# Electric field and tension

1. Feb 13, 2012

### coldjeanz

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

E= Fe/q
F=ma

3. 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?

2. Feb 13, 2012

### coldjeanz

nevermind, figured it out.

3. Feb 13, 2012

### 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)