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 300 from the horizontal, E→1 and E→2 both horizontal) have magnitudes of 2.50×104 NC−1 and 7.00×104 NC−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
−67.5 ε0 C
b
37.5 ε0 C
c
105 ε0 C
d
−105 ε0 C
answer is A.
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Detailed Solution
To find the charge enclosed, we need the flux through the parallelepiped:Φ1=AE1cos600=(0.0500 m)(0.0600 m)(2.50×104 NC−1)cos600=37.5 Nm2C−1Φ2=AE2cos1200=(0.0500 m)(0.0600 m)(7.00×104 NC−1)cos1200=−105 Nm2C−1So, the total flux isΦ=Φ1+Φ2=(37.5−105)Nm2C−1=−67.5 Nm2C−1q=Φε0=(−67.5 Nm2C−1)ε0=−5.97×10−10 CThere 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.