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**1. Homework Statement**

The centre of a small sphere carrying a positive charge of 1.0 x 10^-8 C is fixed at 3cm above the centre of a small conducting ball, which had a negative charge and a mass of 0.05g. If the ball remains suspended, what charge does it have?

Answer is [ 4.9 x 10^-9]

but i can't seem to get that answer

**2. Homework Equations**

**3. The Attempt at a Solution**

Because the ball is suspended i take it that the force between the two charged bodies must be equal but opposite to the force due to gravity so:

F = mg

F = 0.05 x 9.8 = 0.49

F = (1/(4 x pi x epsilon 0)) x ((Q1 Q2) / r^2)

(1/(4 x pi x epsilon 0)) = 9 x 10^9

F = 9 x 10^9 x (Q1 Q2 / r^2)

Q2 = 9 x 10^9 x (Q1 / (r^2 x F)

Q2 = 9 x 10^9 x (10^-8 / (3^2 x 0.49)

Q2 = 20.4

please help