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 Nuclear and Particle Physics Nuclear and Particle Physics Help Forum Dec 29th 2015, 02:38 PM #1 Member   Join Date: Oct 2010 Posts: 30 Nuclear physics questions What is N?, where N = Avogadro's number / mass number How can I intuitevely imagine this N? What do we get? What is fission rate? In my notes it is written as Power / Fission work. Another thing which I can't imagine. Some example might help.   Dec 29th 2015, 07:33 PM   #2
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 Originally Posted by Nforce What is N?, where N = Avogadro's number / mass number How can I intuitevely imagine this N? What do we get? What is fission rate? In my notes it is written as Power / Fission work. Another thing which I can't imagine. Some example might help.
This is way too vague to answer. A variable is not uniquely defined by a particular letter. For example; if you're looking in a text on nuclear physics then N might be the number of nucleons in the nucleus of particular isotope of an atom. An isotope is the term used to distinguish between atoms of the same element but having a different number of neutrons. E.g. there are fifteen "isotopes" of carbon, two of which are stable. A list of then can be found at: https://en.wikipedia.org/wiki/Isotopes_of_carbon

N is then the sum of the number of protons and the number of neutrons.

The rate of fission of an isotope is the number of fissions per unit time.   Dec 30th 2015, 06:33 AM #3 Senior Member   Join Date: Apr 2008 Location: Bedford, England Posts: 668 Avogadro's number is (essentially) the number of atoms in 1 gram of hydrogen. There are some tweaks to the detail of this definition see Avogadro_constant, but I think that for the purpose of your question this definition is satisfactory. The mass number is the number of protons plus the number of neutrons in an atom. Hydrogen will have a mass number of 1 (deuterium 2, tritium 3, helium 4, etc.). Note that for every proton there will be an electron and the mass of a neutron is (essentially) the same as the mass of a proton plus the mass of an electron. Again there are small tweaks due to (for example) the binding energy, but I think these will be small enough to be ignored for the purpose of this question. From this we can see that N in your equation will be the number of atoms in 1 gram of the element being considered. __________________ You have GOT to Laugh ! Last edited by MBW; Dec 30th 2015 at 06:37 AM.   Dec 31st 2015, 01:39 AM   #4
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 Originally Posted by MBW Note that for every proton there will be an electron and the mass of a neutron is (essentially) the same as the mass of a proton plus the mass of an electron.
Note: This only holds for electrically neutral atoms, not for ions.

 Originally Posted by MBW Again there are small tweaks due to (for example) the binding energy, but I think these will be small enough to be ignored for the purpose of this question.
If you don't mind me asking, tweaks to what?

From this we can see that N in your equation will be the number of atoms in 1 gram of the element being considered.[/QUOTE]

Thanks for correcting me pal. I mixed them up.

Z = number of protons
N = number of neutrons (this is different than the OPs N)
A = Z + N

https://en.wikipedia.org/wiki/Mass_number   Dec 31st 2015, 02:28 AM #5 Senior Member   Join Date: Apr 2008 Location: Bedford, England Posts: 668 tweaks The mass number (in its simplest form) assumes that the mass of an atom can be defined completely by counting the number of protons and neutrons. This includes the assumption that the mass of a proton plus the mass of its associated electron is the same as the mass of a neutron. It also includes the assumption that all the mass of the atom is totally accounted for within the neutrons and proton/electron pairs. (which as you point out includes the assumption it is not ionized). However the binding energy between neutrons and protons introduces an additional source of mass. OK it's a very very small effect, which has no particular bearing on the original question, I just included it for "completeness". __________________ You have GOT to Laugh ! Last edited by MBW; Dec 31st 2015 at 02:31 AM.   Dec 31st 2015, 04:09 AM   #6
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 The mass number (in its simplest form) assumes that the mass of an atom can be defined completely by counting the number of protons and neutrons.
I disagree. I believe that you're confusing atomic mass with mass number. The atomic is not directly related to the mass of the nucleus. It's actually defined as the number of nucleons in the nucleus of an atom. which is sum of protons and neutrons. Mass number is unit-less, unlike mass which has units.   Dec 31st 2015, 10:22 AM #7 Senior Member   Join Date: Apr 2008 Location: Bedford, England Posts: 668 No, I was trying to describe the (small) difference between mass number and isotopic mass. Atomic mass includes the effect of the relative abundances of different isotopes of the element. Isotopic mass is for a single isotope of the element. However, simply adding the number of nucleons (mass number) does not give exactly the isotopic mass, due the the different binding energies required to hold nuclei of different compositions together. https://en.wikipedia.org/wiki/Atomic_mass __________________ You have GOT to Laugh !   Dec 31st 2015, 09:44 PM   #8
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 The mass number (in its simplest form) assumes that the mass of an atom can be defined completely by counting the number of protons and neutrons.
May I make a comment which I hope will make your responses better in the future? It's not meant as a criticism but merely a suggestion. First off, what is the definition of isotopic mass? I tried to look it up but ran into trouble.

Second, it's not clear at all that this was what you were trying to do. And third, you can't compare the two. They're like apples and oranges just like I mentioned above. You're confusing atomic mass with mass number.   Jan 1st 2016, 06:25 AM #9 Senior Member   Join Date: Apr 2008 Location: Bedford, England Posts: 668 Atomic mass is based on the mix of isotopes that will be naturally found in a sample (which has not been artificially enriched to favor one isotope). https://en.wikipedia.org/.../Relative_atomic_mass Thus (for example) chlorine, which has an abundance of naturally occurring isotopes, has a standard atomic mass of 35.453. https://en.wikipedia.org/.../Isotopes_of_chlorine Isotropic mass is based on a single isotope of the element https://en.wikipedia.org/.../Relative_isotopic_mass: The mass number of the original post can be related to the mass via the atomic mass unit: https://en.wikipedia.org/.../Atomic_mass_unit However a simple multiplication of the atomic mass unit by the mass number of an isotope will not give exactly the correct mass (except of course for carbon 12), because of the different binding energies: https://en.wikipedia.org/.../Binding_energy_curve - common_isotopes Note also the isotropic masses for chlorine in this table: https://en.wikipedia.org/.../Isotopes_of_chlorine#Table None of them are integer numbers (although all of them are close). __________________ You have GOT to Laugh !  Tags nuclear, physics, questions Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post Pink Ling Nuclear and Particle Physics 2 May 18th 2011 06:14 AM bambamm Nuclear and Particle Physics 0 Apr 6th 2011 09:03 PM jishent Electricity and Magnetism 5 Apr 2nd 2010 09:40 PM roshanhero Nuclear and Particle Physics 1 Jun 8th 2009 03:28 AM bucket head General Physics 2 Jan 22nd 2009 01:02 AM 