# Homework Help: Clueless about electric charges questions; need help!

1. Feb 1, 2004

### Matt Jacques

Oh boy, for some reason I'm just not getting this. I must be fouling up some really simple part. He just dives into problems without explaining the derivations and uses of the problems. Forgive my lack of knowledge on use of units.

My questions:

"A uniform line charge extends from x = -2.5 to x = 2.5 and charge density of 6.0 nC/m. a ) find the total charge

Ok, that's easy. That's just Q = 6.0 nC/m * .05 m = .3 * 10^-9

B) Find the electric field at 4.0 cm

The examples infer I should use:

E = kQ / (xp^2 - (1/2L)^2) where xp = (1/2)L

Is that the right formula?

Next question:

A 2 micro-coulomb charge is .2 m above the center of a square of sides .4 m. Find the flux through the square.

I found the electric field at the square with:

E = kQ/ r^2

E = (9*10^9)(2*10^-6) / (.2^2)
E = 450,000

Flux = E*A
Flux = (450,000)(.4^2) = 72,000

However, the book says 37700 ...

Next...

"The electric field at 250 m is 150 n/C and at 400 m it is 170 N/C. Calculate the volume charge density"

I'm just clueless here.

Any help is appreciated. I'll be back for more help soon :(

2. Feb 1, 2004

### Matt Jacques

no one has a clue?

3. Feb 1, 2004

### himanshu121

Ya for Pro1

For Finite Line of Charge use the formulae

$$E_{n}=\frac{\lambda}{4\pi \epsilon_{0} x}(\sin\alpha+sin\beta)$$

Similarly for
$$E_{t}=\frac{\lambda}{4\pi \epsilon_{0} x}(\cos\alpha+cos\beta)$$

where alpha & beta are the angles which the point makes with the line perpendicular to the line of charge and the line joing the top point of Charge & bottom point of line charge

Last edited: Feb 1, 2004