The electric field E→1 at one face of a parallelopiped is uniform over the entire face and is directed out of the face. At the opposite face, the electric field E→2 is also uniform over the entire face and is directed into that face (as shown in figure). The two faces in question are inclined at 30o from the horizontal, E→1 and E→2 (both horizontal) have magnitudes of 2.50×104NC−1 and 7.00×104NC−1, respectively. Assuming that no other electric field lines cross the surfaces of the parallelopiped, the net charge contained within is
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a
−67.5ε0C
b
37.5ε0C
c
105ε0C
d
−105ε0C
answer is A.
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Detailed Solution
To find the charge enclosed, we need the flux through the parallelepiped:Φ1=AE1cos60∘=(0.0500m)(0.0600m)2.50×104NC−1cos60∘=37.5Nm2C−1Φ2=AE2cos120∘=(0.0500m)(0.0600m)7.00×104NC−1cos120∘=−105Nm2C−1So, the total flux isΦ=Φ1+Φ2=(37.5−105)Nm2C−1=−67.5Nm2C−1q=Φε0=−67.5Nm2C−1ε0=−5.97×10−10C or −67.5ε0CThere must be a net charge (negative) in the parallelepiped since there is a net flux flowing into the surface. Also, there must be an external field, otherwise all lines would point toward the slab.