Electric field for point charge

1. Aug 26, 2013

tuggler

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

You charge a piece of aluminum foil (mass = 4.65 g) by touching it to a charged rod. The charged rod gives the aluminum foil a charge of 10 µC. Your professor brings a charged plate over and tells you to put the aluminum foil on top of the plate. To your surprise the aluminum foil hovers motionless in the air above it! Calculate the value of the electric field from the charged plate (assume it is a uniform field and the aluminum foil is a point charge).

2. Relevant equations

F = Eq and F = ma

3. The attempt at a solution

What I did was first calculate the gravitational force with the equation F = ma = (9.8m/s^2)(4.65g) = 45.6 N. Then I use the equation $$F = Eq \implies E= F/q$$, where E = electric field, q = charge, and F = force.

Thus F = 45.6, q = 10e-6 $$\implies$$ 45.6N/(10e-6 µC ) = 455700 N/C.

But it is wrong. What did I do wrong? Thanks in advance!

2. Aug 26, 2013

voko

Do grams times meters per seconds squared give newtons?

3. Aug 26, 2013

tuggler

No, it does not, but kilograms over s^2 does.

4. Aug 26, 2013

voko

So what's the correct weight then?

5. Aug 26, 2013

tuggler

It is 4557 N/C. THANKS!