EPS 102 Lecture 2 Thursday January 22
We’re going to talk about geochronology and the idea is to apply radioactivity to the
dating of rocks and minerals which will say something about the ages of planetary
surfaces. The first thing we oughta know is that radioactivity is common in nature.
Radioactivity is a very natural process. It allows us to date rocks and planetary systems.
When we think of solid matter, on a small scale we need to ask ourselves if the material
is still solid. It is not. At the atomic scale, the atom itself is sort of empty. 10 million
atoms could fit in the period at the end of a sentence. Most of the atom is empty space,
but there are forces and fields within the atoms. Opposite charges attract, so with a
positive nucleus, it attracts the electrons with negative charge. Rocks and minerals are
made of molecules or groups of atoms held together by chemical bonds. Atoms are made
of electrons, negatively charged and nucleons, or things that make up the nucleus=
protons, positively charged, neutrons which are neutral, and gluons which hold this
together; it is like the glue that holds the nucleus together. In addition to electrical charge,
another factor to understand is a relative weight of these objects. Electrons are really light
compared to protons: they are 1/2000
in mass of a proton. Neutrons are roughly the
same mass as protons.
An element is defined by how many protons are in it’s nucleus, or
how many positive charges are in it’s nucleus. Hydrogen has 1 positive charge, and
helium has 2 positive charges in its nucleus. In addition, a given element can have a
different number of neutrons like Carbon. Radiocarbon dating, for example, is dependent
on the fact that Carbon has several different masses to it, or several different numbers of
neutrons to it. Carbon 12 has 6 protons and 6 neutrons and is the normal form of Carbon.
Carbon 13 is another form of C, but it has 7 neutrons and C-14 still has 6 protons but it
has 8 neutral neutrons. In nature these don’t all occur in the same abundances. A given
element with different masses is an isotope.
Four forces of nature have been known to varying degrees for a long time. Gravity is one
of them and it has been known the longest; ironically it is understood the least well and it
holds masses together like planets, planetary systems, galaxies, and systems of galaxies.
Electromagnetism is the force of electricity and magnetism. 300 years ago people thought
electricity and magnetism were different but until the 20
century they were connected.
Electromagnetism= opposite charges attract. It is characterized by forces that go as 1/r^2.
Gravity also has the same kind of distance dependence for the force, but aren’t they the
same thing? In electromagnetism, you have positive and negative charges but in gravity
you only have positive masses. That is the difference that doesn’t let us know about
gravity too well; it is a weak force. The forces within nuclei became better understood.