Question

A particle of mass 2g and charge $1\mu C$ is held at a distance of 1 metre from a fixed charge of $1mC.$ If the particle is released it will be repelled. The speed of the particle when it is at a distance of 10 metres from the fixed charge is

• A. 100 m/s
• B. 90 m/s
• C. 60 m/s
• D. 45 m/s

Answer 1

Answer: (B)

90 m/s

Answer 2

According to conservation of energy

Energy of moving charge at $A =$Energy of moving charge at B

$9 \times 10^{9} \times \frac{10^{- 3} \times 10^{- 6}}{1} = 9 \times 10^{9} \times \frac{10^{- 3} \times 10^{- 6}}{10} + \frac{1}{2} \times (2 \times 10^{- 3})v^{2} \Rightarrow$ $v^{2} = 8100$ $\Rightarrow$ $v = 90\text{m/sec}$