To find the velocity of the particle

[Clari]

A metal sphere of radius 0.10m carries a positive charge of $$1.0 x 10^{-4}C$$. A particle of mass $$2.0 x 10^{-5}kg$$ carrying a negative charge of $$1.5 x 10^{-10}C$$ is released from rest at a distance of 1.0m from the centre of the sphere. Calculate the velocity of the particle when it srtikes the surface of the sphere. Neglect the gravitational effect.

I cannot get the correct answer, which is 11m/s...anyway, here are my steps:

KE gained = PE loss
1/2 mv^2 = $$\frac{Qq}{4\pi\epsilon r}$$
v = 3.87 m/s

 

[SpaceTiger]

I cannot get the correct answer, which is 11m/s...anyway
v = 3.87 m/s

Your method is right, you're just using the wrong distance in the PE. Remember, the particle goes from r=1 m to r=0.1 m, not from r=infinity to r=1 m.

 

[Clari]

Thank you SpaceTiger!
I get the answer now, I thought that the PE on reaching the surface of the sphere is zero... :tongue: