# Homework Help: Capacitance, Charge Density, and Young's Modulus

1. Oct 17, 2004

### ZedLeppelin

I've done a search for some help on my problem, but I haven't seen anything resemble it. Anyway my question is the following:

Not all dielectrics that separate the plates of a capacitor are rigid. For example, the membrane of a nerve axon is a bilipid layer that has a finite compressibility. Consider a parallel-plate capacitor whose plate seperation is maintained by a dielectric of dielectric constant (kappa) = 3.0 and thickness d = 0.2 mm, when the potential across the capacitor is zero. The dielectric, which has a dielectric strength of 40 kV/mm is highly compressible, with a Young's modulus for compressive stress of 5*10^6 N/m^2. The capacitance of the capacitor in the limit V -> 0 is C(initial). Derive an expression for the capacitance, as a function of voltage across the capcitor.

Well, what I have so far is C = Q/V and C = epsilon(zero)*A/d

I'm pretty sure I should be doing an indefinte integral of the funtion at the end seeing as they give me a limit whereby the constant will just be C(initial) since they want capacitance as a function of voltage.

I have an idea of what I should be doing to bring the voltage to the top of the equation by multiplying C by V/Q so that I get:

C = (epsilon(zero)*A*V)/(d*Q)

and then to get rid of the Q, I'd have the linear charge density relationship:

sigma = Q / A

which when subbed into my equation above will give me:

C = (epsilon(zero)*V)/(sigma*d)

I'm stuck here, on how to relate Young's modulus to the charge density.

The answer given in the back of the book is C(V) = C(initial)[(1 + V^2*epsilon(zero)*(kappa))/(2Y*d)]

The question I'm having to resolve here now is how to relate Young's modulus, Y, to the linear charge density, kappa, and the initial capacitance. Thanks to anyone who can shed some light on this, and if I'm actually headed in the right direction.