The joule (J) is the unit of measurement for energy in the SI system, defined as the energy expended by a force of 1 newton moving an object 1 meter. It is a derived unit and can be decomposed as meters squared times kilograms times meters per second per second. Work and energy can be measured in joules, while power is measured in watts. The unit is named after James Prescott Joule, who discovered the theory of conservation of energy.
The joule (J) is the unit of measurement of 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 derives from a proper noun, even if it is not capitalized if written. A 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; thus, the acceleration is such that the velocity of the mass increases by 1 meter per second with each passing second.
Definition
The joule definition above may seem complicated, but the advantage of SI units is that they allow you to quantify all the many things that scientists might want to measure in terms of a small number of base units. It also means that by sticking to these units, all scientists speak the same language. The joule is said to be a derived unit, as it is derived from the base units kilograms, meters and seconds.
It can be thought of 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 decomposed further as meters squared times kilograms times meters per second per second:
J = m2kgs-2
Although a joule is 1 newton-metre, energy is not normally indicated using the latter unit, to avoid confusion with the unit of torque, or moment of force, which has to do with rotational motion.
Force, Work, Energy and Power
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 the work is done. For example, pushing an object in a certain direction involves applying a force, which works, and energy must be expended. Both work and energy can be measured in joules.
Power is the rate at which energy is consumed and is measured in watts. Using 1 J of energy for 1 second uses 1 watt of power, so 1 watt is 1 joule per second. A 100 watt light bulb consumes 100 J of energy per second. Household electricity consumption is measured in kilowatt hours: one kilowatt is 1,000 watts and 1 kilowatt hour is 3,600,000 J.
Examples
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 required 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 Earth’s gravity is roughly equivalent to 1 J.
History
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 the work is done, but it cannot be created out of thin air or destroyed. For example, when electricity is used to turn an engine, some of the electrical energy is turned into motion and the rest is converted into heat. He was able to show that motion plus heat equaled the original amount of electricity.
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