CBSE Class 12 Physics 2013 Paper

Question : 22 of 29

Marks: +1, -0

(a) Define electric flux. Write its S. I. units.
(b) Consider a uniform electric field

{E}↖{→}=3 × 10^3 {i}↖{\^} {\N} \/ {\C}

. Calculate the flux of this field through a square surface of area

10 {\cm}^2

when
(i) its plane is parallel to the

y-z

plane, and
(ii) the normal to its plane makes a

60^{∘}

angle with the

x

-axis.

Solution:

Electric flux: Number of electric field lines crossing an area in a direction normal to it.

\;\text " SI unit: "\; {\Nm}^2 \/ {\C}

(b)

ϕ =|E| A \cos ϕ

(i) When the plane is parallel to

{\Y}- {\Z}

plane, then

ϕ =0^{∘}\;

∴ \;ϕ \;=3 × 10^3 ×(0.1)^2 \cos 0^{∘}

\;=30 {\Nm}^2 \/ {\C}

(ii) When normal to the plane makes an angle

60^{∘}

with

{\X}

-axis, then

ϕ =60^{∘}

∴ \;\; ϕ \;=3 × 10^3 ×(0.1)^2 \cos 60^{∘}

\;=15 {\Nm}^2 \/ {\C}

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