# Homework Help: Inserting Dielectric and Direction of Force on It

1. Mar 4, 2012

1. The problem statement, all variables and given/known data
A slab of width d and dielectric constant K is inserted a distance x into square parallel capacitor
plates of side L. Determine the magnitude and direction of the force on the slab (Assume V0 is constant.

2. Relevant equations
F= d/dx(U)
U=1/2CV2

3. The attempt at a solution
I am through determining the magnitude of the force (V020*(K-1)/2d, but am confused as to why the direction of this force is toward the left, which is what my book, Giancoli, says. I have attached a picture with this post.

The way I rationalized it was that because the dielectric raises the capacitance and the potential difference is held constant, the stored energy, U, should be higher with the dielectric, and thus you add energy to the system by putting in the slab. Thus, I would think that the electric field would be trying to push the slab out (to the right) to attain a lower energy state. What am I missing?

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