# Thread: Charges on electons and Protons

1. ## Charges on electons and Protons

Can someone point me to (or explain) proof that an electron and proton have exactly the same charge? It's obvious their charges are similar in strength, but how is it known that they are exact?

2. Well the simplest thing I can think of is that an atom with equal numbers of protons and electrons have no net charge.

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Hydrogen atoms having a net zero charge would be a simple test.

Beyond that there are numerous ways to directly measure charge. Look at the wiki page on electric charge

4. Thank you. I didn't realize the measurements were that exact.

5. Originally Posted by AonSao
Thank you. I didn't realize the measurements were that exact.
I don't know how exact it can be measured. I suspect that we'd find it impossible to prove they were the same to one part in 1080; but that difference would be the difference of a hundredth of an electron charge over all of the protons in the universe.

6. Originally Posted by korjik
Hydrogen atoms having a net zero charge would be a simple test.

Beyond that there are numerous ways to directly measure charge. Look at the wiki page on electric charge
Would not the neutral charge on the neutron combined with knowledge of the processes of electron capture by protons and beta decay of neutrons also show the charges to be equal ? I don't know the precision with which the neutron has been shown to be of neutral charge though.

7. Originally Posted by DrRocket
Would not the neutral charge on the neutron combined with knowledge of the processes of electron capture by protons and beta decay of neutrons also show the charges to be equal ? I don't know the precision with which the neutron has been shown to be of neutral charge though.
Good point! If you look e.g. at Uranium 238, there are 92 protons, 146 neutrons in the nucleus. If charges of neutrons werenīt extremely close to 0 we should be able to see an imbalance between the "+" (proton)- and "-" (electron)- charges in an U-atom, let alone in 1 kg of U

8. Originally Posted by dhd40
...If charges of neutrons werenīt extremely close to 0 we should be able to see an imbalance...
Extremely close isn't "exactly".
Mind you, I think they are exactly equal, but I don't know.

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Originally Posted by antoniseb
Extremely close isn't "exactly".
Mind you, I think they are exactly equal, but I don't know.
Neutron stars would be quite a bit different if neutrons werent neutral

10. Originally Posted by antoniseb
Extremely close isn't "exactly".
...
Sigh.

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Because charge for the elementary particles only comes in a few discrete sizes:

-1: electron, muon lepton, tau lepton, W boson
-1/3: down quark, strange quark, bottom quark
+2/3: up quark, charm quark, top quark
+1: W boson

A proton is a baryon, of the fermion (matter) family, in the quark group. They're composed of three quarks, two up quarks (which brings the charge to 4/3), and one down quark (which brings the charge down to 3/3, or 1).

Thus, the electron's charge is -1, and the proton's charge is +1.

Exactly.

By contrast, the neutron is composed of one up quark and two down quarks, for a charge of 2/3 - 1/3 - 1/3 = 0.

Exactly.

12. Originally Posted by mugaliens
Because charge for the elementary particles only comes in a few discrete sizes:

-1: electron, muon lepton, tau lepton, W boson
-1/3: down quark, strange quark, bottom quark
+2/3: up quark, charm quark, top quark
+1: W boson

A proton is a baryon, of the fermion (matter) family, in the quark group. They're composed of three quarks, two up quarks (which brings the charge to 4/3), and one down quark (which brings the charge down to 3/3, or 1).

Thus, the electron's charge is -1, and the proton's charge is +1.

Exactly.

By contrast, the neutron is composed of one up quark and two down quarks, for a charge of 2/3 - 1/3 - 1/3 = 0.

Exactly.
Exactly! But unfortunately they (who?) have chosen the charge of an electron to be -1. Minus 3 (or +3) would have been much more comfortable, because then the quarksī charges would be integers

13. Originally Posted by dhd40
Exactly! But unfortunately they (who?) have chosen the charge of an electron to be -1. Minus 3 (or +3) would have been much more comfortable, because then the quarksī charges would be integers
I believe they chose those values because quarks can't exist on their own. With those values, all of the particles that can exist in isolation have either 1 or -1 charge.

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Originally Posted by dhd40
unfortunately they (who?) have chosen the charge of an electron
to be -1. Minus 3 (or +3) would have been much more comfortable,
because then the quarksī charges would be integers
Originally Posted by phunk
I believe they chose those values because quarks can't exist on
their own. With those values, all of the particles that can exist in
isolation have either 1 or -1 charge.
Originally Posted by IsaacKuo
The charge numbers were chosen long before anyone theorized
Even so, phunk's comment is true. The arbitrary value of
(negative) unity for electron charge was chosen because quarks
can't exist on their own. The people who chose it didn't know
that was why they chose it.

-- Jeff, in Minneapolis

15. Originally Posted by Jeff Root
... The arbitrary value of
(negative) unity for electron charge was chosen because quarks can't exist on their own. The people who chose it didn't know that was why they chose it.

-- Jeff, in Minneapolis
To me that sounds like a contradiction in itself.

