Unlike mass, the charge on an object is said to be unaffected by the motion of the object.

This statement in my text book, is not yet understood by me. I don't know theory of relativity. On expressing my inability in understanding this particular line. My sir said that, the statement understanding needs knowledge about theory of relativity. Sir said that, according to theory of relativity, mass doesn't remain constant during object's motion. And sir expressed their inability to make me understand this particular line because of lack of knowledge on this topic.

I got a doubt from the statement made by my teacher. If mass varies during motion, why do charge remain constant. What I thought is, charge is the property of all the substances. If mass varies, charge should also vary. Because, electron and proton are the particles exhibiting charge property, and they are the part of that particular mass which varies during motion. Then, why is it said that charge remains unaffected during the motion of the object?

EDIT: I tried to answer myself, if we consider a neutral body and assume mass varies discretely by the mass of atom, even though mass varies, net charge remains constant. So, here constancy of charge is maintained. But, if we consider a charged body in motion or uncharged body with variation of mass continuously, charge should also vary with change in mass. I don't know whether I am right in predicting this, but this what I have thought.

  • $\begingroup$ I have just noticed from your profile that you must still be in highschool. It is good that you are interested in physics but if you want to continue no matter your young age you should start by reading some introductory material in modern physics and go on from there. It is not productive to start imagining with a small knowledge data base . If you google "introduction to modern physics" you will get a list of available lectures and articles on line from which you could start ( presuming you have mastered classical physics) $\endgroup$
    – anna v
    Dec 8, 2013 at 7:38
  • $\begingroup$ your addition is wrong, charge does not vary nor does the invariant mass vary. What varies is the relativistic mass, as referenced in my answer, and the relativistic mass is a concept that physicists are using less and less, because it confuses people, as you are confused. One uses the four vector space and the invariants coming from it in order to calculate and predict measurements. $\endgroup$
    – anna v
    Dec 8, 2013 at 7:50
  • $\begingroup$ Thank you for the answer and useful comments. I understood your answer, which says that rest mass remains constant, and thus charge remains constant. If I find difficulty in understanding any thing which I have googled, I will ask doubt for you and others in physics stack exchange. First I will try to know what is rest and relativistic mass. $\endgroup$
    – Sensebe
    Dec 8, 2013 at 10:48

2 Answers 2


Your teacher is correct that the mass of an object if it is moving with very high energies appears to increase according to the formula , it is called the "relativistic mass" .

relativistic mass

Where E is the energy of the particle and c the velocity of light.

But each elementary particle ( these are concepts that apply to elementary particles to start with) is characterized not only by its charge, but also by its "rest mass" or "invariant mass" uniquely, the same way as it is characterized by its charge. The electron has a fixed rest mass and charge -1, and this rest mass is the same whether it is moving or not.

This is the mass that enters the four momentum "length"

four momentum

where p is the momentum of the particle and m the invariant mass, because it is the same in all moving systems, analogous to the length of a three vector (3 dimensiona normal space) space which is invariant in all moving systems

The reason that there does not exist an equivalent to the relativistic mass, which changes with velocity, is primarily experimental. We have not observed such a situation in our experiments, i.e. a relativistic charge with the changes in momentum, which guides us to theories where the charge characterizing a particle is invariant to the motion of the particle, and is not a function of space and time the same way that the rest/invariant mass is not.


Consider a positively charged particle, if it comes in upward motion its velocity increases as a result its mass decreases(according to conservation of momentum i.e. m1v1=m2v2), as mass decreases its number of atoms decrease i.e. equal number of protons and electrons are decreasing which reside in an atom.thus the charge on the particle is unaffected by its motion..

Hope it clears your doubt!!

  • 1
    $\begingroup$ This is not true. The statement has nothing to do with conservation of momentum; it has to do with properties of particles travelling at relativistic velocities. $\endgroup$ Jun 29, 2016 at 2:21
  • $\begingroup$ you seem to be misusing, among other things, the law of conservation of momentum. Unless propellant (I'm using a broad definition of that term) is ejected, momentum conservation doesn't include the loss of mass $\endgroup$
    – Jim
    Jun 29, 2016 at 12:43

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