# Dielectrics Attracted to Capacitors

1. Oct 12, 2012

### Kakashi24142

I have two questions:
1) Is it in general assumed that the parallel plates of a capacitor are fixed to some surface or is there a force that counteracts the attractive force between the plates whenever one considers a typical problem involving capacitors?

2) Consider a dielectric of length L and two capacitor plates of length L. Suppose the dielectric is wedged halfway in between the two plates. Why does the capacitor attract the dielectric into itself? The only force I can think of is the force due to the electric field between the plates, but this force is perpendicular to the direction of displacement into the capacitor, hence cannot be responsible for such a displacement. Is it the fringe field at the ends of the capacitor or is there some other force I missed?

2. Oct 12, 2012

### tiny-tim

Welcome to PF!

Hi Kakashi24142! Welcome to PF!
Yes, the plates are usually (though not always) fixed to the dielectric.
Imagine that the capacitor is cut into two separate capacitors, of length L/2.

For a particular voltage V, what will the charges be on each half?

And what will happen to those charges if you then join the capacitors? And what will the effect be on the dielectric?

3. Oct 12, 2012

### Staff: Mentor

Re: Welcome to PF!

Not to the dielectric, but to something else, in this type of problem.

The field close to the edge of the dielectric material is not perpendicular to the plates - it is deformed "towards" the dielectric, and the polarization of the dielectric feels a force towards that.