What are Van der Waals interactions?

[lha08]

Homework Statement

So basically I'm sort of unclear as to what van der walls interactions are and how they occur...like I know that they're a type of noncovalent bond that occurs between identical groups in two molecules.
Like, how do the partial charges appear? (e.g. let's say there are 2 hydrogen groups, i thought that they both have partial positive charges, how does one get induced with a partial negative charge...? Something to do with their orientation at a certain time?

Homework Equations

The Attempt at a Solution

 

[zhermes]

"Van der waals" interactions apply to a wide range of phenomenon, essentially all molecular scale forces that aren't covalent or ionic bonds.
The most common example--London forces--are due to quantum mechanical effects. Even if an atom is neutral in a molecule, or has a certain partial-charge, due to statistics and quantum mechanics it will occasionally change its charge distribution so as to create a small field that is attracted or repelled by neighboring atoms.

Its only a steady-state phenomenon when you consider a large ensemble of particles. If you have (e.g.) a mole of noble gas X, at any given time a large number of molecules will have an 'instantaneous' dipole, and interact with neighboring molecules. Overall this looks like a constant net interaction in the gas.