Conversion from 5 Nanojoules to Electronvolts

Information about the Nanojoule (nJ)

The nanojoule is a unit of energy equal to one billionth of a joule, or 0.000000001 joules. Its symbol is nJ. It marks the point where mechanics gives way to optics and electronics: far too little to move anything the eye can see, yet still very large compared with the energy carried by a single particle. Almost every quantity written in nanojoules comes off an instrument rather than out of everyday experience.

Light gives the clearest picture of the scale. A photon of green light carries about 3.6 × 10⁻¹⁹ joules, so a nanojoule is roughly 2.8 billion such photons arriving together. Measured against heat it is larger still: the average thermal energy of a single molecule at room temperature is about 4 × 10⁻²¹ joules, which makes one nanojoule some 240 billion times that. The unit therefore sits well above the quantum world and well below the mechanical one.

Pulsed lasers are where it earns its keep. The oscillator in a two-photon microscope typically delivers pulses of a few nanojoules at eighty million pulses a second, which averages out to a few hundred milliwatts of beam power. Fibre lasers, optical coherence tomography scanners and time-of-flight rangefinders are all specified the same way, because what damages a sample or returns a usable echo is the energy in each pulse, not the average power spread across the second.

Digital electronics is measured against the nanojoule from below. A single switching event inside a processor costs femtojoules, a memory access a few picojoules, so one nanojoule pays for hundreds of memory reads or millions of logic operations. Energy-harvesting design inverts the picture: an indoor photovoltaic cell, a piezoelectric pickup or a radio-frequency antenna may gather only a few nanojoules per second, and the whole craft of ultra-low-power engineering consists of fitting useful work inside that budget.

Ordinary objects reach the scale only when they are very small or barely moved. A grain of sand weighing a milligram, falling a tenth of a millimetre, releases about one nanojoule. Lifting a single human hair by its own width costs less than that. Nothing in a kitchen or a workshop is ever priced in nanojoules, which is precisely why the unit belongs to laboratories, laser catalogues and datasheets rather than to bills and labels.

In writing, the prefix competes with scientific notation. Physics papers usually print 10⁻⁹ J and move on, while instrument manufacturers print nJ because it fits on a specification line and reads without arithmetic. Both express the same quantity, and the choice says more about the intended reader than about the measurement. Where a table mixes pulse energies spanning several decades, the prefixed forms are generally easier to compare at a glance.

One nanojoule equals 0.000000001 joules, about 6.24 × 10⁹ electronvolts, or about 2.78 × 10⁻¹³ watt-hours.

Information about the Electronvolt (eV)

The electronvolt is a unit of energy equal to the work done on a single electron moved through a potential difference of one volt. Its symbol is eV. Since the 2019 redefinition of the SI it has an exact value, 1.602176634 times ten to the power minus nineteen joules, because the elementary charge itself is now a defined constant.

It exists because the joule is hopelessly large for the atomic world. A single visible photon carries between about 1.6 and 3.3 electronvolts, and expressing that in joules requires nineteen leading zeros. Working in electronvolts lets a physicist state the energy of a chemical bond, an atomic transition or a subatomic collision as a number between one and a few, which is what a unit is supposed to do.

The scale organises whole disciplines. Chemistry lives at a few electronvolts, the energy of bonds and of the visible and ultraviolet photons that break them. X-rays occupy kiloelectronvolts. Nuclear physics works in megaelectronvolts, the scale of binding energies and radioactive decay. Particle physics uses gigaelectronvolts, where the proton's rest mass is 0.938, and teraelectronvolts at the Large Hadron Collider, whose protons carry 6.8 each.

Mass is quoted in the same unit through the mass-energy relation. A particle's rest mass is given in electronvolts divided by the speed of light squared, and physicists routinely drop the divisor and simply say that the electron is 511 kiloelectronvolts. The Higgs boson is 125 gigaelectronvolts on the same convention.

Semiconductor engineering depends on it. The band gap that determines what a material does electrically and optically is an energy in electronvolts: about 1.1 for silicon, 3.4 for gallium nitride, and this single number decides whether a device is a solar cell, a transistor or a blue light-emitting diode.

Astronomy inherits the convention for high-energy photons, so gamma-ray telescopes report observations in gigaelectronvolts and teraelectronvolts rather than in wavelengths.

Particle accelerators are named after the energies they reach, which is why their names track the prefixes. The Cockcroft-Walton machine that first split a nucleus in 1932 worked at a few hundred kiloelectronvolts; the Bevatron of the 1950s reached billions and found the antiproton; and the Large Hadron Collider brings protons to 6.8 teraelectronvolts each. Cosmic rays put all of this in perspective by arriving with energies up to 10 to the twentieth electronvolts, tens of millions of times more than any machine can produce, though only a handful of such particles strike the Earth in a century over a square kilometre.

One electronvolt equals about 1.602 times ten to the power minus nineteen joules, or roughly 3.83 times ten to the power minus twenty calories.