N/CWhat is the Electric Field Strength to Reduce String Tension to Zero?

[coldjeanz]

Homework Statement

A piece of aluminum foil of mass 4.3 10-2 kg is suspended by a string in an electric field directed vertically upward. If the charge on the foil is 2.7 µC, find the strength of the field that will reduce the tension in the string to zero.

Homework Equations

E= Fe/q
F=ma

The Attempt at a Solution

So what I can gather is I need to find the net force acting on the piece of foil. I know that the T should be equal to 0 and that the foil isn't accelerating. So what forces would I be left with? I'm trying to solve for Fe right here, correct?

 

[coldjeanz]

nevermind, figured it out.

 

[tony24810]

first, equation of electric field strength is force per unit charge E = F / q

or F = qE

the force generated by electric field needs to cancel out the weight of foil exactly, thus

mg = F

mg = qE

rearrange:

E = mg / q

E = 4.3e-2 * 9.81 / 2.7e-6

E = 1.56e5 N/C (1.56 x 10^5)