Capacitors,dielectrics and electrostatic forces

[harshasunder]

hi

in what direction does the electrostatic force act when we insert a dielectric within a capacitor? and when we remove the dielectric? supposedly they act in opposite directions in each case, and either push or pull the dielectric out or into the capacitor. i m not able to understand how the direction is acting perpendicular to the direction in which the charges are. for example, q is the charge on one plate.put a dielectric in and charge on dielectric is -q', where q' is less than q.then force is given by coulombs law, and let it be f. now that f is pulling the dielectric towards the plate no? (attractive force). so where does the force where that pushes or pulls the dielectric out or in come from?
thanks
harsha

 

[EFuzzy]

Firstly, the net horizontal force on the dielectric is going to be zero, since there are TWO forces pulling the dielectric to the side, one on each side of the dielectric.

The reason the dielectric is pulled inward is due to the fringing field of the capacitor, the part of the field that is no longer uniform. You can calculate the force by using the formula for energy, .5*Q^2/C and then differentiating with respect to distance, since F = dU/dx.

 

[optrix]

I think actually you would have to integrate over the distance since F= Ux