Question

A charge of 5 nC is uniformly distributed on a ring of
radius 4 cm. Find the potential at the axis at a
distance of 3 cm from the centre.
(1) 0.09 V
(2) 600 V
(3) 900 V
(4) 300 V
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Answer 1

Answer: (3)  900V

Explanation:

1) We have,  a ring where 5 nC charge is uniformly distributed over it's circumference.

Since,  Potential is a scalar quantity.

Potential at a distance 3cm from the

Centre is given by

$\frac{kQ}{x}$

where x : distance from circumference to that point.

2) We know,  

In Δ AOB,  by Pythagoras theorem,

$AB^2=AO^2+BO^2 \\ \\=>AB^2=4^2+3^2\\ \\=>x= AB = 5cm$

3) Required Potential,  

$V=\frac{kQ}{x}\\ \\=\frac{9*10^9*5*10^{-9}}{5 *10^{-2}}\\ \\= 900V$

Hence,  Requited potential will be 900V .