Question

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.

Answer 1

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.

Answer 2

this is the image
ww w.ux1.eiu.edu/~cfadd/1360/24Gauss/24Hmwk/24.05. html

Answer 3

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

Answer 4

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