Three large identical conducting parallel plates carrying charge +Q,−Q and +2Q respectively, are placed as shown in the figure. If EA,EB, and EC refer to the magnitudes of the electric fields at points A,B and C respectively, then
A
EA>EB>EC
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B
EA=0 and EC>EB
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C
EA=0 and EB>EC
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D
EA=0 and EB=EC
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Solution
The correct option is BEA=0 and EC>EB E1,E2 and E3 are electric fields due to plates 1,2 and 3 respectively. Let A is the surface area throught which the electric field lines pass perpendicularly.
Net electric field at point point A:
EA=E1+E2−E3
Substituting the values of electric field from figure,
EA=Q2ϵ0A+Q2ϵ0A−2Q2ϵ0A
∴EA=0
Now net electric field at point B:
EB=E1−E2−E3
Substituting the values of electric field from figure,
EB=Q2ϵ0A−Q2ϵ0A−2Q2ϵ0A
∴EB=−Q2ϵ0A
Similarly, net electric field at point C:
EC=E1−E2+E3
Substituting the values of electric field from figure,