# Question about parallel plate stuff

## Homework Statement

A cell membrane consists of an inner and outer wall separated by a distance of approximately 10 nm. Assume that the walls act like a parallel plate capacitor, each with a charge density of 10^−5C/m^2, and the outer wall is positively charged. Although unrealistic, assume that the space between cell walls is filled with air.

Part A
What is the magnitude of the electric field between the membranes? Answer: 1*10^6 N/C

Part B
What is the magnitude of the force on a K+ ion between the cell walls? Answer: 2*10^-13 N

Part C
What is the potential difference between the cell walls? Answer: 1*10^-2 V

Question : If released from the inner wall, what would be the kinetic energy of a 7 fC charge at the outer wall? 1fC = 10^−15C.

KE = q*Delta(v)

## The Attempt at a Solution

I have solved each of the other parts of this question. I found that Delta V was 1×10^−2V. The question states that 1fC is 10^-15 C. I plugged in for KE = (7*10^-15)(0.01V), and keep getting 7*10^-17. However the answer is 8*10^-17. Can anyone catch my mistake, or am i using the wrong formula?

Last edited:

NascentOxygen
Staff Emeritus
Hi johnknee. There is a figure to accompany this problem, I believe?

rude man
Homework Helper
Gold Member

## Homework Statement

For part A, I used the equation E = charge density/Permittivity of free space = 10^-5/(8.85*10^-12) = 1*10^6 N/C

NascentOxygen
Staff Emeritus
10^-5/(8.85*10^-12) = 1*10^6 N/C
= 1.13×106 N/C

rude man
Homework Helper
Gold Member
For part A, I used the equation E = charge density/Permittivity of free space = 10^-5/(8.85*10^-12) = 1*10^6 N/C

Yeah I have many decimal points haha, it's just that the answer given by masteringphysics is in 1 sig fig.

rude man
Homework Helper
Gold Member
Yeah I have many decimal points haha, it's just that the answer given by masteringphysics is in 1 sig fig.
Well, yer' not likely to master it with that! LOL

haruspex