Electric field of a Styrofoam ball

[jonno426]

Okay, hypothetically speaking, I have a styrofoam ball that can be suspended by a positive magnetic force pointing straight down at the earth, and I'm only given the mass of the styrofoam ball, is there an equation that can help me solve for Q?

 

[StatMechGuy]

So the ball has a charge but it isn't moving. Why would there be a magnetic force on it?

 

[arunma]

Jonno, I assume that you meant to say electric field, because first of all, a magnetic field will not have any effect on a stationary charged object, and secondly, electric force and electric field are different things. An electric field is the cause, and the electric force is the effect.

To answer your question, yes you can determine the charge. First of all you should think about the sign of the charge. If the field is pointing down, and the resultant electric force opposes gravity, then what must the sign of the charge be? To find an actual number, you need to draw a free body diagram. You know that the only two forces acting on the ball are gravity and the electric force (what is the relationship between electric field and electric force?). Apply the fact that the ball is in static equilibrium, and you should be able to solve for the charge in terms of the field. If you know the field, then you can find a numerical answer as well.