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DarkChapter
2004-Dec-06, 05:08 AM
Hi all, I am a bit confused by some of the units used to describe mass and energy.

according to

http://en.wikipedia.org/wiki/Electron_volts

electron volts are used to measure mass of small particles because of E=mc^2

For example, an electron and a positron, each with a mass of 511 keV, can annihilate to yield 1.022 MeV of energy

yet, according to this

http://en.wikipedia.org/wiki/Neutrino

it says that a neutrino is virtually massless, electron neutrino 2.5 eV, muon 170KeV, tau, 15Mev. does that mean that allthough almost completely massless, the Tau has more mass than the Muon, which has more than the electron neutrino?

I am confused because you see mass and energy expressed in eV, and yet some particles can have energy, yet no mass. Is this because The ratio of mass:energy in a particle can exist in a infiite range of states between All mass no energy and all energy no mass?

StarLab
2004-Dec-06, 05:23 AM
Yeah, anything at the molecular level can be expressed in eV, kg, J, or h-bar.

Matthew
2004-Dec-06, 05:49 AM
Its just convenient to express things in eV due to the extremely small masses. 1 eV is equal to 1.6022e-19 J, so sub that value into E=mc^2 to find mass.

I&#39;ve also done it for you:

E=mc^2
1.6022e-19=mc^2
m=1.6022e-19/(3e8)^2
m=1.78e-36 kg

Which is quite small, actually it is extremely small.

StarLab
2004-Dec-06, 06:01 AM

Matthew
2004-Dec-06, 06:38 AM
W=QV

Where W = work (expressed in Joules
Q = Charge (Coulombs)
V = Electromotive force (or simply, voltage)

As such:

E=mc^2
QV=mc^2
m=(QV)/(c^2)

antoniseb
2004-Dec-06, 12:27 PM
Originally posted by StarLab@Dec 6 2004, 06:01 AM
For charged particles, we normally discuss in units of the charge of the proton.

antoniseb
2004-Dec-06, 12:32 PM
Originally posted by DarkChapter@Dec 6 2004, 05:08 AM
it says that a neutrino is virtually massless, electron neutrino 2.5 eV, muon 170KeV, tau, 15Mev. does that mean that allthough almost completely massless, the Tau has more mass than the Muon, which has more than the electron neutrino?
Anybody can write for Wiki. Sometimes it lies. Find something they have no topic on yet, and submit it. You can make stuff up, and people will think its fact until an expert sees it and bothers to submit a replacement article. That could take years.

The best guess mass for the electron neutrino is currently somewhere in the milli-eV range. The mu and tau neutrinos are also much lighter than the article you found says.

DarkChapter
2004-Dec-07, 12:10 AM
OK, thanks about the heads up on Wiki, but i still dont understand, when the same units are used, how you can tell how much of that eV is the mass measurement, and how much is the energy measurement. If a photon has an energy of 150KeV (for example), why does it have no mass? Why dont you say, a photon has a mass of 150KeV. If mass is energy, then how can you tell whether something has mass or not?? What determines how much of that component is to be described as energy, and how much as mass?

antoniseb
2004-Dec-07, 12:24 AM
Originally posted by DarkChapter@Dec 7 2004, 12:10 AM
What determines how much of that component is to be described as energy, and how much as mass?
For some particles, you find out what the rest mass is by finding one that&#39;s standing still and weighing it [I&#39;m sort of joking here]. Basically, you can tell the mass of the electron by how much momentum it has when its direction is changed by an electric or magnetic field [usually magnetic]. You know how fast they are going, and how much energy they have, so you can find the mass. Many charged particles that can be produced in quantity are measured as they move through a magnetic field.

For neutrinos , they are so low-mass that almost any energy they might have makes them go at nearly the speed of light. It is a challenge to find a way to measure the mass of the neutrino, especially the mu and tau ones, which are less plentiful. The best we can do is say that the mass has to be under some number, and over some other number through various experiments and conjectures.

The neutrinos detected from SN1987A give one of the most understandable bits of evidence about the mass of the neutrino. They travelled 170,000 light years, and were perhaps delayed by their difference in speed from light by as much as a few hours. We also have approximate numbers for how much energy some these neutrinos carried. Hence a rough guess at the mass. There are many other papers showing limits on neutrino mass through different techniques and theories.