# Homework Help: Given conductivity, calculate E, D, and P vectors

1. Feb 15, 2010

### aliaze1

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

The gap between a pair of parallel infinite copper plates extends from z = 0 to z = W > 0 and is originally occupied by vacuum (ε0, μ0). The plates carry equal and oppositely signed surface charge densities and as a consequence we have a constant electric field E = −4ˆz V/m in vacuum in the gap region.

What would be the new equilibrium values of E, D, and P in the gap region if some amount
of salt were dissolved in the fluid in the gap (64ε0) to raise its conductivity to σ = 4 S/m (conductivity of sea water)?

State the values of E, D, and P after a steady-state equilibrium is reached and briey explain your answer.

2. Relevant equations

DE0*E0+P

P=polarization vector

3. The attempt at a solution

I have the equation J = σE, but I do not know how to relate this to D and P, since I am not given J in the problem

Thanks!!!

Last edited by a moderator: Apr 24, 2017
2. Feb 15, 2010

### gabbagabbahey

Try to look at this problem qualitatively first. What does Ohm's Law ($\textbf{J}=\sigma\textbf{E}$) tell you the current will be when the salt is first added? What does that current do to the amount of charge on each plate? What effect does that have on the electric field in the gap?....Can you guess what the electric field will be when equilibrium is reached?