(a) Define electric flux. Write its S. I. ...

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

\vec{E}=3 \times 10^3 \hat{i} \ \mathrm{N}/\mathrm{C}

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

10 \ \mathrm{cm}^2

when

(i) its plane is parallel to the

y-z

plane, and

(ii) the normal to its plane makes a

60^{\circ}

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: }\; \mathrm{Nm}^2 / \mathrm{C}

(b)

\phi =|E| A \cos \phi

(i) When the plane is parallel to

Y-Z

plane, then

\phi =0^{\circ}\;

\therefore\; \phi\;=3 \times 10^3 \times (0.1)^2 \cos 0^{\circ}

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

(ii) When normal to the plane makes an angle

60^{\circ}

with

X

-axis, then

\phi =60^{\circ}

\therefore \;\; \phi \;=3 \times 10^3 \times (0.1)^2 \cos 60^{\circ}

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

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