How to determine the electric field intensity that enables the spheres to be in equilibrium?
Two 4.0 g spheres are suspended by 9.0 cm long light strings. A uniform electric field is applied in the x direction. If the spheres have charges of -4. 10-8 C and +4. 10-8 C, determine the electric field intensity that enables the spheres to be in equilibrium at θ = 8°.
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Apply Coulomb’s law.
Equate the horizontal force due to gravity (Fg) and that due to electrostatic attraction (Fe).
Fg = mgtanθ (θ assumed to be the angle from vertical of the strings)
Fe = Eq
E = mgtanθ/q = 0.004*9.81*tan(8 deg)/4E-8 = 137871 V/m