How Is the Mass of a Charged Particle Determined in an Electric Field?

[adca14]

Homework Statement

A positively charged particle (9.35pC) is suspended in equilibrium in an electric field between two horizontal parallel plates with opposite charges. Each one of the square plates has a charge of 5.5x10^-5C; and are separated by 6.25mm and their sides are 11cm long
what is the mass of the particle?

Homework Equations

E=kq/r^2
n=q/qp
idk...

The Attempt at a Solution

first i divided the charge of the particle by the elemental charge
9.35x10^-12/1.602x^-19
i got 58364544
then i multiplied this number by a protons mass
58364544*(1.673x^-27)
i got 9.764x10^-20

the book says the answer is 4.9x10^-4
soo confused!

 

[LowlyPion]

Consider the E-Field between the plates.

What does that come to?

You know that F = q*E

And you also know F = m*g

So ...

m = (q / g) * |E|

 

[adca14]

wow, i feel like a moron for not seeing that! thanks!