# Homework Help: Electric field problem

1. Nov 9, 2015

### gracy

1. The problem statement, all variables and given/known data
The potential at a point x(measured in µm)due to some charges situated on the x axis is given by $V(x)$=$\frac{20}{x^2-4}$V.The electric field $E$ at x=4µm is ?
2. Relevant equations
$E$=-$\frac{∂V}{∂r}$

3. The attempt at a solution
Actually I have solution but still I am unable to understand.So ,instead of my attempt at a solution .I 'll post solution itself.
$Ex$=-$\frac{∂V}{∂x}$ (1)

= -$\frac{d}{dx}(\frac{20}{x^2-4})$ (2)

=$\frac{40x}{(x^2-4)^2}$ (3)

$Ex$ at x=4µm =$\frac{10}{9}$V/µm
Here I am not able to understand how it proceeded from (2) to (3)

2. Nov 9, 2015

3. Nov 9, 2015

### gracy

And how to proceed from (3) to answer?

4. Nov 9, 2015

### blue_leaf77

You just plug in x=4 into the expression, right?

5. Nov 9, 2015

### gracy

Yes,of course.