Find the charge of the oil drop

[Fatima Hasan]

Homework Statement

Homework Equations

E = σ / 2ε
E = F_el / Q

The Attempt at a Solution

E = σ / 2ε
= 17.7*10^-9 /(2*8.85*10^-12)
E =1000 N/C
ΣF = 0
F_el + mg = 0
F_el = - mg
1000 * q = - 10^-3
q = - 10^-3 / 1000 = 10^-6 C
= -1μC
Could somebody check my answer please ?

 

[kuruman]

If the charge of the drop is negative, is the direction of the electric force towards or away from the plate? What about the direction of gravity, towards or away?

 

[BvU]

Which way is positive ? Then think about the sign of $g$