What is electron rest mass? - Definition from WhatIs.com


electron rest mass

Part of the Electronics glossary:

The electron rest mass, symbolized m e , is the mass of an electron as measured when its speed is zero relative to an observer. The electron has extremely small mass  -- it's almost 2000 times less massive than the proton -- at approximately 9.10956 x 10 -31 kilogram (kg) or 9.10956 x 10 -28 gram (g). Every known electron at rest has the same mass as every other known electron at rest.

When an electron is accelerated to high speed, its mass increases because of relativistic effects. This increase is not significant until the speed of the particle, relative to the observer, becomes an appreciable fraction of the speed of light . If v is the speed of an electron in meters per second (m/s) and c is the speed of light (approximately 2.99792 x 10 8 m/s), then the mass of an electron in motion, m e* , is

m e* = m e / (1 - v 2 /c 2 ) 1/2 = 9.10956 x 10 -31 / (1 - v 2 /c 2 ) 1/2 kg

The exponent refers to the square root .

The relativistic mass increase of high-speed electrons is significant in real-world scenarios, because it increases the effects of the electrons on objects they strike. High-speed electrons are known as beta particle s.

Also see Table of Physical Units and Constants.

This was last updated in September 2009
Contributor(s): Roger Millikan
Posted by: Margaret Rouse

Related Terms


  • laser

    - A laser is a coherent and focused beam of photons; coherent, in this context, means that it is all one wavelength, unlike ordinary light which showers on us in many wavelengths. The acronym laser s... (WhatIs.com)

  • photon

    - In some ways, visible light behaves like a wave phenomenon, but in other respects it acts like a stream of high-speed, submicroscopic particles. (WhatIs.com)

  • saser (sound amplification by stimulated emission of radiation)

    - The SASER is the sonic equivalent to the laser and is often referred to as a sound laser or acoustic laser. SASERs beam concentrated sound waves in the terahertz range. (WhatIs.com)


  • Electronics

    - Terms related to electronics, including definitions about electrical components and words and phrases about computers, laptops parts, digital cameras, televisions and home appliances.

  • Internet applications

    - This WhatIs.com glossary contains terms related to Internet applications, including definitions about Software as a Service (SaaS) delivery models and words and phrases about web sites, e-commerce ...

Ask a Question About electron rest massPowered by ITKnowledgeExchange.com

Get answers from your peers on your most technical challenges

Tech TalkComment



    Contribute to the conversation

    All fields are required. Comments will appear at the bottom of the article.