And after all, the electronīs charge isnīt -1, it is approx. 1,60 Ũ 10−19 C
As you say, -1 is the arbitrary value of unity for the electronīs charge

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Originally Posted by dhd40
Originally Posted by Jeff Root
... The arbitrary value of (negative) unity for electron charge was
chosen because quarks can't exist on their own. The people who
chose it didn't know that was why they chose it.
To me that sounds like a contradiction in itself.
I'm not sure I know what you mean, but if you are saying that
that last sentence doesn't sound quite believeable, how about
an analogy? Diamond cutters learned centuries ago how to break
diamonds along their cleavage planes. They chose those planes
because that is how carbon atoms align in diamond. They did not
know that diamonds consist of carbon atoms, so they didn't know
the alignment of carbon atoms was why they chose the planes
that they chose.

Originally Posted by dhd40
And after all, the electronīs charge isnīt -1, it is approx. 1,60 Ũ 10−19 C
Those are just two different systems of units. They are both
arbitrary. A good argument can be made that the coulomb is far
more arbitrary than the unit electric charge of the electron.

-- Jeff, in Minneapolis

17. Originally Posted by mugaliens
Because charge for the elementary particles only comes in a few discrete sizes:...
We have every experiential reason to believe this, but we are relying on our models of how the world works to predict this. Likewise we are relying on these models to say that there isn't some subtle, perhaps time-varying differences in charge from proton to proton.

Since our models work *very* well, we assume these values are exact.

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Originally Posted by mugaliens
Because charge for the elementary particles only comes in a few discrete sizes...

...Thus, the electron's charge is -1, and the proton's charge is +1.

Exactly.
All you did was re-state the assertion which the original poster
requested "proof" of.

-- Jeff, in Minneapolis

19. Hmm...what if they are slightly different in charge? Even if we can't measure it in the lab, could there be some subtle large scale effects we could see in far away astronomical effects?

20. Originally Posted by IsaacKuo
Hmm...what if they are slightly different in charge? Even if we can't measure it in the lab, could there be some subtle large scale effects we could see in far away astronomical effects?
I doubt it would be observable. If the proton and electron were different charges by one part in 1052, then one extra electron in the Sun would be enough to compensate.

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Originally Posted by IsaacKuo
Hmm...what if they are slightly different in charge? Even if we can't measure it in the lab, could there be some subtle large scale effects we could see in far away astronomical effects?
If you had a 1 part in 10^15 difference in the charges, the solar charge would be rather large.

2x10^30 kg=2x10^33g=~10^57 protons and electrons

using 1/10^15 as the difference gives about 10^22 Coulombs of charge.

Even in just 1 mole of Iron you would need about 10^10 extra/fewer electrons to cancel the charge.

It would pretty fundamentally change materials. Even tenous gases would be slightly conducting because of free electrons.

22. Well, as a physics layman I wouldn't have thought it possible to tell the difference between massless neutrinos and massy neutrinos that travel slightly less than the speed of light. But physicists had clever ways of using observations of solar neutrinos to figure out that they have mass.

I don't know if there could be some subtle physics means of detecting a phenomenon only explicable by slightly different proton/electron charges. I give the neutrino example as one which shows subtle astronomical phenomena might be used in clever ways to detect a difference.

23. The charge numbers were chosen long before anyone theorized about quarks.

24. Originally Posted by IsaacKuo
The charge numbers were chosen long before anyone theorized about quarks.
Yes, that would have been exactly my answer to phunkīs comment, also.

But, honestly, I donīt think it really matters which charge-number you allocate to the electron (or proton, ...)

25. Originally Posted by IsaacKuo
The charge numbers were chosen long before anyone theorized about quarks.
Good point.

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We don't even know for sure that all electrons have exactly the same charge. Maybe congress will fund my investigation.

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Originally Posted by Chuck
We don't even know for sure that all electrons have exactly the same charge. Maybe congress will fund my investigation.
If they weren't identical this would have ramifications on the Pauli exclusion principle. You could fit more than two spin orientations into the same state, since they would be distinguishable by the different charge.

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Originally Posted by swansont
If they weren't identical this would have ramifications on the Pauli
exclusion principle. You could fit more than two spin orientations
into the same state, since they would be distinguishable by the
different charge.
Rather than allowing more than two spin orientations in the same
state, the precise description of the Pauli exclusion principle would
be very slightly different. The problem is that the different charges
are not different enough to be distinguishable by any means used
so far. So the Pauli exclusion principle must be very nearly correct.

-- Jeff, in Minneapolis

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Originally Posted by Jeff Root
Rather than allowing more than two spin orientations in the same
state, the precise description of the Pauli exclusion principle would
be very slightly different. The problem is that the different charges
are not different enough to be distinguishable by any means used
so far. So the Pauli exclusion principle must be very nearly correct.

-- Jeff, in Minneapolis
I'm not convinced that "distinguishable" means "distinguishable by us." Has anyone actually worked through what the PEP would look like if charge were not uniform?

Another problem I think you'd run into would be particle annihilation. You'd have an electron and positron annihilate but their charge wouldn't exactly cancel  what happens to it?

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Originally Posted by swansont
...Another problem I think you'd run into would be particle annihilation. You'd have an electron and positron annihilate but their charge wouldn't exactly cancel  what happens to it?
That's a good indication that the charges for the electron and the positron are opposite and exactly equal, but it doesn't prove the charge on the positron is exactly the same as the charge on the proton. Mind you, I believe they are, but I don't know if it can be ruled out that they differ by one part in 10 to the umpteenth.

Nick

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