# [symbolic] Linear uniform charge density (E field at a point)

1. Nov 29, 2009

### syntroniks

1. The problem statement, all variables and given/known data
Center a rod of length L at (0,0) with the length going horizontally.

Take a point P at (0,y).

Find the electric field at P.

2. Relevant equations
$$\lambda$$=Q/L
$$E= \int k*dQ/R^{2}$$

3. The attempt at a solution
I am integrating from -L/2 to L/2
Since Q=lambda*L, I guess differentially dQ=lambda*dL.

Substituting that into the integral, it becomes:
$$k*\lambda \int dL/R^{2}$$
from -L/2 to L/2 of course.

R is pretty messy so I'll just write what I came up with for $$R^{2}$$:
$$R^{2}=(L^{2}/4)+y^{2}$$

So... Doesn't this seem pretty reasonable? I just want to be double sure that this is OK.

2. Nov 29, 2009

### Delphi51

There may be a confusion with L, used in two different ways here.
I suggest you change dQ=lambda*dL to dQ=lambda*dx, where x is a distance along the x axis. This x runs from -L/2 to L/2.
I think you'll find that R² = x² + y².
Looks like one of those trig substitution integrals.

3. Nov 29, 2009

### syntroniks

Thankfully for this particular problem I get to do the integration by software... kinda. Turns out to involve arctangent and a relative mess of symbols.

Thanks for the suggestion about variables, it is definitely more clear that way.

4. Nov 29, 2009

### Delphi51

Looks like the substitution x = y*tan A really simplifies it!
And the A is a real angle in the problem.