energy for one second uses 1 watt of power, so 1 watt is 1 joule per second. A 100-watt light bulb uses 100 J of energy per second. Household electricity consumption is measured in kilowatt-hours: a kilowatt is 1,000 watts and 1 kilowatt hour is 3,600,000 J.

To get an idea of how much energy a joule actually is, it helps to look at some everyday examples. A joule is the amount of energy needed to raise the temperature of 1 gram of cool, dry air by 1.8°F (1.0°C). An average person sitting quietly produces 100 J of heat every second. The amount of energy required to lift a small apple 1 meter (3.3 feet) against the Earth's gravity is roughly equivalent to 1 J.

The unit is named after the English physicist James Prescott Joule, whose work led to the theory of conservation of energy, and the first law of thermodynamics, in the 19th century. Through his experiments, he was able to determine that energy changes from one form to another when work is done, but cannot be created from nothing or destroyed. For example, when electricity is used to turn a motor, part of the electrical energy is turned into motion, and the rest is converted into heat. He was able to show that the motion plus the heat were equivalent to the original amount of electrical energy.