# Homework Help: Electric Field Velocities from Point A to Point B

1. Mar 2, 2010

### drprevet

1. The problem statement, all variables and given/known data

A proton's speed as it passes point A is 4.90×104 . It follows the trajectory: Point A @ 30V through Point B @-10V. What is the proton's speed at point B?

2. Relevant equations

deltaV=deltaK/q
V=40
vi=4.9*10^4
q=1.6*10^-19
m=1.67*10^-27
deltaK=1/2mvf^2-1/2mvi^2

3. The attempt at a solution

40=(0.5)(1.67*10^-27)(vf^2)-=(0.5)(1.67*10^-27)(4.9*10^4)/1.6*10^-19

velocity final=8.75*10^4

2. Mar 3, 2010

### ehild

The calculation is easier if you factor the right side of the equation:

$$40=\frac{0.5*1.67\cdot 10^{-27}}{1.6\cdot 10^{-19}}(v^2-(4.9\cdot 10^4)^2)$$

and then simplify.

ehild