Units of energy

Author: Marina

May. 27, 2024

Energy Measurement Units

Units utilized in assessing energy quantities

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Energy is measured by its ability to do work, thus the SI unit for energy also represents the unit of work: the joule (J), a name honoring James Prescott Joule and his investigations into the mechanical equivalent of heat. More fundamentally, 1 joule equates to 1 newton meter and in terms of SI base units, this relationship is defined as:

1 J = 1 kg (m/s)^2 = 1 kg m^2/s^2

The electronvolt (eV) is an energy unit widely used in atomic, particle, and high energy physics. One electronvolt equals 1.602176634×10^-19 joules.

In spectroscopy, the unit cm^-1 (which equates to approximately 0.000123986 eV) is often used to denote energy, derived from the equation E=hc/λ, indicating energy's inverse relation to wavelength.

In energy production and consumption discussions, units like the barrel of oil equivalent and ton of oil equivalent are commonly referenced.

British Imperial and US Customary Units

Within British imperial and U.S. customary measurement systems, energy and work are quantified by units such as the foot-pound force (equivalent to 1.35582 joules), the British thermal unit (BTU) approximating 1055 joules, the horsepower-hour (measuring 2.6845 megajoules), and the gasoline gallon equivalent (about 120 megajoules).

Electricity

The kilowatt-hour (kWh) is a primary unit of electrical energy, especially seen in utility billing. Each kilowatt-hour amounts to 3.6 megajoules. Electrical consumption commonly appears as kilowatt-hours per year or other specified periods, reflecting an average power transfer rate. Essentially, one kilowatt-hour annually translates to approximately 0.11407 watts.

Natural Gas

Natural gas transactions may occur by energy content or volume. Energy content sales commonly utilize joules or therms, with 1 therm being roughly 105.5 megajoules. Volume sales use cubic meters or cubic feet, where 100 cubic feet (or 1 Ccf) equals 1 therm in U.S. trading. In contrast, Australian markets use cubic meters, with 1 cubic meter translating to around 38 megajoules. Globally, natural gas is frequently sold in gigajoules.

Food Industry

The calorie, quantifying the thermal energy required to increase one gram of water by 1 degrees Celsius at a pressure of 1 atm, is a fundamental unit in the food industry. The thermochemical calorie equates to 4.184 joules, though variations like the International Steam Table calorie exist (4.1868 joules). Food energy often appears as kilocalories (1000 calories), sometimes denoted as Calories with a capital "C". In the European Union, food labels must display energy in joules mandatorily, supplemented by calorie information.

Atomic Physics and Chemistry

Energy at the atomic scale in physics and chemistry often utilizes the non-SI unit electronvolt (eV). An eV represents the energy gained by an electron passing a potential difference of 1 volt in a vacuum. SI magnitude prefixes like milli- or mega- are typically applied to electronvolts. Due to mass-energy equivalence principles, the electronvolt sometimes measures mass. The Hartree (an atomic energy unit) features prominently in computational chemistry, and historically, Rydberg units were used.

Spectroscopy

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Spectroscopy frequently uses reciprocal centimeter units (cm^-1) to express energy levels. Although not strictly energy units, these units (cm^-1) match energies proportionally with hc ≈ 1.986×10^-23 joules per centimeter being the proportional constant.

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Explosions

When TNT explodes, it releases between 4,100 to 4,600 joules (or 980 to 1,100 calories) per gram. To standardize, the tonne of TNT was set to release exactly 1 kilocalorie (4,184 joules), amounting to 4.184 gigajoules (1 billion calories) for one tonne of TNT.

What does Einstein's equation E=mc² really mean?

In the equation E=mc², "E" signifies energy, and "m" denotes mass. This relation indicates that energy and matter are interchangeable while the total energy/matter in the universe remains constant.

Dr. Seuss's "The Sneetches" provides an illustrative analogy, where sneetches (characters in the book) continuously transform but their total count remains unchanged, akin to the interchangeability of energy and matter while maintaining a constant sum.

The term "c²" in the equation matches the function of the star-on and star-off machines in "The Sneetches". "c" represents the speed of light, a universal constant, demonstrating that energy is matter multiplied by the speed of light squared.

Energy, such as light waves or radiation, travels at light speed, approximately 186,000 miles per second (300,000 kilometers per second). When an atom splits in a nuclear reaction, the resulting energy moves at light speed.

The speed of light squared appears due to kinetic energy, defined as energy of motion, which is proportionally related to mass. Accelerating an object increases its kinetic energy exponentially (proportional to speed squared).

Consider this driving example: doubling your speed quadruples your braking distance, an illustration of speed squared in practical scenarios.

The enormous value of the speed of light squared underscores the colossal energy potential within small matter. For instance, if 1 gram of water converted entirely to energy via E=mc², it would release energy comparable to 20,000 tons (18,143 metric tons) of TNT.

Einstein's equation has catalyzed numerous advancements, from nuclear power and medical applications to the processes occurring within the sun, emphasizing that matter and energy are essentially one.

This article was updated in conjunction with AI technology and subsequently fact-checked and edited by a HowStuffWorks editor.

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