# Find thekinetic energy of the sphere from line of charge

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1. Mar 1, 2017

### wormwoodsilver101

1. The problem statement, all variables and given/known data
A small sphere with mass 5.00×10−7 kg and charge +6.00 μC is released from rest a distance of 0.400 m above a large horizontal insulating sheet of charge that has uniform surface charge density σ = +8.00 pC/m2.Using energy methods, calculate the speed of the sphere when it is
0.100 m above the sheet.

2. Relevant equations
V=integrate(E dl) from a to b
E=σ/2ε
3. The attempt at a solution
σql/2ε=0.5mv^2
(8*10^-12)(6*10^-6)(0.4-0.1)/2ε=0.5(5*10^-7)v^2
v=1.8m/s
But mastering physics keeps saying it is wrong
I don't know what did I did wrong

2. Mar 1, 2017

### BvU

Hello Wormwood,

What if gravity were 19.62 m/s2 instead of 9.81 m/s2 ? Would it go just as fast then ?

3. Mar 1, 2017

4. Mar 1, 2017

### haruspex

Both charges are positive, so why did it fall?

5. Mar 1, 2017

### wormwoodsilver101

Ah, I totally ignored gravity >.<
With gravity the formula is:
mgh=σql/2ε+0.5mv^2
(5*10^-7)(9.81)(0.4-0.1)=(8*10^-12)(6*10^-6)(0.4-0.1)/2ε+0.5(5*10^-7)v^2
v=1.62m/s