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My physics teacher gave me an assignment to work out (theoretically) how Millikan's oildrop experiment works.

The simple principle of the experiment (as far as I know):

[itex]E = \frac{F}{Q}[/itex]

[itex]F = mg[/itex]

[itex]mg = qE \rightarrow q = \frac{mg}{E}[/itex]

However, after reading a bit on Wikipedia it seems as if there is more involved in order to receive an acceptable value of the particle.

Archimedes' principle:

[itex]F = pVg[/itex]

Volume of a sphere (the oildrop):

[itex]V = \frac{4πr^3}{3}[/itex]

The weight of the drop:

[itex]w = \frac{4πr^3}{3}(p - p_{air})g[/itex]

I don't understand why p is being subtracted by p_air, what density does p refer to?

I'm sorry if I'm ambiguous or outright wrong, I'm not particularly good at physics.

You can read more about the method I'm trying to understand here: http://en.wikipedia.org/wiki/Oil_drop_experiment#Method

Thanks in advance!