Homework 14. Helium Synthesis

Name: ________________________ DUE 12/04 ID number: ________________________

When the Universe was just 3 minutes old, atoms had not yet formed. However, the particles which make up the nuclei of atoms were present; in particular, the Universe contained protons and neutrons. At this time, protons outnumbered neutrons by a ratio of 7:1 -- in other words, there were 7 protons for each neutron. By the time the Universe was a quarter of an hour old, all these neutrons had combined with protons to make Helium nuclei.

Show that the result of this process is a mixture in which 75% of the mass is Hydrogen nuclei and the other 25% is Helium nuclei. Using your own words, explain the reasoning behind your answer; this explanation is an important part of the assignment, and you must include an explanation to get credit.

(Hints: a Hydrogen nucleus is just a single proton, while a Helium nucleus contains two protons and two neutrons. For the purpose of this assignment, assume that a neutron has the same mass as a proton, and that a Helium nucleus has four times the mass of a Hydrogen nucleus. Don't confuse this process with Hydrogen burning in stars! In stars, protons are transformed into neutrons, while here the ratio of neutrons to protons does not change.)
























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Joshua E. Barnes (barnes@ifa.hawaii.edu)

Last modified: November 27, 2001
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