Above a Charged Plate

òòclosed area E·dA = 4pkQenclosed .

The four sides have dA’s that are horizontal, while the E field is vertical. Thus the dot product in Gauss’ Law says these do not contribute.

However, both the top and bottom sides have

areas that are parallel

to the field. Hence we

have EAtop + EAbottom =

2EA = 4pkQencl, or E = 2pks.

s=Q/A

x

L/2

E

L

L

dA

dA

dA

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