Use the relation between electric force and electric field and the concept of gravitational force to calculcate the electric field. The electric force is given by
[tex]F_e = qE[/tex]
And the gravitational force is
[tex]F_g = mg[/tex]
For the ball to float above the ground, the magnitude of electric force on the ball must be equal to the magnitude of the gravitational force. That is must be a equilibrium condition, so,
[tex]F_e = F_g[/tex]
[tex]qE = mg[/tex]
[tex]E = \frac{mg}{q}[/tex]
Replacing the values we have that,
[tex]E = \frac{(0.010)(9.8)}{-24*10^{-6}}[/tex]
[tex]E = -4.083*10^{3} N/C[/tex]
Therefore the electric field magnitude that would cause the ball to float above the ground is [tex]-4.083*10^{3} N/C[/tex]
