This one has bothered me for a while (electric field)

[qz27]

basically i have a small charged sphere on the left, and at a certain horizontal distance away from it there's a bigger sphere that is made of a thin shell of nonconducting material. the bigger sphere (B) is hanging from a non conducting, uncharged thread, and that thread makes an angle of 20 degrees with the vertical when sphere B is in equilibrium. centers of the sphere are at the same vertical height and are 1.5m away from each other.

the small sphere(A) has a charge of 120microCoulumns, and the larger sphere has radius of .05m and mass of .025kg

what is the charge on sphere B?
now i think if i set up an equation where the potential energy of sphere B ( since the thread hanging it is not vertical, B has a vertical displacement caused by the electric field b/t the two spheres) is equal to the electric field generated by the two spheres, i can figure out the charge on B. but i wasnt given the length of the thread, so that prevent me from getting the vertical displacement of sphere B.

but i wonder why the mass and radius of the sphere B was given tho..
any clue on where to tackle this problem?

 

[learningphysics]

I don't understand your method...

I'd just use forces... what are all the forces acting on the bigger sphere... The sphere is in equilibrium... so sum of forces in the x direction is 0... and sum of forces in the y-direction is 0. From that you can get the electrical force that the smaller charge exerts on the bigger... and from that you can get the charge.

 

[qz27]

well, the force acting on the bigger sphere is just the repulsion force between the two spheres, but how can i find the electric field between them when i only know the charge of the smaller sphere?

 

[learningphysics]

well, the force acting on the bigger sphere is just the repulsion force between the two spheres, but how can i find the electric field between them when i only know the charge of the smaller sphere?

First find the tension in the cable... use sum of forces in the vertical direction.

 

[Gokul43201]

well, the force acting on the bigger sphere is just the repulsion force between the two spheres

That does not account for ALL forces of sphere B.