Q48 / 57 · JEE Advanced 2021 (Paper 1) · Physics

Two point charges $-Q$ and $+Q / \sqrt{3}$ are placed in the $x y$-plane at the origin $(0,0)$ and a point $(2,0)$, respectively, as shown in the figure. This results in an equipotential circle of radius $R$ and potential $V=0$ in the $x y$-plane with its center at $(b, 0)$. All lengths are measured in meters.
Figure
The value of b is ________ meter.

Correct answer: 3

Solution

Let Ph,k be be a general point with potential to be zero.

Figure

V1+V2=0   (as potential given =0)

K−Qh2+k2+KQ3h−22+k2=0

13h−22+k2=1h2+k2

3h−22+k2=h2+k2

3h2−4h+4+k2=h2+k2

2h2+2k2−12h+12=0

h2+k2−6h+6=0

h2−6h+6+k2=0

h2−6h+9+k2=3

h−32+k2=3

Standard equation of circle is given by

x−x12+y−y12=R2

∴  x1=3, y1=0 and R=3

Hence, R=3 m

and value of b=3 m 

Hence, the correct answer is 3.

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