# Homework Help: Determining electric permittivity from time constant decay

1. Jul 9, 2012

### Dergyll

1. The problem statement, all variables and given/known data

a slab of some insulator with an unknown permittivity ε. To determine ε experimentally I
go to the lab and insert the slab in between the plates of a capacitor whose plate spacing exactly
matches the width of the slab. I observe that the time constant of exponential decay of the
capacitor voltage in an RC circuit that I construct increases by 25% when the slab is inserted
to replace the air spacing. Determine ε in terms of εo. Explain your reasoning carefully.

2. Relevant equations

The problem is, I'm having troube finding a place to start. We are learning about d'Alembert wave solutions, Poynting theorem, TEM waves and such.

3. The attempt at a solution

I first thought of using the solution to the differencial equation with the decay model but that doesn't seem to help me

Can anyone point me in the right direction as to where to start with this problem? Any help is greatly appreciated.

2. Jul 9, 2012

### gabbagabbahey

Well, what part(s) of the RC circuit do you think gets affected when you place a dielectric between the parallel plates of the capacitor in that circuit? Could you calculate that effect if you knew the permittivity?