Sunday
May 19, 2013

# Homework Help: E&M Physics

Posted by Robert on Friday, May 18, 2012 at 5:06pm.

Hello,
I have this number where I don't understand why there's a negative sign to it.

Consider a closed triangular box resting withing a horizontal electric field of magnitude E = 7.8 x 104 N/C as shown here. Calculate the electric flux through

(a) the vertical surface,

with all the measurements I get 1.8, but the answer is -1.8. Why is that? I mean the Electric Field is going through the rectangle and A should be in the same direction, because in the the next question they ask you about how much Flux goes through the slanted surface, and that one was positive.

• E&M Physics - bobpursley, Friday, May 18, 2012 at 5:09pm

on the surface, which way is negative, and which way is positive? Look at the coordinate system. Normally, on a box, outward is +, but that is not standard. If the E field is external, then on one vertical surface it will be going in (neg), and on the bottom, it will be +, going out.

• E&M Physics - Robert, Friday, May 18, 2012 at 5:21pm

this is the image

• E&M Physics - bobpursley, Friday, May 18, 2012 at 5:28pm

I cant access it. Reread my explaination on + or - directions.

• E&M Physics - Elena, Friday, May 18, 2012 at 6:27pm

The flux is the dot product
Ô = (E,A) = E•A•cosα = =7.8•10^4•0.1•0.3•(-1) = - 2340 V•m.

α is the angle made by outward normal to the surface with the direction of the field lines. Here α = 180º, => cosα = -1

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