Ask A Scientist General Science Archive

name         Roberto
status       other
grade        other
location     TX

Question -   The way I understand it, electromagnetism is the force
that allows molecules to attract each other and also prevents atoms from
interpenetrating. But why can the attractive force work at a longer
distance (hold molecules in a liquid state), and repulsion apparently
only works at short distance (prevent interpenetration)?
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As you point out the electromagnetic force is long range. Depending
upon the type of electromagnetic force the distance dependence can vary
with distance as: ~1/R^2 (coulombic), and ~1/R^n where 'n' can have values
3, 4, 5, 6 for charge/dipole, dipole/dipole, ..., induced dipole/induced
dipole (n=6) also called van der Waals forces.
  The repulsive forces arise from a very different type of interaction
between electrons, the Pauli exclusion principle. Electrons are "fermions"
which means that no two electrons can have the same set of quantum numbers,
which is to say they cannot occupy the same space at the same time. This is
in addition to just charge repulsion. The magnitude and range of this
exclusion force is a very complicated, but computable, function of the
detailed electronic structure. Under "normal" conditions this causes the
strong repulsion between molecules that "try" to occupy the same space. I
use the term "normal" because the Pauli exclusion repulsion is strong but
not infinite and at sufficiently high density can be forced to break down.
     That electrons are fermions and behave this way is essentially an
experimental observation so it is difficult to give an "explanation".

Vince Calder
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