# Homework Help: Electric flux of surface

1. Feb 3, 2009

### katta002

1. The problem statement, all variables and given/known data
A flate surface with area 0.14m^2 lies in x-y plane in a uniform electric field given by E=5.1i+2.1j+3.5k kN/c

find the flux through this surface

2. Relevant equations

I know that flux= EA which E and A are given so I multiply The area with the victor E but I got a wrong answer

3. The attempt at a solution
0.14m^2(5.1i+2.1j+3.5k kN/c)=7.14e-1i+2.94e-1j+4.9e-1z

but the correct answer is 490Nm^2/C,

1. The problem statement, all variables and given/known data
3)A 10-{ nC} point charge is located at the center of a thin spherical shell of radius 8.0 { cm} carrying -20 {{ nC}} distributed uniformly over its surface.

A.What is the magnitude of the electric field 2.0 {cm} from the point charge and the direction of the field?

B. What is the magnitude of the electric field 15 { cm} from the point charge?

2. Relevant equations
E outside spherical =Q/4pi$$k\ =\ 1.3806503(24)\ \times\ 10^{-23}\ J\ K^{-1}$$r^2

and E inside= Qr/4pi$$k\ =\ 1.3806503(24)\ \times\ 10^{-23}\ J\ K^{-1}$$r^3

3. The attempt at a solutionR^3
A)and since 8.0cm>2.0cm it's outside so I used the outside equation
(10e19)(2e-2)/(4pi(9e-2)(8e2^3)=3.45e3

but the correct answer is 230kN/C

b) since 15cm>8.0cm the E is inside therefore
(.08)/4pi(9e-12)(.15)=4.19.e3

1. The problem statement, all variables and given/known data
2)A sock comes out of the dryer with 10^12 excess electron.
What's the electric flux through a surface surrounding the socks?

2. Relevant equations
Flux= EA
but since the area was not given
flux= Qenclosed?$$k\ =\ 1.3806503(24)\ \times\ 10^{-23}\ J\ K^{-1}$$

3. The attempt at a solution
10^12.9e-12=1.e23

please help me because I have a test on flux tomorrow and without understanding this might cause me. thanks.

2. Feb 3, 2009

### tiny-tim

Welcome to PF!

Hi katta002! Welcome to PF!
You need, not the flux through the area, but the component of flux in the direction normal (perpendicular) to the area.

So the normal to the area is … ?, and the component of flux in that direction is … ?

(and how are you doing on the others?)​

3. Feb 3, 2009

### katta002

Re: Welcome to PF!

Thank you for welcoming me PF and boy am i glad to find this website
So I multiply the area by the i direction, j direction and K direction and find the sum of all direction but still didn't get the answer
i=.714
j=.294
k=.49
the sum is 1.498, I still don't get it.

and the rest that I posted I'm still facing difficulties so could u break it down for me, thanks and God bless you

4. Feb 3, 2009

### tiny-tim

that's not the component in the normal direction …

it's not even a dot-product of anything.

What is the normal direction (of the area)?​

5. Feb 3, 2009

pi*r^2

6. Feb 4, 2009

### tiny-tim

uhh?

The plate lies in the x-y plane …

so the normal direction is the z-direction (k) …

so the question is asking what is the component of flux in the k-direction?

(btw, did you find out how to post in the academic guidance forum?)

Last edited: Feb 4, 2009