Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 19/10/2014 10:22:56

Title: What is the mass of a neutron? How would we measure it?
Post by: chris on 19/10/2014 10:22:56

How much do neutrons "weigh" and how would one go about "weighing" a neutron?

Title: Re: What is the mass of a neutron? How would we measure it?
Post by: PmbPhy on 19/10/2014 10:48:58

Quote from: chris on 19/10/2014 10:22:56

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.

Title: Re: What is the mass of a neutron? How would we measure it?
Post by: alancalverd 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. 

Title: Re: What is the mass of a neutron? How would we measure it?
Post by: PmbPhy 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

Title: Re: What is the mass of a neutron? How would we measure it?
Post by: chris on 26/10/2014 09:07:20

Thanks, Alan, for that excellent and clearly-explained piece of reasoning.

Title: Re: What is the mass of a neutron? How would we measure it?
Post by: PmbPhy on 26/10/2014 20:40:38

Quote from: alancalverd 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.

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.

Title: Re: What is the mass of a neutron? How would we measure it?
Post by: alancalverd 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.