# Question on electric filed.

1. Jun 19, 2010

### seto6

NOT HOME WORK QUESTION (itsfrom last year exam question)

1. The problem statement, all variables and given/known data
electric field between two disk of radius of R and at distance D=a/2 is given by the following
n=charge density (Q/A) E_0= the constant 8.85*10^-12

E=(n/E_o)((1- (a/2)/(((a/2)^2)+(R^2))^1/2

for what value of the ratio of the plate radius to separation between the plates does the electric field at point x=a/2 deffer by 1percent from the result n/E_o for infinite sheets?
R/a=......
2. Relevant equations

the equation for infinite sheet is E=n/2E_o

3. The attempt at a solution
not sure how to approach this question
is it R/2a=0.01 then R/2a*0.01=50?
please can some one explain this to me!!

2. Jun 20, 2010

### hermy

What the question means is that E = 0.99 n / $$\epsilon$$

or it could also be E = 1.01 n / $$\epsilon$$.

E is given in terms of a and R, so finding the ratio should not be a problem.

Are you sure, though, that it is " 1 - a/2 " in the numerator? In this case, finding the ratio would be impossible.