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Why is eV used instead of proton V even if they would give the same value? I have this view: from electrochemistry's Faraday's law, scientists first calculated the charge of an electron. Then, they introduced a unit of eV for measuring really small work in electrochemistry. But, scientist didn't know that proton would be equal in charge to electron. Actually, when they calculated it, nothing special was known about the attributes of proton. eV was already established as a unit. So, scientist didn't change it. Am I right?

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In this context, the letter e stands for the quantum of charge, not for the particle called the electron. That's why eV is notated as a multiplication, and that's why it has a positive value.

Stoney introduced the term "electron" in 1891 to refer to hypothetical positive and negative charges that were permanently attached to atoms. The absolute value of the charge of these particles was known, albeit with poor precision, from electrolysis combined with approximate knowledge of the atomic mass scale. In 1897, Thomson measured the charge to mass ratio of cathode rays. Its value was negative and very large, and he interpreted this as an indication that there was a light, negatively charged particle that was a universal building block of matter. In Thomson's paper, he uses the notation $e$ for the charge of these particles:

Let m be the mass of each of the particles, e the charge carried by it. Let N be the number of particles passing across any section of the beam in a given time; then Q the quantity of electricity carried by these particles is given by...

Later, it was shown by Millikan that charge was quantized in units of $e$. The proton was discovered later still.

So the concept of $e$ as a fundamental charge originated in 1891, and the notation $e$ was used for it by Thomson in 1897. It would have been at some point during that six-year period that the notation $e$ was first used. The choice of that letter of the alphabet could have been motivated by "electron" or by "quantity of electricity."

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