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NCERT Class XII Chapter
Electric Charges and Fields
Questions With Solutions

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Question : 15
Total: 34
Consider a uniform electric field
→
E
 = 3 × 109
^
i
NC−1.
(a) What is the flux of this field through a square of 10 cm on a side whose plane is parallel to the yz plane?
(b) What is the flux through the same square if the normal to its plane makes a 60° angle with the x-axis?
Solution:  
(a) Area of square, A = a2 = 102 = 100 cm2 = 100 × 10–4 m2 = 10–2m2
As the plane is along yz plane, so area vector
→
A
 is directed along x-axis
i.e.,
→
A
 = (10−2
^
i
)m2
∴ Electric Flux through the square is
ϕ =
→
E
.
→
A
 = (3×103
^
i
) . (10−2
^
i
) = 3 × 101 or ϕ = 30 V-m
(b) When normal to plane i.e.
→
A
 makes an angle of 60° with
→
E
 , then ϕ' = EA cos 60° = 3 × 103 × 10−2 × 1/2 = 1.5 × 101 or ϕ' = 15 V-m
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