In figure (a) below, an electron is shot directly away from a uniformly charged plastic sheet, at speed vs = 2.13 × 10^5 m/s. The sheet is nonconducting, flat, and very large. Figure (b) below gives the electron's vertical velocity component v versus time t until the return to the launch point. What is the sheet's surface charge density?
There is also a graph that shows Velocity at various times. It is 0 at 7 ps.
V = Vo + at
a = QE/m (Q = 1.609 x 10^-19, m = 9.109 x 10^-31)
E = σ/ε₀ (ε₀ = 8.85 x 10^-12) -> σ = Eε₀
The Attempt at a Solution
So I found acceleration with the first formula, then found E with the second, and found σ with the third. My answer was correct to the second decimal point, but when I did my friends' problems, the answer required me to subtract .4 from both of them to get the correct answer. I'm not sure if I did something wrong or if the homework website was wrong. If its any help, my solution was about 1.5. One of my friends problem had initial velocity of 2.03 x 10^5, all other numbers used were the same (give the same graph), and I calculated 1.4, but the correct answer was a little under 1.
Thanks for any help.