# Rest Energy concept ?

1. Nov 10, 2008

### mkbh_10

What exactly do we mean by rest energy of a particle ?

It is known that electron has rest energy of .511Mev but what is meant by this ?

An electron is never at rest (except in its own frame of reference) then why it is said rest energy ?

2. Nov 10, 2008

### tiny-tim

Hi mkbh_10!
erm … what part of the word "never" are you finding confusing?

A photon is never at rest …

Millikan's electron was certainly at rest …

energy is the t part of a 4-vector (the energy-momentum 4-vector) … it has to have a non-zero value at zero speed, or it wouldn't make a valid 4-vector.

3. Nov 10, 2008

### Jonathan Scott

Surely Millikan's oil drop was at rest macroscopically, but the electrons in it were behaving in the usual way?

I'd say that the rest energy of something is the energy that something would have if it were at rest, equal to its total energy minus its kinetic energy. Another way to think about it is that it is the part of its total energy which is due to its rest mass, equal to the rest mass times c squared.

4. Nov 10, 2008

### MeJennifer

You seem to forget the principle of relativity.

5. Nov 10, 2008

### mkbh_10

Can i get proper answers ?

6. Nov 10, 2008

### Naty1

Read the following Wikipedia references...quick one liners below...

per wikipedia, http://en.wikipedia.org/wiki/Rest_energy

and
per wikipedia, http://en.wikipedia.org/wiki/Energy

Last edited: Nov 10, 2008
7. Nov 10, 2008

### Naty1

Also for perspective,
via Wikipedia, http://en.wikipedia.org/wiki/Total_energy

8. Nov 10, 2008

### StatusX

The point is that the thing that's conserved is the time component of the four momentum, which has a non-zero value even when the particle is at rest, this being called its rest energy. Then there are processes allowed that convert a massive particle at rest into things with smaller (or zero) rest masses but non-zero kinetic energies, thereby conserving energy but converting some of the particle's rest energy into kinetic energy.

9. Nov 10, 2008

### clem

$$E^2=p^2+m^2$$ (with c=1) so energy increases with momentum.
For an electron at rest, its energy=m, which is thus called the "rest energy".
The relation is also written as $$m^2=E^2-p^2$$, with m here being the
"invariant mass", or in modern terminology just the "mass", because this m is the same in any Lorentz frame. The "invariant mass" and the "rest energy" are equall in magnitude, but have slightly different definitions. In modern terminology it is best to just use "mass" for m.
This avoids the confusion in your question.

10. Nov 11, 2008

### bernhard.rothenstein

It is the energy of my wrist watch measured by myself.

11. Nov 11, 2008

### Fredrik

Staff Emeritus
Its energy in the frame where it's at rest.

That its mass is 0.511 MeV/c2.

I would think that the name explains itself. If you disagree, see my answer to the first question.

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