Help about millikan oil drops apparatus?

[astri_lfc]

Homework Statement

in a millikan type apparatus, the horizontal plates are 2 cm apart. with the field switched off, an oil drop is observed to fall with a steady velocity of 2 x 10^-4 m/s. when the field is on, the upper plate being positive and the drop just remains stationary when the pd between two plates is 1150 v

a/ calculate the radius of the oildrop
b/ how many electrons charges does it carry

Homework Equations

a/ w=fd
m g = 6 phi η r v

b/ fe = w
q Δv/d = m g

The Attempt at a Solution

a/ i don't understand how much is m, and η
it is not mention in the question. what should i do? where should i put V?

b/how much is m?

 

[mfb]

η and the density of oil are something you have to look up (in the problem statement or somewhere else).
m can be calculated as function of the radius of the oil drop and the density.

"phi" is pi ($\pi$)?

where should i put V?

What do you mean here?
v as velocity? Exactly where it is in the formula.
V as volume? That is related to the mass.
V as voltage? Exactly where it is in the formula.