# Homework Help: Electric energy and speed

1. Dec 7, 2012

### Lolagoeslala

1. The problem statement, all variables and given/known data
An electron is released from rest from the negative plate of a parallel-plate apparatus, what is the electrons speed one third of the way between the plates. It also asks for the speed at which electron will hit the positive plate if a 450 V potential difference is applied.

3. The attempt at a solution

a)
Eel = V x q
Eel = ( 450 V ) x (1.6 x 10^-19C)
Eel = 7.2 x 10^17 J

Eel = 1/2mv^2
V = √((2 x (7.2 x 10^17 J))/9.1 x 10^31Kg)
V = 1.26 x10^7 m/s

b) i don't know how to do this?

2. Dec 7, 2012

### Staff: Mentor

I don't understand your answer's division into a) and b). The a) section looks like a response to the second question in the problem statement. What happened to the first question? Or is that now part b)?

Also, watch out for the signs associated with exponents; there's a rather large difference between 10^17 J and 10^-17 J. Same goes for the electron's mass. Your final velocity for the 450V excursion looks okay though.