# Electron Volts ?

1. May 14, 2006

### eggman

Help me to wrap my brain around this notion of 'Electron Volts'

The lamp on the table has 120 Volts...

explain this MeV...in comparison to the Lamp Bulb please.

Is every type of energy measured in the form of electricity
in the world of Physics?

if i get this info i can move deeper in QM i feel.

Thanks

2. May 14, 2006

### Hootenanny

Staff Emeritus
The electron volt is the energy required to accelerate an electron through a potential difference of one volt. 1 eV = 1.6 x 10-19[\sup]. Electron Volts are usually used in atomic and particle physics where it is more useful to determine energy in terms of eV's. Electron volts are also used to measure mass againt in particle physics where the masses are very small.

~H

3. May 14, 2006

### AKG

Electron-Volts are just a unit of energy, like Joules or ergs. 1 V = 1 J/C (one Joule per Coulomb) so multiplying an electron, which is a fraction of a coulomb (i.e. a coloumb is an amount of charge equal to some large constant amount of electrons) gives a fraction of a Joule, so eV are very small amounts of energy compared to Joules. This is kind of like "kilowatt-hours" as it's the amount of energy used by something with a power of 1 kilowatt over 1 hour. Watts are J/s (Joules per second) so kilowatts are thousands of joules per second. A kilowatt-hour is 1000 joules per second times 1 hour, i.e:

1000J/s * 1hr
= 1000J/s * 3600s
= 3600000 J
= 3.6 MJ

4. May 15, 2006

### chroot

Staff Emeritus
If you allow an electron to accelerate from rest through a potential difference of 1 V, it will gain 1 eV of kinetic energy.

- Warren

5. May 15, 2006

### Staff: Mentor

And the reason why the electron volt is the main unit of energy in atomic, nuclear, particle physics, etc., is that it relates directly to the way that we usually produce particles with a specified amount of kinetic energy: we accelerate it with an electric field produced by a potential difference.

For example, set up two electrodes with a potential difference of 10,000 volts between them. Produce some protons near the positive electrode. The electric field accelerates them towards the negative electrode. Put a hole in the negative electrode so the protons can fly through, and voilà, a proton beam with a kinetic energy of 10,000 electron volts! (or 10 keV)

You can of course convert the units to the equivalent $1.6 \times 10^{-15}$ joules, but 10 keV is a much nicer number to work with. :!!)

And thanks to Einstein's formula

$E^2 = (pc)^2 + (mc^2)^2$

where m is the invariant mass a.k.a. "rest mass" of the particle, we can see that $E$, $pc$ and $mc^2$ all must have the same units. So (especially) particle physicists use electron volts for mass and momentum also, which can be a bit confusing at first. They say things like "the mass of the electron is 511 keV" when they really mean "$mc^2$ for the electron is 511 keV", and they say "the momentum is 1 MeV" when they really mean "$pc$ is 1 MeV."