Electronvolt (eV)
The electronvolt, symbolized as eV, is primarily used in atomic, nuclear, and particle physics. It is the energy gained by an electron moving through a one-volt potential. The electronvolt enables scientists to measure very small energy changes at the atomic scale. Introduced in the early 20th century, it is standard for describing energy in atoms, photons, and particles. The electronvolt adds the necessary precision in modern physics and works in conjunction with larger units, such as joules and kilojoules.
BTU (International) (BTU)
The British Thermal Unit (BTU), symbolized as BTU, is an imperial unit of energy used to measure heat. One BTU is the heat needed to raise one pound of water by 1°F. The BTU originated in the 19th century, during the Industrial Revolution, for heating in the UK and the US. It is still used in HVAC systems and energy production. The BTU connects imperial and metric energy systems, especially in countries that use imperial units for industry.