- #1

- 575

- 47

## Homework Statement

1. A positively charged object with a charge of +85 nC is being used to balance the downward force of gravity on a 1.8-gram balloon which has a charge of -63 nC. How high above the balloon must the object be held in order to balance the balloon? (NOTE: 1 nC = 1 x 10^-9 C)

## Homework Equations

F = k • Q1 • Q2 / d^2

k = 9.0 x 10^9 N • m2 / C2

## The Attempt at a Solution

I turned the grams into kilograms, converted the kilograms into weight to equal the force and put all the other numbers back into the equation but no matter what I do, I can't get the answer the website gives as the answer, which is 0.155 meters.

I noticed the "explanation" of the answer didn't include the constant k. Here's the explanation: Use the mass to determine the force of gravity (m • g). The force of gravity on the balloon is 0.0176 N. Thus, the upward electrical force is 0.0176 N. Now that F, Q1, and Q2 are known, Coulomb's law can be used to determine the distance d in the equation. Algebraic rearrangement leads to d = Sqrt [ (Q1• Q2) / F ]. Substitution leads to the answer.

It leaves out k in the formula. But even when I leave out k, I still don't get their answer.

The answer I got was 1.65x10^-5 m. That sounds a little too close.

Thanks.