CBSE Class 12 Physics 2019 Delhi Set 2 Paper

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Question : 5
Total: 6
(a) Draw equipotential surfaces corresponding to the electric field that uniformly increases in magnitude along with the Z-direction.
(b) Two charges −q and +q are located at point (0,0,−a) and (0,0,a). What is the electrostatic potential at the points (0,0,±z) and (x,y,0) ?
Solution:  
(a)

Vz‌=‌
kq
(z−a)
+‌
k(−q)
(z+a)
‌ if
‌z
>a

‌=Kq[‌
1
z−a
−‌
1
z+a
]
=Kq‌
2a
z2−a2

Potential at pt. (0,0−z)
V(−z)‌=‌
Kq
(z−a)
+‌
K(−q)
(z+a)

‌=Kq[‌
1
z−a
−‌
1
z+a
]

‌=Kq‌
2a
z2−a2

∵r for the point is √x2+y2+a2
∴‌‌V(x,y,0)‌=‌
Kq
(x2+y2+a2)1∕2
+‌
K(−q)
(x2+y2+a2)1∕2

‌=0.
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