Electronvolts and all that

An electronvolt, or eV, is the amount of energy an electron or proton gains when accelerated by a one-volt battery. It is a convenient unit for particle physicists to use when they use electric fields to make beams of high energy: If they know the strength of the battery, they know the energy of the beam.

TeV. GeV. eV. You don’t have to have read many Fermilab Today articles to have encountered these terms. So what the heck are they?

First, the good news: The basic idea is really easy. The term “eV” is just shorthand for electronvolt. (Note that both letters are pronounced: “ee-vee.”) So what is an electronvolt? It is a measurement of energy, specifically, the amount of energy a particle with the charge of an electron gets when it is accelerated by the equivalent of a one-volt battery. That’s where the term comes from: one electron accelerated by one volt gets one electronvolt of energy.

To be more precise, one eV is the amount of energy a particle with the same magnitude of electric charge as the electron gets when accelerated by the equivalent of a one-volt battery. So a proton, which has the same charge as an electron (with the opposite sign), also will gain an electronvolt of energy when it encounters a one-volt battery. But the name comes from the electron, and the unit is very convenient when scientists want to quickly know the beam energy generated from an accelerator that employs batteries of a certain voltage.

So what about those other terms, keV and all that? This is just a metric system thing. For the case of keV, it is short for “kiloelectronvolt,” where “kilo” means thousand. The table shows the rest:

 

Symbol Metric prefix Value
k kilo thousand
M mega million
G giga billion
T tera trillion

 

The Fermilab Tevatron, with its TeV energy, was an accelerator that produced beams with a trillion electronvolts and contained protons with the energy they would have received if they had been accelerated by one trillion one-volt batteries (or, equivalently, a single trillion-volt battery).

So just how much energy is an eV? Well, it’s actually really, really tiny. For those of you who took introductory physics, 1 eV is equal to 1.6 × 10-19 joules. If you dropped a small apple from about the height of your chest onto your foot, it would have one joule of energy. An electronvolt would be about 10 and a half billion billion times smaller than that, so we see that an electronvolt is very small. Even the TeV energy of the Tevatron is pretty small. A proton carrying a TeV of energy has about as much energy as a flying mosquito.

So how is it that we physicists can study such amazing things with a mosquito’s worth of energy? In particle physics collisions, this energy is concentrated into a really, really tiny volume. When one squeezes that energy into such a small space, it is possible to heat matter to a hundred thousand times hotter than the center of the sun and to discover new particles, such as the Higgs boson, and possibly even something unexpected that will require us to rewrite the textbooks.

Don Lincoln

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