Millikan Experiment: Calculating Charge and Excess Electrons

[zmike]

In Millikan type experiment, 2 plates parallel 1 cm apart. Sphere mass of 2.2x 10^-31 Kg doesn't move when potential diff is 350V with upper plate negative

a) What charge is sphere + or -
b) magnitude of charge on sphere?
c) How much excess or deficit of electrons does it have.

MY ATTEMPT:

a) I am guessing it's + b/c of gravity and the negative upper plate.
b) uhhh Fe=Fg? What do I use for plates, I know how to do this if it was ALL point charges but not with plates
c) ...no idea how to get charge on the sphere so can't get this one


thanks -please help I have my exam tmr morning

 

[rock.freak667]

a) correct. Since the electric force exerted needs to oppose the weight
b) $F_E=W \Rightarrow EQ=mg$

Do you know how to find the electric field strength between two parallell plates?

 

[zmike]

thanks for the reply

how do you find electric field strength for plates?

 

[rock.freak667]

E=V/d