What is the mass of a neutron? How would we measure it?

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Offline chris

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How much do neutrons "weigh" and how would one go about "weighing" a neutron?
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline PmbPhy

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Re: What is the mass of a neutron? How would we measure it?
« Reply #1 on: 19/10/2014 10:48:58 »
How much do neutrons "weigh" and how would one go about "weighing" a neutron?
See http://en.wikipedia.org/wiki/Neutrino#Mass

I don't know what you mean by "weighing" a neutron but if you're referring to measuring it's mass then see the above link.

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Offline alancalverd

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Re: What is the mass of a neutron? How would we measure it?
« Reply #2 on: 19/10/2014 17:15:12 »
We know the charge of an electron from Millikan's experiment, and the charge/mass ratio from Thompson's apparatus, hence the mass of an elecgtron.

Now we know there are 6.23 x 10^23 atoms per gram of hydrogen (Avogadro) and each consists of an electron and a proton, so we know the mass of a proton.

Helium atoms consist of two electrons, two protons, and two neutrons, and we know the density of helium relative to hydrogen, so we can calculate the mass of a neutron. 
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Offline PmbPhy

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Re: What is the mass of a neutron? How would we measure it?
« Reply #3 on: 20/10/2014 02:07:04 »
Quote from: chris
How much do neutrons "weigh" and how would one go about "weighing" a neutron?
The mass of the neutron is not measured directly since it's uncharged and therefore we can't use the techniques of mass spectroscopy. So instead we use indirect measurements. For a detailed description please see
http://en.wikipedia.org/wiki/Neutron#Mass

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Offline chris

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Re: What is the mass of a neutron? How would we measure it?
« Reply #4 on: 26/10/2014 09:07:20 »
Thanks, Alan, for that excellent and clearly-explained piece of reasoning.
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline PmbPhy

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Re: What is the mass of a neutron? How would we measure it?
« Reply #5 on: 26/10/2014 20:40:38 »
We know the charge of an electron from Millikan's experiment, and the charge/mass ratio from Thompson's apparatus, hence the mass of an elecgtron.

Now we know there are 6.23 x 10^23 atoms per gram of hydrogen (Avogadro) and each consists of an electron and a proton, so we know the mass of a proton.

Helium atoms consist of two electrons, two protons, and two neutrons, and we know the density of helium relative to hydrogen, so we can calculate the mass of a neutron.
You left out the mass-energy associated with the strong force between the neutron and the proton and the electric force between the electron and proton.

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Offline alancalverd

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Re: What is the mass of a neutron? How would we measure it?
« Reply #6 on: 27/10/2014 12:13:05 »
In the case of neutron mass, the e-p force cancels as we are comparing hydrogen with helium.
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