Rest Energy Concept: What Does it Mean?

[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 ?

 

[tiny-tim]

Hi 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 ?

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.

 

[Jonathan Scott]

Millikan's electron was certainly at rest …

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.

 

[MeJennifer]

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

You seem to forget the principle of relativity.

 

[mkbh_10]

Can i get proper answers ?

 

[Naty1]

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

The rest energy E or rest mass-energy of a particle is its energy when it is at rest relative to a given inertial reference frame.

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

and

While one form of energy may be transformed to another, the total energy remains the same...Although the total energy of a system does not change with time, its value may depend on the frame of reference. For example, a seated passenger in a moving airplane has zero kinetic energy relative to the airplane, but non-zero kinetic energy relative to the earth.

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

 

[Naty1]

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

In modern physics, the total energy of an object is the sum of its rest energy, its total kinetic energy, and its potential energy.

 

[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.

 

[Meir Achuz]

$$E^2=p^2+m^2$$ (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.

 

[bernhard.rothenstein]

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 ?

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

 

[Fredrik]

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

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

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

That its mass is 0.511 MeV/c².

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

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