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The definition of electronvolt (eV) from wiki is

In physics, the electronvolt (symbol eV; also written electron volt) is a unit of energy equal to approximately 160 zeptojoules (symbol zJ) or 1.6×10−19 joules (symbol J). By definition, it is the amount of energy gained (or lost) by the charge of a single electron moved across an electric potential difference of one volt.

Masses of particles are often expressed in eV. But to say that the energy or mass of a particle such as a neutrino is measured in eV is weird to me since it cannot be accelerated by an electric field. Why do people use the electronvolt to describe the masses of all particle, even those that don't carry electric charges?

innisfree
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user6760
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    The invariant mass of the photon is zero. I don't see what charge has to do with this. – John Rennie Jan 24 '16 at 06:11
  • @JohnRennie: sorry my mistake I mean the energy of photon vs definition of electronVolt. – user6760 Jan 24 '16 at 06:14
  • I corrected the title according to your reply to John. The content is still strange. – anna v Jan 24 '16 at 06:33
  • @annav: actually I look up mass of say neutrino, higgs boson, neutron etc I mean these particles usually can't be accelerated in electric field unlike electron. Then why use eV as unit for their mass. – user6760 Jan 24 '16 at 06:54
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    eV is simply a unit of energy, just like Joule. It does not presume that a system has any electrodynamic properties. The rest mass of a photon is zero, so its rest mass energy is 0eV. Its 4-momentum can be large, which means that its energy is given by the 0th component of the 4-momentum vector: http://physics.stackexchange.com/questions/186019/4-momentum-of-photon. – CuriousOne Jan 24 '16 at 07:00
  • Are you really worrying of how we can find the mass of particles if they have no charge? there are other methods and they depend on using energy conservation equations. see as an example determining the mass of the neutron https://en.wikipedia.org/wiki/Neutron#Mass – anna v Jan 24 '16 at 09:38
  • @annav no, the q is about the eV unit of mass – innisfree Jan 24 '16 at 09:45

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One electron-volt $=1.6 \times 10^{-19} joules$ and is a unit of energy that is equal to the energy acquired by an electron falling across a 1 volt potential difference. The particle (neutrino) doesn't need a charge to have some energy. Instead of expressing the mass of a particle in kg, we can express it as $mc^2$ which is an energy (joules or eV... your choice). This makes it easy to calculate the total energy of a particle $E_{Total}=T_{KineticEnergy}+mc^2$ since all the quantities are now energies.

Gary Godfrey
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  • Sorry I replaced photon with neutrino but I could also have used other particles which don't carry electric charge such as higgs boson etc. – user6760 Jan 24 '16 at 07:41
  • @user6760: Gary's answer still applies. An electron-volt is just a unit of energy like a joule (only smaller!). You could use it anywhere energy is measured e.g. the potential energy stored in a spring or kinetic energy of a baseball. – John Rennie Jan 24 '16 at 07:50
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Although the eV is defined with respect to a particle of unit charge (an electron) in an electric field, it is simply a unit of energy.

There are simple relations between thr eV and everyday units of energy, such as the Joule or calorie.

Thus energies of all sorts of things, in fact any energy, can be expressed in eV, even if it has nothing whatsoever to do with electric charge or electric fields. With that in mind, the energy and mass (by mass-energy equivalence) of neutral particles, such as the neutrino, may be expressed in eV without any contradiction.

innisfree
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