Volt is a term invented in 1861 as a unit of electromotive force. The Ohm measured resistance, and the Farad measured capacitance, e.g. 300 Farads x 12 Volts & 12 Ohms could run a current for 1 hour. See the Farad below. Amp was defined in 1881 as the unit of current (i.e. charge movement per second), and the Coulomb as the unit of charge. Current is equal to the voltage divided by the resistance or I = V/R. This is known as Ohm's law. In the 1850s Ohm's law was widely known and considered proved. "I" stands for "Intensité de Courant" (French), or Current Intensity. So, Voltage is measured in volts, current is measured in amps and resistance is measured in ohms. Watt was invented the following year, in 1882, as the unit of power with the related term Joule suggested the same time, as a unit of heat. The joule and the watt were adopted in 1889. W (joules per second) = V (joules per coulomb) x A (coulombs per second) where W is watts, V is volts, and A is amperes of current. Farad is a derived unit where volts (of force) x farads (of capacitance) = coulombs (of charge). Named after physicist Michael Faraday for storage of charge in capacitors. One Farad describes a case of two oppositely charged plates, each with a coulomb of charge and a potential difference of one volt between them. Why is your answer in seconds when you multiply ohms times farads using T=RC? By Ohm’s law, resistance is inversely proportional to current which is coulombs per second (charge movement per second). C measures total units in coulombs. What’s left after the "units" math is seconds. Photon In 1900 Max Planck quantified the smallest unit of heat change or "action" as 6.626 × 10−34 joule per hertz (joule seconds). In 1905 Albert Einstein associated this "quantum" or minimal element of the energy, to the electromagnetic wave itself. Between 1925-1930 the photon became its favoured name, a force carrier having no mass but possessing momentum, with visible light measured at 10−19 joule. In modern physics, its repulsion of two electrons is described by a scattering process, where the two electrons bounce off one another through the passing of an intermediate photon. Electronvolt An associated term that also developed in France in 1925-1930 was the electronvolt or eV, defined as 1.6 x 10−19 Joule. One eV corresponds to an infrared photon with a wavelength of 1,240 nm nanometres and a frequency of 241.8 Terahertz THz Trillion Hz.
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