A point charge, q = -0.35 nC, is fixed at the origin. Where must a proton be placed in order for the electric force acting on it to be exactly opposite to its weight? (Let the y axis be vertical.)

Question

I’m confused on how to get this question started. We haven’t done one like it in class. I just need at least a place to start and a bit of an explanation of what to do. If someone could help, that would be great, thanks.

Answer 1

The fixed charge is negative which means that the proton will be attracted to it. If the attraction is to balance the gravitational force (weight), the proton needs to be located below the negative charge.

-mg = k x Q_neg x Q_proton / y² where y is the coordinate below the negative charge and m is the mass of the proton in appropriate units, and k is the coulomb constant. Solve for y, the negative root is applicable.

I got about -5.6 x 10³ m if my math is correct. This number shows how much greater the electrical force is compared to the gravitational force on the proton.