# Mass is the amount of substance, what's Electromagnetic Mass?

We usually learn mass is the amount of the substance contained in it. But who might have guessed then that it was just one side of the story. That was just "mechanical mass"! There is one more type namely Electromagnetic Mass. It is all because computation tells us that momentum(due to em fields) comes out to be proportional to velocity. And what must be the coefficient of proportionality? You guessed it right. It's our deary Mass. It sometimes becomes $\frac{2{e}^{2}}{3a{c}^{2}}$ where ${e}^{2} = \frac{{q}^{2}}{4\pi\epsilon}$, $a$ is the radius, $c$ is of course speed of light in vacuum.

But it comes out to be wrong since here we have not considered the "Poincare stresses". And we have to add the two contributions.

My question however, is how you would explain this "Electromagnetic mass" physically. That will be your own opinion since I think it has not been explained. This is just a mathematical consequence. What's the physics? Note by Kartik Sharma
6 years, 1 month ago

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Kartik, this is a huge subject. If you want a short answer, the idea of an "electromagnetic mass" has been superseded by special relativity, it's become largely irrelevant. But if you want the long answer, wait for it. It's a very interesting and instructive subject, which should serve to show that numerous physicists in the late 19th century were well aware of the issues that ultimately led to Einstein's theory of relativity.

- 6 years, 1 month ago

Oh, thanks! I will surely want the long answer and wait. :)

- 6 years, 1 month ago

@Michael Mendrin I am not able to think of other names(my memory just went away).

- 6 years, 1 month ago