Q.

Three infinitely long wires with linear charge density λ are placed along the x-axis, y-axis and z-axis respectively. Which of the following denotes an equipotential surface          [2025]

1 xy + yz + zx = constant  
2 (x + y)(y + z)(z + x) = constant  
3 (x2+y2)(y2+z2)(z2+x2) = constant  
4 xyz = constant  

Ans.

(3)

V=E·dr=2kλrdr=2kλ ln r+c

Net potential due to all wires

V=2kλ ln x2+y2+2kλ ln y2+z2+2kλ ln z2+x2+c

for V to be constant

(x2+y2)(y2+z2)(z2+x2)=c'

 (x2+y2)(y2+z2)(z2+x2)=c

where c = constant