Physics HELP!!
posted by Maria .
Two iron spheres each have a volume of 10 cm^3 and a mass of 78g. How many electrons must be placed on each sphere so that the electrostatic force of repulsion between them just balances the force of gravitational attraction? Assume the distance between the spheres is more than 2 meters.

Physics HELP!! 
drwls
You can treat the spheres as point charges at R >2m separation. If e is the charge on each sphere, for the electrical repulsion to equal the gravitational attraction,
G M^2 /R^2 = k Q^2/R^2
GM^2 = k Q^2
G is the Newton gravity constant and k is the Coulomb law constant
Solve for Q. The number of electrons is Q/e.
e is the elctron charge.
Respond to this Question
Similar Questions

physics
how many excess electrons must be placed on each of two small metal spheres placed 3.00 cm apart in a vacuum if the force of repulsion betwen the spheres is to be 0.000000008 N? 
physics
Two spheres are placed so that their centers are 2.6 meters apart. The force between the two spheres is 2.75x10^12N What is the mass of each sphere if one sphere is twice the mass of the other sphere? 
Physics
1. The electrostatic force of attraction between two small spheres that are 1.0 meter apart is F. If the distance between the spheres is decreased to 0.5 meter, the electrostatic force will be a) F/2 b) 2f c) F/4 d) 4F I know when … 
Physics
Two very small spheres are initially neutral and separated by a distance of 0.50 m. Suppose that 2.50 x 10^13 electrons are removed from one sphere and placed on the other. What is the magnitude of the electrostatic force that acts … 
physics
Two spheres are placed so that their centers are 2.9 m apart. The force between the two spheres is 2.60 multiplied by 1012 N. What is the mass of each sphere if one sphere is twice the mass of the other sphere? 
physics
1.) A small sphere of charge 2.4 μC experience a force of 0.36 N when a second sphere of unknown charge is placed 5.5 cm from it. What is the charge of the second sphere? 
Physics
Two insulated charged metallic spheres A & B have their centres separated by a distance of 50cm in air. Calculate the force of electrostatic repulsion between them. If the charge on each is +5.0*10^8c. The radii of A & B are negligible … 
Physics
Two iron spheres each have a volume of 10 cm^3 and a mass of 78g. How many electrons must be placed on each sphere so that the electrostatic force of repulsion between them just balances the force of gravitational attraction? 
Physics
Two small spheres spaced 20.0 centimeters apart have equal charge. How many excess electrons must be present on each sphere if the magnitude of the force of repulsion between them is 4.57×10 −21 newtons? 
Physics
Two electricallycharges spheres are suspended from insulated threads a certain distance from each other. There is a certain amount of electrostatic force between them. Describe specifically (not just increase or decrease) what happens …