Question

A circular ring of radius $r$ made of a non- conducting material is placed with its axis parallel to a uniform electric field. The ring is rotated about a diameter through ${180}^{\circ }$. The flux of electric field,

• A. increases
• B. decreases
• C. does not change
• D. is zero

Answer 1

The correct option is B decreases


When a circular ring is placed with its axis parallel to uniform electric field, the angle made by positive normal to it with electric field is $\theta =0^{\circ }$, as shown in the above diagram.

So, initial electric flux is,
${\varphi }_i=\overrightarrow{E}.\overrightarrow{A}=EA\cos 0^{\circ }=EA=E\pi r^2$

But when circular ring is rotated, about a diameter, through ${180}^{\circ }$, the angle made by positive normal to it with electric field is $\theta ={180}^{\circ }$.

So, the final electric fluxis , ${\varphi }_f=\overrightarrow{E}.\overrightarrow{A}=EA\cos {180}^{\circ }=-EA=-E\pi r^2$

Hence, flux of electric field decreases.


Hence, $\left(B\right)$ is the correct answer.