I am wondering how an electron can gain and lose mass. What is the electron actually gaining and losing that causes its mass to change?
Specifically, my application is in an electron beam travelling from a cathode to an anode.
I am not a physicist or electrical engineer, so I am lacking some basic background knowledge. It's hard for me to understand how an electron, which is a fundamental particle that cannot be broken into anything smaller, can gain or lose mass.
I would very much appreciate any rudimentary explanations!
Hi Nancy,
an electron, like any other "material" particle, does have a rest mass. This is a lower limit value, so under arbitrary circumstances there cannot be an electron with less mass.
The key fact is that energy has mass, too. Electromagnetic radiation (which is "pure" energy and does not have a rest mass) transfers momentum to a mirror when it is reflected, for example.
So, the "effective" mass of a particle or even of a macroscopic body is the sum of its rest mass and the mass of the energy it contains.
The mass of a macroscopic body increases if its temperature rises. The mass of a compressed spring is larger than that of the same spring relaxed. And the mass of an electron increases if the speed of the electron increases.
While it is virtually impossible to measure the mass increase of macroscopic bodies in everyday life, electrons can be easily accelerated to very high velocities in electric fields. That's the case you mentioned: The anode is usually at an electrical potential some 100 V or even some 10 kV higher than the potential of the cathode, so the (negatively charged) electrons which escape the cathode are accelerated toward the anode, and thereby gain energy and hence mass. And this increase can have noticeable effects, for example when electrons are deflected in a magnetic field.
Hope this helps a bit.
In addition to Prof. Fricke's description:
Rest 'constant' mass is based on Newton's second law. Einstein's relativity theory describes that mass of a body increases with velocity and is given by the relation:
m = m0/[1- (v2/c2)]0.5
where m0 = rest mass, v = velocity of the body, c = speed of light.
The change in the mass of electrons is 'realized' because of its smaller mass and very high velocity.
The effective mass of electron in crystals could be related to the scattering process and direction of travel with reference to the electric field. If the acceleration of electrons in a crystal is easy and large in magnitude (at high mobilities), the effective mass is considered to be fraction of the rest mass. Even a negative mass is possible if the direction of travel is opposite to the field!
Hello Nancy,
How are you
This file is helpful for your question you can check this PDF
Thank you all for taking the time to help with detailed explanation and the awesome reference! That is very interesting that energy has mass.
I was thinking further ... is this considered quantum physics since we're discussing subatomic particles?
I am used to a continuum perspective, where mass, energy and momentum are conserved. What fundamentally changes when going from a continuum to a subatomic perspective?
I just read an interesting explanation, which states that m is relativistic mass and then proceeds to discuss the location of the observer relative to the system. Can anyone elaborate? I'm a bit confused.
https://plato.stanford.edu/entries/equivME/#1
Thanks for any further information!
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