# 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.

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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.

7. 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 quarks´ charges would be integers

8. 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?

9. 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.

10. 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.

11. 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.

12. 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.

13. 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 weren´t 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

14. 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 quarks´ 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.

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

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

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

17. Originally Posted by dhd40
...If charges of neutrons weren´t 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.

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

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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.

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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

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Originally Posted by korjik
Neutron stars would be quite a bit different if neutrons werent neutral
Quite frankly, I find this comment somewhat electrifying, if not altogether charged...

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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 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

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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 quarks´ 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

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It is not possible to measure physical quantities with infinite precision.

The charge of an electron can be measured in a laboratory to better
than one part in a trillion (1 part in 10^12), and probably to about one
part in a quadrillion (1 part in 10^15) as korjik indicated. And as
he indicated, an imbalance even smaller than that would likely have
significant observable consequences.

When electric charges are created, they are always created in matched
pairs of positive and negative. The notion that the two exactly balance
each other over the entire Universe is obvious, and has a great deal of
observational support, but of course it can never be proved.

-- Jeff, in Minneapolis

26. Is it obvious that they exactly balance? We can't even observe the entire universe, so how could we know? There seems to be an imbalance between the amounts of matter vs antimatter. Why not an imbalance of charge?

Like I said, I'm a physics layman, so I honestly don't know. I don't know why we have an apparent imbalance of matter vs antimatter.

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I said "The notion that the two exactly balance each other over the
entire Universe is obvious". There could be a very tiny imbalance,
but there is no indication of one. In any case it would have to be
awfully tiny. Probably much less than one part in a quadrillion.

There does seem to be an imbalance between ordinary matter and
antimatter, but then, we have no way of distinguishing the two from
a distance. The Universe could be half and half and we'd have no
way of ever knowing it.

-- Jeff, in Minneapolis

28. I don't doubt that it's balanced--or at least very closely balanced--but I am not a physicist so I don't know what evidence we have that it's (closely) balanced.

Heck, I'm not entirely sure how we know/suspect there's an imbalance between matter/antimatter. My guess is that we'd expect to see regions of gamma radiation where the majority matter and majority anti-matter zones collide.

So my question is--what sort of indication of an imbalance would be evident from a distance?

You don't seem confident of the evidence for matter/antimatter imbalance. What makes you so certain about the evidence for charge balance?

I'm not saying there's no such evidence--I'm just a non-physicist asking for what the evidence is. Let's say there's some far away galaxy with some excess positive charge. What would be the blatant smoking gun?

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I did read about the balance between electric charges but it has been a
very long time-- decades-- and I don't recall where I read it, how deeply
I read, or exactly what the arguments were. All I can think of is Millikan's
experiment which is one way of measuring the charge of electrons.
A Google search on "Millikan" and "electric charge" should certainly find
discussion of the balance between plus and minus.

I feel confident of the balance between charges because it just
looks right to me. Nothing objective about that at all. The obvious
imbalance between ordinary matter and antimatter is a theoretical problem
for me: Given the example of apparently perfect balance between electric
charges, why would there be an imbalance between ordinary matter and
antimatter? I solve that problem by hypothesizing a phenomenon which
has never been observed: Symmetry between attractive and repulsive
gravitational forces. That hypothesis is based on similarity of the force
of gravity to the electric force. Someone will come up with evidence
either supporting or refuting the hypothesis before long. Maybe me, but
more likely someone who is farther along on the problem, such as the
people at CERN, who have said that they would build a device capable
of measuring the effect of Earth's gravity on antihydrogen atoms. If it
is up to me to provide evidence, I will most likely search for gravitational
lensing involving antimatter in distant galaxies or quasars.

-- Jeff, in Minneapolis

30. Originally Posted by AonSao
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?
I have changed the name of the thread from +/- to Charges on electrons and protons as the first name was not clear was what the subject matter was. I would advise that thread titles can be made clear as to their subject matter

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